Determination of Trace Elements

Determination of Trace Elements Edited by Zeev B. Alfassi Balaban Publishers 3 +b VCH Weinheim . New York Base1 Ca...

Author: Zeev B. Alfassi

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Determination of Trace Elements Edited by Zeev B. Alfassi

Balaban Publishers

3

+b

VCH

Weinheim . New York Base1 Cambridge Tokyo

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Determination of Trace Elements Edited by Z. B. Alfassi

Balaban Publishers

3

0 VCH Verlagsgesellschafi mbH, D-69451 Weinheim (Federal Republic of Germany), 1994 ~~

~

Distribution: VCH, P.O. Box 101161, D-69451 Weinheim, Federal Republik of Germany Switzerland: VCH PO.Box, CH-4020 Basel, Switzerland United Kingdom and Ireland: VCH 8 Wellington Court, Cambridge CB1 lHZ, United Kingdom USA and Canada: VCH, 220 East 23rd Street, New York, NY 10010-4606, (USA) Japan: VCH, Eikow Building, 10-9 Hongo 1-chorne, Bunkyo-ku, Tokyo 113, Japan ISBN 3-527-28424-9

Prof. Zeev B. Alfassi Department of Nuclear Engineering Ben Gurion University Beer Sheva 84 102 Israel

This book was carefully produced. Nevertheless, authors, editors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Published jointly by VCH Verlagsgesellschaft, Weinheim (Federal Republic of Germany) VCH Publishers, New York, NY (USA)

Editorial Director: Miriam Balaban Production Manager: Dip].-Wirt.-Ing. (FH) Bernd Riedel

Library of Congress Card No: applied for A catalogue record for this book is available from the British Library.

Die Deutsche Bibliothek - CIP-Einheitsaufnahme

Determination of trace elements / ed. by Zeev B. Alfassi. Rehovot (Israel): Balaban Publ.; Weinheim; New York; Basel; Cambridge; Tokyo: VCH, 1994 ISBN 3-527-28424-9 NE: Alfassi, Zeev B. [Hrsg.]

0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim (Federal Republic of Germany), 1994 Printed on acid-free and low chlorine paper.

All rights reserved (including those of translation in other languages). No part of this book may be reproduced in anyform - by photoprinting, microfilm, or any other means - nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printing: stmuss offsetdruck gmbh, D-69509 Morlenbach Bookbinding: IVB Heppenheim GmbH, D-64646 Heppenheim Printed in the Federal Republic of Germany

Preface Elements in low concentration, "trace elements" are very important in various fields of science and technology. In medicine the concentration of several trace elements is very crucial since some elements are essential in low concentration while a little higher concentration is hazardous. Consequently, trace element determination is essential for food analysis, water analysis, criminology, etc. In the semiconductor industry, very minute amounts of impurities can drasticall change the electrical properties of the devices. In order to understand the effect of trace elements in biology or to obtain purer materials for the semiconductor industry, it is important to be able to measure accurately and precisely the concentiation of them. Various analytical methods are sensitive enough to measure these low concentrations; however, each method exhibits its highest sensitivity for different particular elements and has its own set of interferences. Consequently, it is important to know the most important methods in order to decide which one is the most appropriate for a special problem. It is the feeling of the editor that most analytical chemists are mainly involved in only a few techniques from the large number available nowadays. Consequently, it is almost impossible for one scientist to write on all the methods, as each method should be written by an expert. The book opens with four chapters dealing with general topics concerning all methods: systematic errors, quality control, sampling and preconcentration. The following seven chapters describe the main methods for trace element analysis. The last two chapters describe the application of the various methods in two areas; one dealing with speciation and the other with biological samples. Beer Sheva, July 1994

Zeev B. Alfassi

Errata Determination of Trace Elements edited by Z. B. Alfassi By mistake, the following texts have not been included:

Dedication To Sabina with love Many adventures with the new ICP

Contents (p. VII-XIV) 1 Systematic errors in trace analysis by G. Tolg and P. Tschopel . . . . . . . . . . . . . . . . . . 2 Limits of detection and accuracy in trace elements analysis by C. J. Kirchmer . . . . . . . . . . . . . . . . . . . . . . 3 Sampling and sample preparation by J. R. W. Woittiez and J. E. Sloof . . . . . . . . . . . . . . 4 Separation and preconcentration of trace elements by K. Terada . . . . . . . . . . . . . . . . . . . . . . . . 5 Determination of trace elements by atomic absorption spectrometry by 1. Z. Pelly . . . . . . . . . . . . . . . . . . . . . . . . 6 Plasma optical emission and mass spectrometry by J. A. C. Broekaert . . . . . . . . . . . . . . . . . . . . 7 Instrumental neutron activation analysis (INAA) by Z. B. Alfassi . . . . . . . . . . . . . . . . . . . . . . . 8 Radiochemical neutron activation analysis by Z. B. Alfassi . . . . . . . . . . . . . . . . . . . . . . . 9 Determination of trace elements by electron spectroscopic methods byM. Polak . . . . . . . . . . . . . . . . . . . . . . . . 10 l h c e element determination by electrochemical methods by R. von Wandruszka . . . . . . . . . . . . . . . . . . . . 11 Determination of trace elements by chromatographic methods employing atomic plasma emission spectroscopic detection by P. C. Uden . . . . . . . . . . . . . . . . . . . . . . . 12 Speciation of trace elements with special reference to the use of radioanalytical methods by H. A. Das . . . . . . . . . . . . . . . . . . . . . . . . 13 Trace elements in environmental and health sciences by G. V. Iyengar and V. Iyengar . . . . . . . . . . . . . . . .

1 39 59 109 145 191 253 309 359 393

425

46 I

543

1.1

1.2

1.3

1.4 1.5 2.1 2.2 2.3 2.4 2.5

2.6 2.7 2.8 3.1 3.2

3.3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 General aspects of extreme trace analysis . . . . . . . . . . 1.1.2 Direct instrumental determination methods . . . . . . . . . 1.1.3 Multi-stage procedures . . . . . . . . . . . . . . . . . . . 1.1.4 Further general important statements . . . . . . . . . . . . Systematic errors and their avoidance . . . . . . . . . . . . . . . . . 1.2.1 Volatilization . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Blanks from vessels, vessel materials and working tools . . 1.2.4 Blanks from the reagents . . . . . . . . . . . . . . . . . . 1.2.5 Blanks from airborne dust . . . . . . . . . . . . . . . . . 1.2.6 Contamination by sample handling . . . . . . . . . . . . . 1.2.7 Problems due to changes of the valency state . . . . . . . . Systeinatic errors during the analytical procedure . . . . . . . . . . . 1.3.1 Sampling, sample storage and pretreatment . . . . . . . . . 1.3.2 Decomposition . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Separation . . . . . . . . . . . . . . . . . . . . . . . . . Basic rules for the recognition and elimination of systematic ei-rors . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Errors in analytical results . . . . . . . . . . . . . . . . . . . . . . Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measuring trace concentrations . . . . . . . . . . . . . . . . . . . . The problem of detection . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Random error of blank responses . . . . . . . . . . . . . . 2.5.2 Errors of the first kind- the critical level (aposteriol-i detection) 2.5.3 Errors of the second kind - the limit of detection (a priori detection) . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.4 Limits to the use of the definitions of L, and L, . . . . . . Regression theory approaches to the problem of detection . 2.5.5 Practical applications . . . . . . . . . . . . . . . . . . . . . . . . . Reporting results at small concentrations . . . . . . . . . . . . . . . Conclusions and recommendations . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes in trace element composition . . . . . . . . . . . . . . . . 3.2.1 Element specific changes . . . . . . . . . . . . . . . . . . 3.2.2 Sample specific changes . . . . . . . . . . . . . . . . . . Pre-sampling considerations . . . . . . . . . . . . . . . . . . . . . VII

2 2 3 3 4 4 6 7 9 13 15 18 19 20 20 22 27 29 31 39 40 41 42 42 42 43

46 48 50 52 53 56 59 62 63 76 77

3.4

3.5 4.1

4.2

4.3

4.4

4.5

5.1 5.2 5.3 5.4 5.5 5.6

Aspects of sampling . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Establishment of analytical control . . . . . . . . . . . . . 3.4.2 Sampling error in a test portion . . . . . . . . . . . . . . . Uniformity of laboratory samples . . . . . . . . . . . . . . 3.4.3 3.4.4 Uniformity of subsamples . . . . . . . . . . . . . . . . . 3.4.5 The gross sample . . . . . . . . . . . . . . . . . . . . . . Sample decomposition . . . . . . . . . . . . . . . . . . . . . . . . Separation and preconcentration of trace elements by coprecipitation 4.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Coprecipitation with inorganic precipitants . . . . . . . . . 4.1.4 Coprecipitation with organic collectors . . . . . . . . . . . Separation and preconcentration of trace elements by flotation . . . . 4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 General procedures . . . . . . . . . . . . . . . . . . . . . Preconcentrauon and separation of trace elements by solvent extraction 4.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Extraction of trace elements . . . . . . . . . . . . . . . . Separation and preconcentration of trace elements by ion-exchange . 4.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Ion-exchange resins . . . . . . . . . . . . . . . . . . . . . 4.4.3 Equilibrium and selectivity . . . . . . . . . . . . . . . . . 4.4.4 Practical column operation . . . . . . . . . . . . . . . . . 4.4.5 Preconcentration . . . . . . . . . . . . . . . . . . . . . . 4.4.6 Ion chromatography . . . . . . . . . . . . . . . . . . . . Separation and preconcentration by sorption . . . . . . . . . . . . . 4.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Activated carbon . . . . . . . . . . . . . . . . . . . . . . 4.5.3 Porous polymers . . . . . . . . . . . . . . . . . . . . . . 4.5.4 Complex-forming adsorbents . . . . . . . . . . . . . . . . 4.5.5 Natural polymers . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quality of results . . . . . . . . . . . . . . . . . . . . . . . . . . . Standards and chemicals . . . . . . . . . . . . . . . . . . . . . . . Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII

83 89 89 92 93 93 94 110 110 110 112 113 114 114 116 117 118 118 121 128 128 129 130 132 133 134 137 137 137 138 140 141 146 147 148 150 152 154

5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 6.1

6.2

6.3

6.4

6.5

7.1 7.2

Major components of the instrument . . . . . . . . . . . . . . . . . 155 Radiation sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Wavelength selection system . . . . . . . . . . . . . . . . . . . . . 159 Atomization by flame . . . . . . . . . . . . . . . . . . . . . . . . . 164 Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Electrothermal atomization . . . . . . . . . . . . . . . . . . . . . . 173 Hydride generation . . . . . . . . . . . . . . . . . . . . . . . . . . 178 Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 Instrumental background corrections . . . . . . . . . . . . . . . . . 183 Modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Atomic spectrometry with plasma sources . . . . . . . . . . . . . . 192 6.1.1 Historical development . . . . . . . . . . . . . . . . . . . 192 6.1.2 Optical emission spectrometry . . . . . . . . . . . . . . . 194 6.1.3 Plasma mass spectrometry . . . . . . . . . . . . . . . . . 199 Plasma sources and sampling . . . . . . . . . . . . . . . . . . . . . 201 6.2.1 Arc and spark sources . . . . . . . . . . . . . . . . . . . 202 6.2.2 Flames . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 6.2.3 DC plasma jets . . . . . . . . . . . . . . . . . . . . . . . 204 6.2.4 Inductively coupled plasmas . . . . . . . . . . . . . . . . 205 6.2.5 Microwave discharges . . . . . . . . . . . . . . . . . . . 207 6.2.6 Sample introduction for plasma spectrometry . . . . . . . 209 6.2.7 Discharges under reduced pressure . . . . . . . . . . . . . 215 Plasma optical emission spectrometry . . . . . . . . . . . . . . . . . 217 6.3.1 Atomic emission spectrometry . . . . . . . . . . . . . . . 217 6.3.2 ICP-Atomic emission spectrometry . . . . . . . . . . . . . 220 6.3.3 MIP-Atomic emission spectrometry . . . . . . . . . . . . 223 6.3.4 Glow discharges . . . . . . . . . . . . . . . . . . . . . . 224 6.3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 226 Plasma mass spectrometry . . . . . . . . . . . . . . . . . . . . . . 226 6.4.1 ICP mass spectrometry . . . . . . . . . . . . . . . . . . . 227 6.4.2 Glow discharge mass spectrometry . . . . . . . . . . . . . 239 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 6.5.1 Power of detection . . . . . . . . . . . . . . . . . . . . . 241 6.5.2 Interferences . . . . . . . . . . . . . . . . . . . . . . . . 242 6.5.3 Economic aspects . . . . . . . . . . . . . . . . . . . . . . 243 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 Basic nuclear physics . . . . . . . . . . . . . . . . . . . . . . . . . 255 IX

7.2.1 Nuclides . . . . . . . . . . . . . . . . . . . . . . . . . . 255 256 7.2.2 Radioactive decay . . . . . . . . . . . . . . . . . . . . . 7.2.3 Kinetics of decay of radioactive nuclides . . . . . . . . . . 260 7.2.4 Kinetics of formation of radioactive nuclides by irradiation 261 7.2.5 The chart of the nuclides . . . . . . . . . . . . . . . . . . 265 267 Gamma detection systems . . . . . . . . . . . . . . . . . . . . . . . 7.3 7.3.1 NaI(T1) - scintillation detector . . . . . . . . . . . . . . . 268 7.3.2 Solid-state ionization detector . . . . . . . . . . . . . . . 268 7.3.3 The shape.of y spectrum . . . . . . . . . . . . . . . . . . 273 278 7.4 Irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 7.4.1 Neutron sources . . . . . . . . . . . . . . . . . . . . . . 279 7.4.2 Samples introduction . . . . . . . . . . . . . . . . . . . . 7.5 Instrumental neutron activation analysis (INAA) . . . . . . . . . . . 280 7.5.1 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 280 7.5.2 Calculation . . . . . . . . . . . . . . . . . . . . . . . . . 280 284 7.5.3 Nuclear interferences . . . . . . . . . . . . . . . . . . . . A test case INAA of trace elements in silicon . . . . . . . 285 7.5.4 288 7.5.5 EpithermalINNA . . . . . . . . . . . . . . . . . . . . . . 296 7.5.6 Fast neutrons INAA . . . . . . . . . . . . . . . . . . . . 7.5.7 CyclicINAA . . . . . . . . . . . . . . . . . . . . . . . . 299 7.5.8 Prompt Gamma Neutron Activation Analysis (PGNAA) . . 301 7.5.9 Depth profiling by INAA . . . . . . . . . . . . . . . . . . 302 309 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Samples dissolution . . . . . . . . . . . . . . . . . . . . . . . . . . 312 8.2 8.2.1 Geological samples . . . . . . . . . . . . . . . . . . . . . 312 312 8.2.2 Metalsamples . . . . . . . . . . . . . . . . . . . . . . . 8.2.3 Biological samples . . . . . . . . . . . . . . . . . . . . . 313 . . . . . . . . . . . . . . . . . . . . . . 316 Radiochemical separations 8.3 8.3.1 Radiochemicalseparationsinmaterialsciences . . . . . . 317 8.3.2 RNAA of geological and environmental samples . . . . . . 326 RNAA of biological samples . . . . . . . . . . . . . . . . 340 8.3.3 359 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 General review of X-ray photoelectron and auger electron spectroscopies362 363 9.2.1 Basic principles . . . . . . . . . . . . . . . . . . . . . . . 9.2.2 Surface sensitivity and depth profiling . . . . . . . . . . . 369 9.2.3 Chemical-state information . . . . . . . . . . . . . . . . . 374 9.2.4 Quantitative analysis . . . . . . . . . . . . . . . . . . . . 375 9.3 XPSjAES applicationsintraceelementdetermination . . . . . . . . 384 X

9.4 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Anodic and cathodic stripping voltammetry . . . . . . . . . . . . . . 10.2.1 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Stripping waveforms . . . . . . . . . . . . . . . . . . . . 10.2.3 Film stripping . . . . . . . . . . . . . . . . . . . . . . . . 10.2.4 Cathodic stripping . . . . . . . . . . . . . . . . . . . . . 10.2.5 Interferences . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Non-stripping methods . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Potentiometric stripping . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Adsorptive stripping voltammetry . . . . . . . . . . . . . . . . . . . 10.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Analytical information from interfaced chromatography with specific element detection . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Interelement selectivity . . . . . . . . . . . . . . . . . . . 11.2.2 Elemental sensitivity and limits of detection . . . . . . . . 11.2.3 Dynamic measurement range . . . . . . . . . . . . . . . . 11.3 Element-selective gas chromatographic detection . . . . . . . . . . . 11.3.1 Non-spectroscopicdetectors . . . . . . . . . . . . . . . . 11.3.2 The Flame Photometric Detector (FPD) . . . . . . . . . . 11.3.3 Atomic spectroscopic detectors . . . . . . . . . . . . . . . 11.3.4 Flame emission detection . . . . . . . . . . . . . . . . . . 11.3.5 Atomic absorption detection . . . . . . . . . . . . . . . . 11.3.6 Atomic plasma emission spectroscopy (APES) . . . . . . . 11.4 Classes of atomic plasma emission chromatographicdetectors . . . . 11.4.1 The microwave-introduced electrical discharge plasma (MIP) detector . . . . . . . . . . . . . . . . . . . . . . . 11.4.2 The Inductively Coupled Plasma (ICP) discharge . . . . . 11.4.3 The Direct-Current Plasma (DCP) discharge . . . . . . . . 11.4.4 The Alternating-Current Plasma (ACP) discharge . . . . . 11.4.5 The Capacitively Coupled Plasma (CCP) discharge . . . . 11.4.6 The plasma electrodeless discharge afterglow . . . . . . . 11.5 The plasma-chromatographinterface . . . . . . . . . . . . . . . . . 11.5.1 Gas chromatographs . . . . . . . . . . . . . . . . . . . . Analytical GC applications . . . . . . . . . . . . . . . . . . . . . . 11.6 11.6.1 GC-AED detection of non-metallic elements . . . . . . . . 11.6.2 GC-MIP detection . . . . . . . . . . . . . . . . . . . . .

389 393 394 395 397 399 400 400 405 405 408 416 426 427 428 428 428 429 429 431 431 432 432 433 434 434 435 435 436 436 436 436 436 438 438 444

11.6.3 GC-DCP and ICP detection . . . . . . . . . . . . . . . . . 11.7 Liquid chromatographic applications . . . . . . . . . . . . . . . . . 11.7.1 HPLC-ICP detection . . . . . . . . . . . . . . . . . . . . 11.7.2 HPLC-DCP detection . . . . . . . . . . . . . . . . . . . . 11.7.3 HPLC-MIP detection . . . . . . . . . . . . . . . . . . . . 11.8 Supercritical fluid chromatographic (SFC) applications . . . . . . . . 11.9 Chromatographic detection by plasma-mass spectrometry . . . . . . 11.9.1 HPLC-plasma mass spectrometry (MS) . . . . . . . . . . 11.9.2 GC-plasma mass spectrometry . . . . . . . . . . . . . . . 11.10 Future directions for trace analysis . . . . . . . . . . . . . . . . . . 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.2 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.3 Inventarisation . . . . . . . . . . . . . . . . . . . . . . . 12.1.4 Organization of this chapter . . . . . . . . . . . . . . . . 12.2 Principles of radiotracer methods . . . . . . . . . . . . . . . . . . . 12.2.1 Basic equations of radiotracer experiments in a closed system and their applications . . . . . . . . . . . . . . . . . . . . Isotopic exchange in one phase . . . . . . . . . . . . . . . 12.2.2 Isotopic exchange between a solid and an aqueous solution 12.2.3 in a closed system . . . . . . . . . . . . . . . . . . . . . 12.2.4 Net mass transport in a closed system . . . . . . . . . . . 12.2.5 Combination of net mass transport and isotopic exchange in a closed system . . . . . . . . . . . . . . . . . . . . . . . 12.2.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Spatial (surface) speciation by nuclear techniques . . . . . . . . . . 12.3.1 Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3.2 Ion-beam applications . . . . . . . . . . . . . . . . . . . 12.3.3 Thermal neutron depth profiling . . . . . . . . . . . . . . 12.3.4 Depth profiling by activation analysis . . . . . . . . . . . 12.3.5 Proton induced X-ray analysis (PIXE) and proton induced ?-ray spectrometry (PIGE) . . . . . . . . . . . . . . . . . 12.3.6 Depth profiling by radiotracer methods . . . . . . . . . . . 12.4 Phase speciation and the use of radioanalysis . . . . . . . . . . . . . 12.4.1 Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4.2 Sampling and separation of sea- and surface water and the determination of trace elements in the isolated fractions . . 12.4.3 Determination of exchangeable phosphate in fresh water sediments . . . . . . . . . . . . . . . . . . . . . . . . . . XI1

448 448 448 450 451 452 453 453 454 455 462 462 463 463 471 471 471 474 474 476 477 478 479 479 479 485 488 489 490 493 493 495 499

12.4.4

12.5

13.1 13.2

13.3

13.4

13.5

13.6

Leaching experiments on granular solids by means of radiotracers and a previously radioactivated aliquot . . . . . . . 499 12.4.5 Measurement of the in situ diffusion coefficient and distribution constant in (partly) wetted soils and granular wastes . . 501 Chemical speciation of trace elements . . . . . . . . . . . . . . . . 504 12.5.1 Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . 504 12.5.2 Radiometric determination of conditional extraction constants 510 12.5.3 Trace element speciation in human serum . . . . . . . . . 513 12.5.4 A case in point: Arsenic speciation in aqueous samples by selective As(III)/As(V) preconcentration and hydride evap518 oration AAS . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544 Need for trace element analysis of biomaterials . . . . . . . . . . . . 545 13.2.1 BTER. a multi-disciplinaryscience . . . . . . . . . . . . . 546 13.2.2 Trace element speciation and bioavailability . . . . . . . . 548 Biological standardization . . . . . . . . . . . . . . . . . . . . . . 549 13.3.1 The “Bio” sources of analytical errors . . . . . . . . . . . 549 13.3.2 Presampling factors . . . . . . . . . . . . . . . . . . . . . 550 Analytical standardization . . . . . . . . . . . . . . . . . . . . . . 556 13.4.1 Analytical quality assurance . . . . . . . . . . . . . . . . 557 13.4.2 Harmonization of measurements . . . . . . . . . . . . . . 559 13.4.3 Trace element determinations . . . . . . . . . . . . . . . . 559 13.4.4 Multianalyte determinations . . . . . . . . . . . . . . . . 559 13.4.5 Matrix related problems in sample treatment . . . . . . . . 560 13.4.6 Sample preservation and storage . . . . . . . . . . . . . . 560 13.4.7 Contamination by trace elements . . . . . . . . . . . . . . 562 13.4.8 Losses of trace elements . . . . . . . . . . . . . . . . . . 562 Clinical specimens from human subjects . . . . . . . . . . . . . . . 563 13.5.1 Special features of biofluids . . . . . . . . . . . . . . . . 563 13.5.2 Medico-legal implications . . . . . . . . . . . . . . . . . 564 13.5.3 Sampling and preparation . . . . . . . . . . . . . . . . . . 565 Environmental biomonitoring for toxicants . . . . . . . . . . . . . . 567 13.6.1 Chemicals in the environment . . . . . . . . . . . . . . . 567 13.6.2 Bioenvironmental surveillance . . . . . . . . . . . . . . . 570 13.6.3 Real time and long-term biomonitoring . . . . . . . . . . . 571 13.6.4 Human specimens for biomonitoring . . . . . . . . . . . . 571 13.6.5 Environmental Specimen Bank (ESB) . . . . . . . . . . . 572 13.6.6 Proven applications of ESB . . . . . . . . . . . . . . . . . 574 XI11

13.7 Biomineral imbalances and health effects . . . . . . . . . . . . . . . 13.7.1 Nutritional and metabolic factors . . . . . . . . . . . . . . 13.7.2 Nutritional surveillance of trace elements . . . . . . . . . . 13.7.3 Recommended dietary allowances (RDA) . . . . . . . . . 13.8 Trace elements and high altitude populations . . . . . . . . . . . . . 13.8.1 Iodine and selenium . . . . . . . . . . . . . . . . . . . . 13.9 Referencevaluesfor traceelement s i n humanspecimens . . . . . . . 13.9.1 Reference values vs normal values . . . . . . . . . . . . . 13.9.2 Referenceconcentrationsinclinicalspecimens . . . . . . . 13.9.3 Trace element content in Reference Man . . . . . . . . . . 13.10 Reference parameters for data interpretation . . . . . . . . . . . . . 13.1 1 The future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

575 575 576 576 577 579 580 580 580 581 586 586

CHAPTER 1

Systematic errors in trace analysis

G.ToLG'i2 and P.TSCHOPEL~

'I~istiturfur Spektrochcniie utid atigewatidte Spektroskopie 2 ~ ~ - P I a t i c k - l t i s t ifur t u tMetallforschung. Luboratoriumfur Reinststoflatialytik Butisen-Kirchhoff-Strasse13. 0.44139 Dortmutid I . Germany

Contents 1.1

1.2

1.3

1.4 1.5

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 General aspects of extreme trace analysis . . . . . . . . . . . . . . . . . . . 1.1.2 Direct instrumental determination methods . . . . . . . . . . . . . . . . . . 1.1.3 Multi-stage procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.4 Further general important statements . . . . . . . . . . . . . . . . . . . . . systematic errors and their avoidance . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Volatilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Blanks from vessels. vessel materials and working tools . . . . . . . . . . . 1.2.4 Blanks from thereagents . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.5 Blanks from airborne dust . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.6 Contamination by sample handling . . . . . . . . . . . . . . . . . . . . . . 1.2.7 Problems due to changes of the valency state . . . . . . . . . . . . . . . . . Systematic errors during the analytical procedure . . . . . . . . . . . . . . . . . . . . 1.3.1 Sampling. sample storage and pretreatment . . . . . . . . . . . . . . . . . . 1.3.2 Decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic rules for the recognition and elimination of systematic errors . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 2 3 3 4 4 6 7 9 13 15 18 19 20 20 22 27 29 31

2

G . TOLG AND P. TSCHOPEL

1.1 Introduction Today, modern analytical chemistry serves as a significant indicator for the optimization of the balance between the technological progress and the inevitable risks accompanying it. Therefore, steadily growing challenges are a constant driving force for improving the three most important analytical figures of merit, namely power of detection, reliability and economy [l-31. The numerous tasks of analytical chemistry of today are of an extreme variety. Many of them are focusing to trace analysis. In this contribution, only the determination of trace elements can be treated. The determination of organic trace compounds must be excluded, because organic trace analysis needs often other or additional strategies. The main problems in elemental trace analysis and particularly in extreme trace analysis, are caused by systematic errors. They are limiting factors which may occur in all steps of the whole analytical procedure, such as sampling, sample preparation, decomposition, separation and preconcentration, and the determination of the elements. At first, these problems will be discussed under more general aspects. Hence it follows that special strategies must be treated as a prerequisite for their understanding and their solutions.

1.1.1 General aspects of extreme trace analysis In principle the concentrations or absolute amounts of all the naturally occurring elements have to be determined within a very broad range and in innumerable kinds of very different organic or inorganic matrices, e.g. in technical products such as high-purity metallic or ceramic materials, glasses, plastics, as well as in solid waste, waste waters or in very complex natural occurring materials, e.g. rocks, soils, biological and medical tissues and body fluids, etc. In the past, mainly the concentrations of the elements in the bulk of the sample had to be determined, which only yields integral information about relative concentrations of elements in large sample amounts. Today, however, the scientific interest is focused far more on trace analysis in micro regions, on the distribution of the elements on the surface of the sample, in microregions or in phase boundaries (micro distributional analysis), also at trace concentration levels (micro-trace analysis) [4,

51. Furthermore, speciation is also required more and more. When only small amounts of sample are available, this challenge in ultra-trace, micro-distribution and species determination asks for methods of highest possible absolute detection power.

SYSTEMATIC ERRORS IN TRACE ANALYSIS

3

I . 1.2 Direct instrumental determination methods

In routine analysis all tasks should be solved in a simple, quick and inexpensive manner. Moreover, the procedures have to be free of errors and should enable it to determine low concentrations of the trace elements down to the pg/g level or absolute amounts in the pg-level or less in all occurring matrices. All these demands cannot be simultaneously fulfilled. For example, in routine analysis the most economic approach seems to be the application of direct instrumental multi-element methods and direct sample excitation. With these techniques, the sample to be analysed undergoes only a short pretreatment for surface cleaning, matching of the shapes, etc. and then is inserted into the instrument for signal generation. Thus, chemical sample preparation is avoided and the time required for analysis is really short. The demand for a reliable determination of absolute quantities at the ng and pg-level, however, is inconsistent with the demand for low costs, because with decreasing concentrations or absolute amounts of elements to be determined, the reliability of the results decreases enormously. Depending on the corresponding elements, systematic errors may falsify the analytical results by up to orders of magnitude, as it can be demonstrated by the results of inter-laboratory comparative analyses. In these studies even data of relatively high contents sometimes suffer from alarming discrepancies [6]. The main reason is that instrumental direct determination methods are relative physical methods requiring calibration, where considerable systematic errors may occur as a result of spectral and non-spectral interferences. The least problematic methods are neutron activation analysis (NAA), sputtered neutral mass spectrometry (SNMS), and X-ray fluorescence spectrometry (XRFA),if one applies very thin samples. Consequently, the most important requirement for the applicability of direct instrumental methods is that suitable standard reference materials must be available for calibration. Unfortunately, these materials are not yet existing at the extreme trace concentration level and for the whole variety of matrices mentioned. 1.1.3 Multi-stage procedures

The strategy to be followed for overcoming this problem in extreme trace analysis is the use of wet chemical multi-stage procedures (combined multi-step procedures), which include sampling, sample preparation, sample decomposition, separation and pre-concentration of the trace elements and finally their determination [&lo]. When the traces which are isolated accordingly are determined in smallest volumes or collected in very thin films on target areas, being as small as possible, effects of sample inhomogeniety and matrix effects are avoided or reduced to a minimum and the power of detection can be improved by orders of magnitude. However, the most important advantage is the easy calibration of a wet chemical procedure with

4

G.TOLG AND P.TSCHOPEL

aqueous standard solutions, by which the problem of the lack of reliable standard reference materials is overcome. The advantages have to be seen together with some disadvantages. Sample preparation essentially is very time consuming, laborious and expensive. The very intensive work requires much expertise and skill as well as well trained personnel. Furthermore, such procedures are hampered by risks for different systematic errors [ 11-15], which are very complex and insidious. The various causes of errors stem to a different extent from the various stages of an analytical procedure. Contamination as well as losses of elements and compounds are the main sources. In spite of these difficulties, multi-stage procedures are indispensable in extreme trace analysis and we have to trace and to eliminate their systematic errors. When the results of these procedures prove to be accurate, one can make use of them to produce reliable standard reference materials with which finally the direct instrumental methods can be calibrated and accordingly one can compensate for their systematic errors. 1.1.4 Further general important statements

In ultra-trace analysis, no generalizations or extrapolations are allowed. Many detection limits of analytical methods described in the literature were mainly obtained by extrapolation. Therewith, it very often is overlooked, that real detection limits in many cases are determined by blanks and their fluctuation. Especially for the omnipresent elements the “real” detection limits can be many times as large as the “theoretical” ones. Optimal absolute power of detection and optimal reliability can only be achieved when the trace element to be determined is available for analysis in an isolated form within the smallest possible target area or excitation volume. “A single method is no method at all.” Only when the results of two or more absolutely different methods agree, one can assume accuracy.

1.2 Systematic errors and their avoidance The causes for systematic errors can be traced back mainly to insufficient qualifications of the analysts and/or inadequate equipment in the laboratory which make any optimal analytical strategy impossible. In the first case, high-quality analytical training would be an answer and in the second case, there is a call for a growing awareness of the fact that false analytical results may finally prove far more expensive than is more advanced equipment and operation. As already mentioned, systematic errors as a rule become evident at the pg/g concentration range and increase enormously with decreasing absolute amounts or concentrations of the elements to be determined. They can exceed several orders of magnitude, depending on the omnipresence and the distribution of the elements in


5

our environment and in the laboratory. Most difficult is the determination of traces of elements, which are of high abundancy in the earth crust e.g. Si, Al, Fe, Ca, Na, K, Mg, Mn, Ti [S, 161 or of elements which are introduced into our environment as a result of anthropogene pollution (e.g. Hg, Cu, Cd,Pb, As, Ni, Zn)[6,7,9-181. There exists no chance to discern systematic errors by statistic evaluation of the analytical data, especially because the most important condition for a statistical treatment of data, which are supposed to display a normal distribution, very often does not apply. Further, no other simple means for the detection of systematic errors are available (see section 4). Systematic errors depend strongly on the element to be determined, on the matrix, on the method and procedure used, on the conditions of the laboratory and on some other parameters. The most important sources of systematic error [6] are: (a) Inadequate sampling, sample handling and storage, inhomogeneity of the sample; (b) Contamination of the sample and/or the sample solution by tools, apparatus,

vessels, reagents and airborne dust during the analytical procedure; (c) Adsorption and desorption effects at the surface of the vessels and phase boundaries (e.g. filters or precipitates); (d) Losses of elements (e.g. Hg, As, Se, Cd, Zn) and components (e.g. oxides, halides, hydrides of the elements) due to volatilization; (e) Unwanted or incomplete chemical reactions (e.g. change of the valency of ions, precipitation, ion exchange, formation of compounds and complexes); (f) Influences of the matrix on the generation of the analytical signals (e.g.

incomplete atomization, overlap of peaks); (g) Incorrect calibration and evaluation as a result of incorrect standard materials, unstable standard solutions or the use of false calibration functions or unallowed extrapolations, respectively. This contribution will mainly deal with the most serious sources of systematic errors of multi-stage procedures: element losses due to volatilizationand adsorption as well as the contamination due to the three most important blank sources: vessels, reagents and dust [ 17-24].


6

Table 1-1. Elements and compoundswhichcanbe separated by volatilization(20-1,000' C) (after Bachmann and Rudolph [25]) Elements Oxides of Fluorides of

Gaseous elements, Te. Sn, Pb, T1, P, As, Sb, S, Se, Br, J, Zn,Cd, Hg As, S , Se, Te, Re, Ru, Os, Zn,Cd,Hg B, Si, Ge, Sn, P, As, Sb, Bi, S , Se, Te, Ti, Zr, Hf, V, Nb, Ta, Mo, W,Re, Ru, Os, Ir, Hg

Chlorides of Al, Ga, In, T1,Ge, Sn, Pb, P, As, Sb, Bi, S, Se. Te, Ti, Zr, Hf, Ce, V, Nb, Ta, Mo, W,Mn, Fe, Ru, Os, Au, Zn, Cd, Hg Hydrides of

Si, Ge, Sn, Pb, P, As, Sb, Bi, S , Se. Te

I .2 .I Volatilization Losses of elements by volatilization mainly occur at high temperatures. However, for very volatile elements these interferences can already be remarkably high at room temperature (Table 1-1) [25, 261. Especially Hg is well known to be extremely volatile. It can be lost during sampling, storage and sample preparation, when aqueous solutions are stored in open vessels or vessels made of organic polymers. By means of the radioactive isotope '03Hg it could be proved that at the ng/ml-concentration level within a few hours, Hg losses of up to 25% from an acidic solution out of an open quartz dish can occur 1271. In addition, Hg quickly penetrates through sample containers made of plastics such as polyethylene or polypropylene. Therefore samples in which Hg is to be determined should not be stored or transported in plastic containers so as to avoid Hg losses from the sample or contamination by the Hg present in the environment. During the dissolution of a metal sample with non-oxidizing acids the hydrides of elements such as S, P, As, Sb, Bi, As, Se or Te may escape. Also when drilling or cutting metal samples such as Al or Fe, the well known smell of H,S or PH, and other volatile hydrides often indicates the loss of these elements. The number of elements and compounds which can be lost as a result of volatilization increases with temperature. This must be considered when evaporating solutions or when performingdecomposition procedures [25] (see Table 1-1). Volatile chlorides of H P , As3+,Sb3+,Sn4+,Ge4+,Se4+,Te4-, e.g. may be lost during the evaportion of acid solutions, and @ tends to be lost as chromyl chloride when perchloric or sulfuric acid is present at temperatures above 150"C. Slight differences in the volatility of the chlorides, e.g. of As3+,Sn", Rh, and 0 s are found when samples are fumed off with perchloric or sulfuric acid. Systematic errors resulting from volatilization can be eliminated by using closed systems (see 1.3.2.2) and by working at low temperatures. One also should avoid all


7

chemical reactions, by which volatile compounds can be formed (e.g. the formation of Cr,OCl,). For the determination of elements which can be relatively easily volatilized (e.g. Hg, Se, As, Bi, Zn, Cd) from non-volatile matrices, we can make use of the very advantageous separation in the vapour phase (see section 1.3.3.4) by trapping these compounds in suitable devices.

I .22 Adsoiptioii The concentrations of the trace elements of very diluted solutions may change very quickly as a result of adsorption and desorption [6, 12, 22, 28-30]. By these processes ions or compounds of trace element are bound onto the surface of the container and may be released later on when the composition of the solution changes (Fig. 1-1). The element losses as a rule become noticeable at concentrations

Figure 1-1. Adsorptionof %o(II)-ions (adapted from [6]);c = 2 x 10-5mo~;AA glass, (7. ooquartz, AOOpH 1.5, A.OpH9.

PTFE,

below 10-6mol/l and they are of the order of 10-9-10-12mol/cm2 [31]. A general statement of the actual error in a special case or an extrapolation from the knowledge of similar analyticalproblems is not possible. However, the losses can be monitored very easily with the aid of radioactive isotopes, provided they are available [32].

G . MLG AND P. TSCHOPEL

8

The amounts of the elements adsorbed very strongly depend on numerous factors which hardly can be specified together. Therefore, an estimation of the losses and gains of trace elements as a result of adsorption and desorption is not possible. The most important factors to be taken into account are: (a) The trace element to be determined, its concentration and its valency; (b) The accompanying elements and the inorganic and organic compounds in the analyte solution (especially the major components but also minor and trace elements), their concentrations and valency, and the pH-value; (c) The composition and the purity of the vessel material, the dimensions and the constitution of the surface of the vessel, as well as its pretreatment and the cleaning procedures applied; (d) Duration of the contact and the temperature.

As a result of adsorption, strong losses of elements especially occur when the sample solution comes in contact with large surfaces. This is the case during filtration [33], ion exchange techniques etc., or even when only changing the vessel. In order to minimize element losses due to adsorption the following precautions should be taken. (a) Vessels made of quartz, PTFE or glassy carbon should be used; glass is not a suitable material in extreme trace analysis as it is linked with the highest adsorption losses; (b) The surface and the volume of the vessel as well as of the sample solution should be as small as possible in order to increase the concentration of a given sample solution; (c) The concentration of the elements to be determined should be as high as possible; (d) The time of the contact between the vessel and the solution should be as short as possible, which can be reached by working up small sample volumes; (e) Sample solutions should be acidified whenever possible, as the losses are then often lower than those occurring with neutral or alkaline solutions;

(0 Cleaning and preconditioning of the vessels should be performed by a treatment with acid vapours. This technique considerably reduces blanks as well as adsorption losses and is more effective than other techniques.


9

Trace elements can also be lost by electrochemical cementation [6] during sampling or sample preparation. This occurs when the trace elements dissolved in an electrolyte come in contact with the surface of a metal which is more elecuonegative than the trace element. Then the trace metal (e.g. Pt, Au, Ag, Hg, Cu) is precipitated onto the metal surface from where it cannot be easily removed. Element losses due to cementation may occur during sampling, milling, cutting, mixing, etc. This is the case when aqueous samples or biological fluids and tissues such as fish, muscle, blood, fruits, etc. come in contact with metal tools. In order to avoid or reduce cementation, freezing of the sample in liquid nitrogen and a treatment of the frozen sample is recommended [6].

1.2.3 Blatiksfiont vessels, vessel materials atid workittg tools No vessel material is absolutely resistant even not to water. Accordingly, as soon as a solution comes into contact with acontainer or any solid substance, each element present in this material will be found at a more or less high level in the solution [6-13,34-361 (Table 1-2). Especially glass, which contains a number of elements as major or minor components and a lot of other elements at a very high trace level, is very impure as compared to quartz, PTFE, polypropylene and polyethylene [6]. Table 1-2. Impurities (pg/g) in different materials according to the literature and investigation [6]

Element B Na

Mg A1 Si

Ca Cr Mn Fe co

cu

zn As

cd Sb Pb

Glassy carbon (sigradur G)

FTE

0.1 0.35 0.1 6.0 80-90 70-90 0.08 0.1 2.0 0.002 0.2 0.3 0.05 0.01 0.01 0.4

25

0.03

0.01 0.002

0.02 0.01 -

4 x 10-~

-

Quartz suprasi@ 0.01 0.01 0.1 0.1

main 0.1 0.003 0.01 0.2 0.001 0.01 0.1 1 x 10-4

-

0.001

Borosilicate glass main main 600 main main lo00

3 6 200 0.1 1 2-4 0.5-22 1 7-9 3-50

10


In addition, the losses of elements due to adsorption are very high. Therefore, glass vessels should not be used in extreme trace analysis. Quartz is the most pure material which is commercially available in different purity classes but unfortunately quartz containers are also the most expensive ones. They definitely deliver negligeable blanks (except for Si) and should be preferred whenever it is possible. Poytetrafluoroethylene (PTFE), other plastic materials and glassy carbon are substantially less pure [21,37-391. Nevertheless, they are much more cheaper than quartz and therefore they are preferred in most of the routine laboratories. After a pretreatment by steaming, glassy carbon releases only a few impurities, because diffusion of impurities from the bulk to the surface is avoided by its structure, which has only a minimum of pores. PTFE,Polypropylene (PP) and Polyethylene (PE) however, are permeable for many substances [40], e.g. for gases or Hg. PP, PTFE and glassy carbon especially are used for solutions containing hydrofluoric acid. To avoid a diffusion of acid solutions to diffuse into the pores and tissues of PTFE, the surface layers of cleaned vessels can be molten by a gas flame and cooled down so as to provide for a dense surface [34]. Contamination by particles, enclosed in deeper layers of the material, as well as losses due to adsorption are reduced by this technique. As compared to PTFE, other fluorinated polymers such as FEP or PFA in some respect are more pure and in addition translucent,and therefore more suitable for ultra trace analysis. Especially during sampling, one has to avoid that the sample comes into contact with other materials causing severe contamination. Therefore, e.g. rubber is not a suitable material, because of its relative high contents of Sb, As, Zn, Cr, Co and even Sc [41]. Nylon contains Co and PVC Zn,Fe, Sb, Cu at the higher trace levels. In addition, we have to note that all half stuff and plastic ware are not produced and manufactured within clean rooms, but in rather dusty factory halls and thereby come into contact with different metals and materials. During the past years it appeared that the purity of plastic materials even became worse. With regard to the vessels, their bulk material is not the only source of contamination. Also the effective cleaning of the surface of the vessels is very important [22, 34, 42). The conventional cleaning technique for laboratory glassware consists of its rinsing and leaching with high-purity acids and pure water [20, 29-31, 42-53]. In addition, leaching can be supported by applying ultrasonic treatment [35]. However, leaching is very expensive and time-consuming (several days or weeks) and it requires large volumes of pure, high-purity or even ultrapure acids, which in the future also will become a waste problem because of environmental contamination. Another disadvantage is the fact that the cleaned vessels remain in contact with the acids now enriched with impurities, by which they are again contaminated. Therefore, in many cases these procedures are not effective enough so as to guarantee for residual blanks down to the lower pdml-region.

11


A very effective and much less time consuming cleaning of laboratory ware can be achieved by a steaming procedure, during which the vessels to be cleaned are brought upside down on the top of quartz tubes located inside of a glass container (Fig. 1-2) [6-13, 541. Vapours of nitric or hydrochloric acid are passing through 1 1

1

Figure 1-2. Steaming apparatus for vessel purificationwith nitric acid steam (after 161);(l)condenser, (2) steam chamber, (3) steam tubes, (4) overflow, (5) round-bottomed flask,(6) heater.

the quartz tubes by which mainly the inner surfaces of the vessels are washed continuously. During 4-6 h, acid vapour treatment is used and subsequently, water vapor is introduced for another 1-2 h. By this process, the surfaces are not only cleaned (Fig. 1-3)but also the adsorption of traces of elements during the subsequent procedure is considerably decreased. Only a few exceptions should be mentioned where the steaming technique may result in relatively high blanks, e.g. for the case of Fe-traces in PTFE. The effect of the purification process can be easily checked by TXRF (total reflection X-ray fluorescencespectrometry) [S,551. Figure 1-3b shows the impurities on a quartz plate before and Fig. 1-3a after steaming the surface with acid vapour. 1.2.3.1 Contamination by tools Especially during sampling [56] and sample preparation there is a inevitablerisk of contamination by tools for cutting, drilling, milling, sieving, crushing, grinding, pulverizing [57, 581. Metal contaminations of biological tissues and fluids (e.g. blood [48]) with Cr, Ni, Co, Fe, Mn, Cu and others due to use of scalpel blades

G . TOLG AND P.TSCHOPEL

12

0

20

10

Energy IkeV

1

Figure 1-3. TXRFA spectra of a quartz plate (adapted from [81); (a) quartz plate cleaned by steaming, (b) uncleaned.

2

L

6

lime I h l

Figure 1-4. Contamination of 150 ml of 2 M HCI with Fe by 20 micropipette tips [121.

and syringe needles are reported. Therefore, the use of forceps, knives, spatulas and needles made of plastic, titanium or quartz is recommended [20, 48, 50, 59, 601. Metallic support material (also when this is made of stainless steel), drying ovens, gas burners, electrical furnaces, washing agents and much other equipment and products in a laboratory [61] may be sources of contamination, which must be taken into account. Figure 1-4 shows the contamination of 150 m12 M HC1 caused by 20 plastic tips of usual micropipettes [12].

SYSTEhWTIC ERRORS IN TRACE ANALYSIS

13

1.2.3.2 Contamination due to man The operator himself also represents a very serious source of contamination. The number of particles emitted per minute by a person amounts up to millions. They are released by the skin, hair, clothes, jewellery, cosmetics, disinfectants, talc, etc.

161. 1.2.4 Blanbfrom. the reagents

The introduction of blanks from the reagents into the sample solution during a wet chemical procedure cannot be avoided, as no substance is absolutely pure. This is especially true when solid reagents have to be used in a large excess as is the case in decomposition by fusion. Unfortunately, the possibilities for lowering the blank contributions introduced by the reagents are very limited [6-17,20-23,62481- For most of the solid substances, the available procedures -which are always separation methods such as solvent extraction, ion exchange, chromatographic techniques, coprecipitation, electrolytic deposition in a flow through system [69] etc., are very laborous and sophisticated. In addition, they are effective as a rule for only a very few elements, whereas for others they even might be increased. Accordingly, in extreme trace analysis we often have to use only those reagents which can easily be purified, such as gases and liquids. The preparation of ultrapure water, e.g. by distillation in a quartz still or by membrane filtration and its permanent quality control is of greatest importance. Especially the sub-boiling distillation technique, described by Kuehner et al. [6, 62, 63, 65-68], is extremely efficient for most of the acids used in the laboratory (e.g. HNO,, HC1, H2S04) and for some organic solvents. The distillation still is made of quartz (see Fig. 1-5a), stills made of PTFE should only be used for the purification of hydrofluoric acid (Fig. 1-5b) [70]. In this technique a liquid is evaporated without boiling with the aid of an IR radiator, which is not allowed to dip into the liquid, and accordingly one avoids the formation of aerosols, which in the conventional distillation technique contaminate the distillate. The residual impurities for subboiled liquids are at the pdml level (Table 1-3) which is sufficient for most of the ultra trace procedures. The yield of such a subboiling still amounts to some 100 ml per day. This is sufficient for most purposes in ultra trace analysis purposes due to unavoidable contamination during storage. Therefore, only that volume of acid required for the immediate use should be prepared. Apart from this technique, no other universal (single) purification procedure is capable of removing all metallic or cationic impurities to such a low extent. Ultrapurificationof some other reagent such as H202,hydrazine, and AsCl, can be performed with the aid of sublimation at low temperature [65,66].

G.T ~ L GAND P.TSCHOPEL

14

An

6

Figure 1-5. Subboiling distillation [6,11,62], (a) quartz still, (1) IR heater, (2) cooling finger; (b) €TFE still for HF (adapted from [70]),(1) PTFE body, (2) HF, (3) PFA foil (transparent),(4) condensing area,(5) water cooling,(6) to receiver, (7) IR lamp.

Table 1-3.Residual impurities in different acids [ng/ml] [6]

Cd

Cu

0.01

Fe

A1

Pb

Mg

Zn

0.04 0.3 0.07 0.6 1 100

5 0.05 0.07 10

0.02 < 0.05 0.5

5 0.02 0.2 14

0.2

HNO3 15Msubb. 0.001 HNO3 15 Mp.a. 0.1

0.25 2

0.2 25

5: 0.005 10

5 0.002 0.5

0.15 22

HF54% subb. HF 48% p.a.

0.5 2

1.2 100

2 5

0.5 4

1.5

1

3

5

HzO subb. HCI 10M subb. HCI 12 Mp.a.

0.01 0.1

0.01 0.06

50.04

8 0.04

3


15

1.2.5 Blanksfi.oni airborne dust

The atmosphere of the laboratory is loaded with particulate matter from different sources such as the environment, the floor, the walls, the ceiling (paint), the furniture, the equipment, the clothes, the analyst himself and so on. Accordingly, various inorganic and organic compounds are present and in principle, any element can be found, depending on the environment, the laboratory itself and its history. Again the dust particles will contain relatively high concentrations of those elements which show a high abundancy in the earth’s crust (e.g. Si, Al, Fe, Ca, Na, K., Mg, P). In addition, all elements of anthropogenic pollution (e.g. Mg, Cu, Cd, Pb, Ni, Co, Zn, Mn) are always present. When the dust comes in contact with the sample, it is a severe source of contamination. Sometimes protection may be achieved with cheap and very simple means, such as closed vessels and apparatus or glove boxes. More efficient and convenient are clean rooms and clean benches [5,6,8, 13, 17,42,48,51,71,72,73] which are flushed with “dust-free’’air. A clean room (Fig. 1-6) is an area which is hermetically separated from the outside atmosphere, and which is only accessible through an air

Figure 1-6. Scheme of aclean room with a clean bench, (1) air conditioningsystem, (2) HEPA-filter, (3) clean air to clean bench, (4) clean air to clean room,(5) clean bench, (6) turbulent air flow, (7) laminar air flow, (8) clean working area,(9) air out, (10) water drain, (1 1) window, (12) door to air lock.

lock. A filter assembly provides for pure air at an overpressure against the outside atmosphere and for a circulation with a turbulent flow, as shown in the example of Fig. 1-6. The most important part of this assembly is the so-called HEPA-filter (high

G.TOLG AND P. TSCHOPEL

16

efficiency particulate air filter), which has a definite pore size of 0.3 pm. The dust removal efficiency by the deposition on the filter is about 99.97-99.95%. At the right side of Fig. 1-6 a cross section of a clean bench is shown, which is the actual working place in a clean room laboratory. Here a laminar air flow from above passes along the working space of the bench. As a result of the laminar flow, the dust content of the air in the clean bench is about one order of magnitude lower than in the room, where turbulent air takes up dust (Table 1-4). The air quality is classified according to the number of dust particles. For instance, the U.S. Federal Standard 209 specifies the cleanness of the air by the number of particles per cubic foot having a diameter between 0.5 and 5 pm and one distinguishes between the classes 100, 10,000 and 100,000. In a clean room with a turbulent air flow, e.g. a cleanness of class 1,000 to 10,000 can be maintained, whereas in a laminar flow clean bench class 100 must be reached. The use of a clean bench in a clean room illustrates only one possibility. Another more expensive alternative is to keep the whole clean room as dust-free as possible. This can be achieved with the aid of a laminar air flow which enters the room through a HEPA-filter installed over the whole area of one wall or the ceiling. For advanced technologies, however, still much higher purity demands now exist. For instance, the electronic industry claims a much lower dust content for the production of megabyte chips (class 10 or 1 according to the US-Federal Standard). For trace Table 1-4.Particle concentration in laboratory air [6] ~

Room

Normal lab

~~

Particle size

Particle concentration

[FI

[m-31

>0.5 >1

7 ~ 1 6 - 2 xi07 1 . 5 ~ 1 -62 x106 1 xio4-4 xi04 2 xld-7 x l d

2 xlo6 3.5 x 10’ 1.5~10~ 5 xld

3 . 5 ~ 1 -07~ x104 7 xld -1.5~10~ o - 1 xi03 0 - 250

5.5 x lo4 1 xi04 350 100

2 xld-3.5~10~ 350 - 700 0-70 -

0.5 x 103

>3 >5

Clean room

>0.5

(turbulent)

>1 >3

2 persons

>5

Clean bench

>0.5

(laminar)

>1 >3 >5

Mean value [m-31

3 xld

-

17


analysis, however, this enormous effort to get such a high air quality is only rarely necessary. Numerous rules have to be taken into account for working effectivelyin clean benches and clean rooms. They concern the arrangement of the bench, an ingenious way of proceeding and a highly pure working style. It is obvious that a high degree of cleanness also has to be maintained. This applies both for the bench, for the devices, and for the operator himself who spreads most of the “uncleanness” in the clean room. Also the laminar air flow may not be disturbed by tools or labour at those places, which are to be protected from dust. All openings of the vessels and devices, e.g. are such places and have to be continually flushed with pure air. For monitoring the blanks caused by the dust content of the air, dust particle counting with different types of stray-light photometers can be used. Further carefully cleaned small quartz beakers can be placed uncovered at different places of the laboratory and the clean bench for several hours during an analytical procedure. After exposure the dust is dissolved in a small volume of acid and the element content can be measured, e.g. by ICP-MS or ETAAS. Figure 1-7 exemplarly shows the contamination for Fe in an AAS laboratory at different days [ 121. Also TXRF is an efficient method to indicate the elemental composition of dust collected on small quartz plates, which are deposited in the area to be controlled. The dust content of the air is not constant but it is influenced by several factors, including the condition and the history of the laboratory, the type of air supply and ventilation, the number of persons in the laboratory, and the type of their activity. Also the weather plays an important role; when it is raining the air generally is essentially purer than when it is dry (see Fig. 1-7).

I

I

lime

lhl

L

6

Figure 1-7. Contaminationof 10 ml HCI (subb.) with Fe by airborne dust in an AAS laboratory [123; (1 and 2) rainy weather, (3 and 4) dry weather.

18


Figure 1-8. Contamination of 0.1 M HCI during storage in vessels of different materials (after 111, 121); (1) vessel without cleaning, (2) vessel cleaned by rinsing with H20, (3) vessel cleaned by rinsing with 2 M HCI, (4) vessel cleaned by steaming with H N a (6 h).

12.6 Contamination by sample handling In spite of the fact that we have available very effective techniques for the cleaning of the vessels and equipment, for the purification of the reagents and for maintaining a clean laboratory and working place, further contamination, e.g. arising from storage of the sample solution (Fig. 1-8) [12], from sample handling and from the analytical procedure cannot be totally excluded. At the ppb- and still much more at the ppt-concentration level, even a very simple working step such as pipetting, shaking, evaporating, filtering a.s.o., already can increase the blanks considerably [6,8, 11-13, 18, 33,429 481. Alarming is not the real value of the blank of one single step but the fact that blanks occur even during very simple operations and accumulate during the whole analytical procedure to amounts in the ng/g-range. In extreme trace analysis the scattering of the blanks can exceed several orders of magnitude. In principle, the various sources of systematic errors described above are present in all steps of an analytical procedure [6] such as sampling, transport, storage [74], sample pretreatment, decomposition and separation (Table 1-5). Accordingly, the

19


Table 1-5. Frequency of sources of systematicerrors in trace analytical procedures [ 6 ] ; f positivehegativeerror, + + +/ - - - large; + + / - - medium; +/- small ~

Type of error

Contamination

Step of operation

Reagents, tools, Tools, laboratory air interfaces

Sampling Sample preparation Dissolution. decomposition Separation, preconcentration

~~~

~

Adsorption Volatilization Cross interferences by elements Chemical reactions, signal-interferences, -coincidence -background

+++

--

-

+++

-

-

f f

+++

-

---

f

++

---

-

fff

general statement can be made that with decreasing absolute amounts of the elements to be determined, systematic errors increase dramatically and that they are the main problem in extreme trace analysis. Unfortunately,this fact will often not be realized in the daily work of routine laboratories and may then be the reason of wrong results with dramatic consequences with respect to economy, safety and health.

12.7 Problems due to changes of the valency state Severe losses of elements can be caused by the change of the valency state of the trace elements during the analytical procedure. This becomes very effective when there is only a partial change and when special chemical reactions such as the formation of insoluble precipitates of organic chelates, the sorption on an ion exchange resin and others are used for the separation of the trace element from the sample matrix or when special ion sensitive techniques are used for the determination of the trace element (e.g. voltammetric methods). There are many possibilities, but only a few examples can be mentioned here. Hg-ions can be easily reduced to the volatile metallic mercury by dust particles, organic impurities in the sample solution or by the plastic container. Also Au and the metals of the platinum group are subject to similar reactions. Especially irreversible adsorption at the container materials such as PE, PP, PTFE ect. may cause severe element losses. Cementation during sampling and sample preparation also causes losses of trace elements. The oxidation of small amounts of sulphide ions in natural waters is described in section 1.3.1.

20

G . TOLG AND P.TSCHOPEL

1.3 Systematicerrors during the analytical procedure 1.3.1 Sampling, sumple storage and Pretreatment

Sampling, sample storage, transport, drying and processing as well as sample pretreatment are associated with a significant amount of very different types of systematic errors when absolute amounts of elements 5 1 pg are to be determined [28,43,46,47, 56,60,73-851. Sample inhomogeneity, adsorption and desorption effects, biological and photochemical reactions which are changing the original bonding of the trace elements, volatilization and impurities introduced by the tools as well as containers are the main keywords which should be mentioned here. These topics represent a specialized area of analytical chemistry with numerous problems, rules and techniques. The theory and the praxis of these topics must be well known by the analyst so as to avoid or minimize the inherent errors. But once again: Generalization is not allowed since all effects mentioned strongly depend on the matrix, on the respective elements, on their concentration and on the type of procedure. Therefore, also in this context the cited literature and this contribution can only present examples, which demonstrate the multiplicity and complexity of the sources of errors [56,86-891. For samples which are unstable or difficult to handle, special techniques are necessary 1901. When sampling compact solid material, inhomogeneity may cause severe errors mainly with respect to geochemical matrices (e.g. rocks and ores). But also in the case of metals and alloys, concentration gradients and clusters must be taken into consideration [91]. As the sample analysed must represent the composition of the whole material to be characterized, the sample mass and its grade of disintegration (particle size) plays an important role [92]. In the case of compact solid samples, contamination can only occur on the surface and it can be subsequently removed or greatly reduced for instance by etching. The limitations of such cleaning processes are today only investigated exemplarly as it is e.g. described for the determination of non metals in refractory metals [93]. Particularly in the case of high-purity materials (e.g. metals, semiconductors) the main impurities are enriched on the surface. Here we strictly have to distinguish between surface and bulk contamination, as it was shown for the case of carbon impurities in high-purity metals [94]. Considerable amounts of impurities may be introduced during disintegrating the sample with common tools (e.g. sawing, drilling, spark erosion). Then one has to use laser beams or other tricky sampling techniques as mentioned in the following example of brittle materials (e.g. glass, quartz or ceramics). Here one can use a simple colouring technique [95]. The material to be disintegrated will be completely coated with ink from a marking pen, subsequently the sample is wrapped in paper,


21

shattered by a hammer and only those pieces which have absolutely no colour are selected under a microscope and used for analysis. As a general conclusion it must be emphasized that in extreme trace analysis, all common techniques for sample disintegration, sieving and homogenization should be avoided whenever it is possible. For each special case, one has to find out its own way. For sample disintegration, the lowest contamination is caused by tools made of agate, monocrystals of aluminum oxide or boron nitride. Preparation tools (tweezers, needles, spatula, etc.) should be made of plastics, quartz, high-purity titanium, gold or silicon nitride instead of stainless steel or nickel. Also in the case of sampling liquids such as aquatic samples, many rules must be followed, which depend on the water type [83-861. Especially for seawater, the inherently low levels of trace metals are known to cause severe problems in sampling [43,96-981. Similar conditions exist for fresh water and rain water of which errors also resulting from contamination and adsorption during sampling and storage must be avoided [23,44,49,99, 1001. Special precaution is necessary for sampling arctic and antarctic ice or snow when extremely low concentrations of trace metals should be determined [72,1011051. Severe problems arise when storing high-purity liquids [43,101, 1021. During storage of high-purity solutions, the blank content increases with time, depending on the pretreatment of the vessel, the composition of the solution and the element Ell, 12, 31,791. Changes in element content and/or valency and in the bonding of the elements, e.g. by oxidation or by biological or photochemical reactions can be illustrated by the determination of ng/ml concentrations of S2- ions in diluted sulfide solutions. The Sz- concentration changes as a result of the oxidation by the oxygen content of the solution. A stabilization can be achieved by adding reducing agents, such as ascorbic acid @H 2 12) and storing the samples in the darkness. However, this is only true for synthetic and sterile solutions and not for samples of natural water, such as river water or mineral spring water. In these samples we have to add Zn2+ up to a concentration of 20 mg/l [pH 81 to avoid the disintegration of S2-by bacteria E1061. Similar reactions induced by bacteria (methylation process) can also convert Hgz+to elemental Hg or organic compounds. All these problems are also relevant in the preparation, storage and use of highly diluted standard solutions in the ng/ml range and lower. In the latter case it is essential to store stock solutions only with concentrations not below lo-’ molar and to renew them in short intervals. The risk of instability increases dramatically at lower concentration ranges. The dilution of stock solutions into the ng/rnl range or below with the aid of common techniques using pipettes and volumetric flasks is unreliable and it must be substituted by micro techniques. When absolute amounts of an element at the ng or pg level are to be dosed, stock solutions in the stable

22


concentration range should be handled with ultra-micro burettes or pipettes, as these still allow the measurement of volumina in the sub- pl-range with relative errors below 1% [107]. Many further sources of error may occur during each treatment of the analyte solutions, e.g. when suspended matter is removed from liquid samples by filtration, traces of the dissolved elements will be lost by adsorption and blanks will be introduced. Here separation by centrifugation is an alternative [33]. The determination of components in gaseous samples [ 1081 aerosols and dust of air [109-1 111 needs much experience and special sampling strategies too. Considering environmental and biological samples [24, 1121 additional severe systematic errors are involved in the sampling step and during storage, which must be avoided from case to case. There exists a lot of information about the reliable storage of biotic samples in environmental specimen banks. It can be performed over longer periods by using liquid nitrogen temperature conditions [1131. During disintegration or homogenization of food or biological matter, losses of elements by adsorption or cementation onto the metal of the cutting tool can be well avoided by using a special mixing apparatus made of PTFE and cooled with liquid nitrogen, as already mentioned [6]. Also during the drying step of biological samples at temperatures of about 110"C losses of volatile elements (e.g. Hg, Se, Cd, Pb) may occur. Therefore, freeze drying is an alternative [ 114,1151. Many further experiences and directions are also given, e.g. for soils [116], for slurries 11171, for plants [118, 1191 and for tissues and body fluids [120, 1211. As another example, the determination of Se in hair for medical or toxicological purposes should demonstrate other severe problems. One has to differentiate between living and already mortified hair [ 1221. Also external contaminations must be removed [1231. Nevertheless it is impossible to correlate the Se-concentration found to its concentration in the blood, because by washing, the hair surface may be strongly contaminated by selenium present in all shampoos as they contain sulfur [1241. Summarizing, it should be mentioned that in this chapter we could only stress that severe errors in a trace analytical procedure may occur within the sampling and sample preparation steps. But it makes not much sense to deepen the considerations, because the sources of systematic errors are so numerous and vary from one task to another, so that no generalization is possible. We can only demonstrate that the trace analyst has to reflect very critically each step or manipulation, also when it seems to be very simple. 1.3.2 Decomposition

The dissolution, decomposition or combustion of the sample is the next step in a multi-stage procedure [ 125-1321. The aim of this step is to obtain a clear solution or


23

in some cases a gaseous sample containing all elements and compounds of interest in the sample without change of their absolute amounts. For special analytical purposes (e.g. sample preparation for XRFA) homogenisation or isoformation of the sample material can be performed by melting the sample with a flux. According to the very huge variety of different matrices with a more or less complex composition, many different decomposition principles were developed in the past, of which, however, only a few are useful in extreme trace analysis with respect to minimal systematic errors. To reduce them the following points must be considered: (a) In most cases the decomposition of the sample has to be complete. All

inorganic materials have to be transformed into easily soluble compounds, organic substances have to be totally mineralized. This demand is especially important, when an electrochemical method is used for the subsequent determination; (b) Residues of a combustion should be quantitatively soluble in a volume of a high-purity acid being as small as possible;

(c) The decomposition procedure should be as simple as possible, and should not involve large expenditure of apparatus, reagents and time; (d) The decomposition procedure must be adaptable in an optimal way to the whole analytical procedure, especially with respect to the subsequent separation or determination step;

(e) Decomposition procedures such as combustion, pyrolysis or hydropyrolysis should be preferred when easily volatile compounds are formed, which can be used for quantitative separation. Then decomposition and separation can be achieved within one step; (f) Clean vessels made of an inert and high-purity material with a possibly small

volume and minimal amounts of high-purity reagents should be used. Dust should be excluded by using clean benches and clean rooms and precautions should be taken so as to avoid losses of trace elements by adsorption or volatilization; (g) The decomposition should be checked with the aid of radioactive tracers. There are mainly two problems with decomposition: (1) The decomposition may be incomplete and parts of the sample or of the reaction products may remain as an insoluble residue. This can be the case for complex matrices, such as geological samples or organic materials containing high concentrations of inorganic compounds. Then we have to apply a second or even a third different decomposition procedure introducing additional systematic errors. (2) The amounts of trace

24

G. TOLG AND P. TSCHOPEL

elements or compounds to be determined and present in the decomposition solution do not correspond to their content in the sample as a result of systematic errors. With decreasing trace element concentrations these problems are of great weight.

1.3.2.1 Decomposition by fusion As a rule, decomposition by fusion with solid reagents is not suitable for extreme trace analysis as it requires an up to tenfold excess of flux reagents causing enormous contaminations. In addition the very impure crucible materials are attacked by the flux which introduces further contamination. Moreover, element losses can occur as a result of volatilization, adsorption or reaction with the crucible material. 1.3.2.2 Wet decomposition Acid decomposition in an open vessel or system is only suitable if losses of volatile trace elements are not to be expected. Wet decomposition in a closed system, e.g. in a pressure bomb, is performed at high temperature [133-1351. PTFE vessels allow temperatures up to 200°C which very often is not sufficient for a complete mineralization of the organic matter [136-1401. Only at temperatures above 300"C the oxidation potential of NHO, will be high enough to mineralize resistant compounds such as fats. These problems have been optimally solved in the high-pressure ashing method with nitric acid in quartz vessels as developed by Knapp et al. [126, 141-1443. A complete mineralization and a complete recovery of volatile elements as well as low blanks can be reached with temperatures up to 320°C and pressures of up to 100 bar. To avoid breaking of the quartz glass vessel as a result of the high pressure inside the vessel, a somewhat higher pressure must be applied outside the decomposition vessel. This can be achieved in a microprocessor controlled autoclave (Fig. 1-9). With this technique, even 300 mg amounts of sample materials such as PVC, polypropylene, rubber, coal and others can be decomposed without contamination within 2-4 hr and with not more than 2 ml of HNO,. Furthermore, the extreme purity of the quartz glass and its tightness in contrast to commonly used PTFE vessels lead to optimal conditions. With respect to the systematic errors introduced into the sample by the heating device, there is no essential difference between a conventional electric and the microwave heating technique. The main advantage of the microwave energy is a very fast increase of the temperature and thus a short time consumption [ 145-1491. A considerable reduction of contamination can be obtained by the use of quartz containers instead of the commonly used PTFE vessels. 1.3.2.3 Combustion with oxygen For organic samples the combustion with pure oxygen in a closed system of high-purity quartz should be preferred over wet decomposition procedures due to

SYSTEMATICERRORS IN TRACE ANALYSIS

25

Figure 1-9. Scheme of a high pressure asher (adapted from Knapp [126,141-1441); (1) autoclave, (2) furnace,(3) sample and decomposition acid, (4)quartz vessel. (5) closure. (6)compressed air; (Pop,).

the lower risk of contamination. This can be performed either in dynamic systems (e.g. Trace-0-Mat) (see 1.3.2.4; Fig. 1-10) [131, 1501 or with a low-temperature ashing technique with a microwave or hf oxygen plasma [150,1513. In order to avoid losses of trace elements the very gentle combustion at low temperatures being about 100-20O0C should be recommended when low concentrations of volatile elements or compounds are to be determined in organic samples. In both techniques the combustion residue (ash) should be dissolved in a minimal volume of diluted acid in order to reduce the blank contribution of the reagents. 1.3.2.4 Thermal volatilization When high temperatures are applied to the sample, volatile elements and compounds can be lost (see Table 1-1). One can make use of this fact very advantageously for a quantitative separation. In this way the decomposition of the sample and the separation of the trace elements can be performed within one step which might considerably reduce the systematic errors. As an example of such a technique, the Trace-0-Mat [131, 1501 should be mentioned here (Fig. 1-10). The quartz apparatus consists of a small burning chamber (ca. 75 ml) flushed with pure oxygen and a liquid nitrogen cooling unit. The sample is ignited with the aid of an IR lamp outside the vessel. The combustion can be controlled by the flow of the oxygen. The volatile components subsequently are transported into the cooling trap, are frozen out completely, and then can be taken up by refluxing in 1-2 ml of high-purity acid. Non-volatile elements remain in the sample holder from which they can be dissolved.

G.TOLG AND P. TSCHOPEL

26 5.

Figure 1-10. Scheme of a “Trace-O-Mat@)”(after [131, 1501);(1) oxygen in. (2) sample, (3) liquid nitrogen, (4) focussed IR lamp, (5) condenser.

The combined decomposition and separation via the gas phase to a very high degree meet the requirements of the extreme trace analysis. Contamination is minimal due to a closed quartz system, to high-purity oxygen and to minimal amounts of high purity acids. Losses of elements are avoided, provided the combustion and separation are complete. Examples are the determination of Hg or Se. However, for the decomposition of insoluble inorganic refractory materials [127, 128, 1521 such as high-temperature ceramics and many other technical matrices, there exists no universal method having the low blank levels required for extreme trace analysis. Here this problem has a good chance to be solved by using elementary fluorine or gaseous fluorides as decomposition agents, as these reagents possess the highest known oxidation potential at relatively low temperatures [153]. A fast and very reliable decomposition of such problematic matrices, such as silicon nitride, silicon carbide, boron carbide, zirconium oxide can be performed in a closed Niautoclave system. This approach enables an on-line determination of main and minor components such as Si, B, P and S via their volatilized fluorides by FTIRspectrometry [153]. In the case of trace elements, the volatilized fluorides can be determined by mass spectrometry in an on-line system. The elements which form non-volatile fluorides can be determined in the residue after its dissolution with special purified agents, such as ammonium fluoride. Up to now the impurities of the nickel will be the main limiting factor, which influences the power of detection for the elements to be determined [ 1541.


27

1.3.3 Separation

For chemical separation and pre-concentration, there are many useful principles available [ 155-1581 such as coprecipitation, electrolytical deposition, volatilization, liquid-liquid extraction, ion exchange, sorption, etc. [5, 127, 157-1611. The main aims of separation and pre-concentration are to reliably remove all interfering matrix compounds from the trace elements of interest and to concentrate the latter and their compounds to be determined within a possibly small volume or onto a possibly small target area and to submit them in this form to be determination procedure. Thus matrix effects are avoided, the sensitivity and the power of detection are increased and an optimal determination method can be selected. The requirements of a separation method which will be suitable for ultra trace analysis are very similar to those for sample preparation and decomposition. Therefore, they must not be repeated here. Additional demands are: (a) The yield must be 100%; (b) The procedure must be simple;

(c) Only small amounts of high-purity reagents should be required; (d) Quartz vessels or in the case of HF-containing solutions PTFE, PP or glassy carbon vessels must be used. Generally, separation should be performed in closed micro-systems using the on-line flow through or flow injection principle, as it has been recently demonstrated in the literature [144,162-1721. In extreme trace analysis these techniques must permit it to transfer dropwise the micro-volumes of solutions containing the preconcentrated elements onto a possibly small target area in order to drastically reduce systematic errors in the whole multi-stage procedure as well as to achieve very intense analytical signals in e.g. voltammetry, GDMS or TXRF. The working time can be reduced significantly by automation. For instance by combining HPLC or micro adsorption columns for separation and preconcentration with direct high pressure injection of the solution fractions into flames or HF-plasmas for atomic spectrometrical determination of the isolated elements to be determined (e.g. FAAS, ICP-AES, ICP-MS) [173, 1741 blanks and time may be furthermore reduced considerably. 1.3.3.1 Precipitation, co-precipitation, electrolysis In trace analysis the separation of the trace elements by precipitation is suitable only in a few cases because the formation of the precipitates very often is incomplete. If matrix components are separated as insoluble compounds [175], interesting trace elements may also be lost due to adsorption or co-precipitation.


28

In this respect only a co-precipitation [ 176, 1771 by which the trace amounts are fixed in the precipitate of a small amount of a well-defined inorganic or organic compound as a result of co-crystallization, sorption or occlusion will not introduce severe systematic errors [ 155, 1571. Precipitation exchange [178] in *in layers of a insoluble compound such as ZnS should be briefly mentioned as a special separation technique which shows less errors than precipitation techniques. The sample solution is filtered through a membrane filter carrying a freshly prepared layer of about 300 pg ZnS. Trace elements (e.g. Ag, Cu, Pb, Bi, Cd, As, Sb, Sn, Se and Te) of which sulfides have a solubility product below the one of ZnS, will be exchanged very quickly against Zn during filtering and then can be reliably determined in the now very well defined new matrix. Electrolysis is a special case of precipitation by which special groups of elements can be separated from each other r160.179-1851. Trace elements with a concentration of less than a few pg/ml can only be deposited completely if either a Hg-cathode or a hydrodynamic flux system [160, 1791 is used. In the latter apparatus, made of PTFE, the sample solution is rapidly moved by a PTFE pump through a small graphite tube, which serves as cathode. The Pt/Ir-anode is situated along the axis of the cylinder. Up to -5 V can be applied without evolution of hydrogen, ng- and pg-amounts of elements such as Cu, Co, Fe, Zn and Bi, can be deposited inside the graphite tube and directly determined by e.g. ETAAS. Another innovative electrochemical preconcentration method [ 162, 186-1 891 should be mentioned here, because of very low blanks introduced by the method, because of the excellent separation yield and because of its on-line character (Fig. 111). The flow-through cell is made of PTFE and contains a small crushed vitreous 5

8

2

10 rnm

I

Figure 1-1 1. Scheme of a flow-through electrochemicalcell (after ([162]);(1) working electrode, (2) polyethylene frits, (3) graphite counter elecrode, (4) leads, (5) reference electrode, (6) body (plexiglas), (7)graphitecontact electrode, (8) sample solution.


29

carbon tube as cathode. Depending on the volume of the porous working electrode many elements behave according to the Nerstian law and display electrochemical yields of 100%. Also here the elements separated by electrolysis can be leached into a small volume of an acid or directly determined by ETAAS using the carbon tube as an atomizer. 1.3.3.2 Liquid-liquid extraction Extraction methods are available in a large variety. Different organic solvents and an enormous variety of inorganic and organic complexing reagents are at our disposal. The possibilities of separation procedures are extended by the variation of the pH and by using additional masking agents. For ultra trace determinations, the extraction methods can be optimized and systematic errors can be minimized by their miniaturization, by their adaptation to the single vessel principle 161 and by using quartz containers whenever possible. After an acid decomposition, e.g. the extraction step should be carried out directly in the quartz decomposition vessel which has a volume of a few ml instead of in a separation funnel. Then pipettes can be employed to separate the two liquid phases after centrifugation. 1.3.3.3 Separation by volatilization Due to the fact that every step of an analytical procedure may introduce systematic errors, such procedures which enclose two steps in one have enormous advantages. This is the case when volatile elements or compounds (see section 1.2.1) are released during decomposition or combustion, provided the formation of the volatile compound is quantitative [6, 11-13, 25, 1561. Such a procedure is possible, e.g. with the “Trace-0-Mat,” described in section 1.3.2.4. 1.3.3.4 Other separation procedures A wide variety of other separation and pre-concentration procedures are at our disposal. They meet to a different extent the requirements of ultra trace analysis of which a comprehensivereview is not possible in the frame of this chapter [103,1901. Many of the procedures are based on ion exchange, adsorption or sorption techniques using common ion exchangers, modified cellulose, charcoal or membranes. The latter can be impregnated, for example, with immobilized chelating agents or metal hydroxides [170, 191-1971. As all these techniques make use of large surfaces of the separation media, great care has to be taken in order to avoid losses of elements as a result of irreversible adsorption or contamination due to impure exchangers. 1.4 Basic rules for the recognition and elimination of systematic errors Especially because of the lack of standard materials at the extreme trace concentration level, there are no absolutely sure methods to recognize and avoid systematic errors. However, at least some basic rules can be summarized so as to minimize these problems [6-131:


30

(a) The reproducibility of the results of an analytical procedure gives absolutely no information on its accuracy. A high standard deviation only might give a hint that systematic errors might be present. However, a low standard deviation does not at all prove that the results are reliable; (b) The accuracy of a result must be confirmed by at least one or two independent analytical procedures, which differ in all steps such as decomposition, separation, preconcentration and determination. A further way to confirm analytical results is to make use of interlaboratory comparison; (c) Storage of samples should be short as possible and should be performed at temperatures below - 15"C; (d) The number of manipulations and working steps has to be restricted to a minimum; (e) Monitoring of the different steps of a combined procedure can be well done with radioactive tracers; (f) Microchemical techniques, which use small apparatus and vessels should be

preferred. It would be the optimum when all steps of the procedure could be performed in one vessel ("single vessel principle"); (g) To avoid losses of elements by volatilization, closed systems should be used and the temperatures applied should be as low as possible; (h) To reduce blanks and losses of elements by adsorption, all apparatus, vessels and tools should be made of materials which are as pure and inert as possible. These requirements are met to a high degree by quartz and to a lesser extent by glassy carbon, PTFE and PP; (i) For the cleaning of apparatus and vessels, a treatment with vapours of nitric acid and water is most effective; (i) Reagents have to be very pure. If possible, liquids which can be purified by subboiling distillation such as acids or organic solvents are to be preferred; (k) Contamination by the dust of the air can be excluded to a high degree by using clean benches and clean rooms; (1) All kinds of generalizations and extrapolations are not allowed;

(m) Perseverance and self-criticism of the analyst are most important prerequisites for obtaining reliable analytical data.


31

Systematic errors also may occur during the classical or instrumental determination of the isolated elements. These are the last step of a multi-step procedure and cannot be considered here. Finally only one important source may be mentioned here and is due to calibration. The standard addition technique often seems to be favoured and to be very sure. However, one should be aware that the use of this technique requires careful critical treatment too, regarding e.g. nonlinear calibration functions, unknown background and blank levels and different binding forms of the element to be determined in samples and standard solutions.

1.5 Conclusion With this contribution only some of the problems and possibilities of elemental extreme trace analysis (near to the detection limits which can be reached today) could be reviewed. They cannot be generalized. Regarding the determination of higher trace levels the difficulties are decreasing and the extreme claims, which are demonstrated here, can be often reduced. But to do this, each analyst should be aware that no sources of systematic errors arise due to the simplifications he is performing. Also the role of the multi-stage procedures in modem elemental trace analysis treated here in some detail cannot be generalized. The importance of these procedures for analyses at the pg-concentration level has diminished in favour of direct instrumental methods. But up to now the multi-stage procedures are indispensable for the determination of ndg- and even more so for pdg-contents as long as standard reference materials at this concentration level are not available for the calibration of the more economical and comfortable instrumental methods. Therefore, further progress in sensitivity and reliability requires an enormous effort of the analysts. The careful elaboration of such procedures proves to be very time consuming and can only be performed by laboratory teams having extended knowledge and experience in this field. Furthermore, high instrument and operation costs are involved in the analytical strategy discussed. However, when considering the economical aspects we have to keep in mind that wrong analytical data, which result of poor knowledge and lack of analytical criticism are not only useless, but moreover, they often lead to wrong conclusions and to unforeseeable consequences. References 1 TOlg, G. and Garten, R.P.H., Angew. Chemie, 97 (1985) 439.

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CHAPTER 2

Limits of detection and accuracy in trace elements analysis

CLIFF J. KIRCHMER

Manager, Quality Asswatice Section, Washirigtoil State Departmetlt of Ecology, Munchester, WA 983534488. USA

Contents 2.1 2.2 2.3 2.4 2.5

2.6 2.7 2.8

Introduction.. . . . . . . , . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . 39 Errors in analytical results . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . 40 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Measuring trace concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 The problem of detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Random error of blank responses . . . . . . . . . . . . . . . . . . . . . . . 42 2.5.1 2.5.2 Errorsof thefirst kind-thecriticallevel (aposrerioridetection) . . . . . . . 43 2.5.3 Errors of the second kind - the limit of detection (a priori detection) . . . . . 46 2.5.4 Limits to the use of the definitions of LC and LD . . , . . . . . . . . . . . . 48 2.5.5 Regression theory approaches to the problem of detection . . . . . . . . . . 50 Practicalapplications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Reporting results at small concentrations . . . . . . . . . . . . . . . . . . . . . . . . 53 Conclusions and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

2.1 Introduction A survey of the literature reveals that different meanings have been given to the expression “limit of detection.” However, some of the differences have been semantic rather than substantial. In order to avoid adding to this historical situation, it is appropriate to start by defining some relevant terms that will be used in this chapter to explain the concept of “detection” and its relation to the “accuracy” of measurement.

CLIFF J. KIRCHMER

40

The following definitions are those proposed by D.T.E. Hunt and A.L. Wilson [13. A determiitand is that which one determines, and is synonymous with the term

arialyte when referring to chemical analyses. The term arzalyticafniethod denotes a set of written instructions specifying an analytical procedure to be followed by an analyst in order to obtain a numerical estimate of the concentration of a determinand in each of one or more samples. The term urzulytical systeni denotes a combination of analyst, analytical method, equipment, reagents, standards, laboratory facilities, and any other components involved in carrying out an analytical procedure. The analytical system does not include the sample(s). The term analytical result denotes a numerical estimate of the concentration of a determinand in a sample, and is obtained by carrying out once the procedure specified in an analytical method. Note that a method may specify analysis of more than one portion of a sample in order to produce one analytical result. All analytical systems include a measurement sub-system (e.g. spectrophotometric measurement of the absorbance of a solution)whose function - on presentation to it of a portion of a sample or treated sample - is to produce a numerical observation whose magnitude is related to the amount or concentration of the determinand in that portion. This numerical observation is termed an analytical respowse. One or more analytical responses (as specified by a method) are used, in conjunction with a calibration curve or factor, to produce an analytical result. The phrase “to have detected the determinand” is understood as meaning “to have obtained experimental evidence that the determinand concentration is greater than zero.” 2.2 Errors in analytical results The absolute error, E of an analytical result, R has been defined as the difference between that result and the true value, T :

E=R-T This error may thus be positive or negative, and its units are the same as those used for T and R. Currie has stated that the basic question facing the analyst is: What is the relationship of R to T [21? The structure of the errors, which provides an answer to this question, is given by Eqn. (2.1).

R = T + A +a+h+f(t) where

A

= bias or constant portion of the systematic error

(2.1)

LIMITS OF DETECTION

41

6

= random error, that part of the variability which can be described by the laws

b

= an erratic blunder or mistake (e.g. in the recording of a final result)

of probability

f ( t ) = the “lack of control” term, that is a variation which depends on some external variable t [the notation .f(t) is not meant to imply an analytic relationship, but rather a “variation with”; time, for example, often plays the role of the surrogate for other causative factors.] Systematic error encompasses all contributions that are not “random,” and is therefore A + b + .f(t).Effective quality control should be able to eliminate blunders and nonrandom variation [b, f ( t ) ] ,in which case the systematic error is constant and equal to the bias, A . The chemical measurement process (CMP) is used to determine the result, R, which Currie described as follows:

E * R=:r+4 -1-6 11

Note that for the CMP the errors are completely characterized by constant (bias) plus random components. As indicated in Eqn. (2.2), the total error E of an individual result equals A + 5, and the limiting mean p of the measurement population equals T + A. If the bias A can be eliminated, then the population mean p equals the true value T.

2.3 Accuracy From the above discussion, it is clear that accuracy is the absence of the two components of error, one fixed (bias) and one random. Some authors have used the term accuracy to denote only the bias component. For example, the commonly used expression “accuracy and precision” implies that precision (i.e. random error) is separate from accuracy. However, for individual analytical results, accuracy is affected by both bias and random error. Only if results are calculated as the average of a large number of individual determinations would random error be negligible and bias be a measure of accuracy.

42

CLIFF J. KIRCHMER

2.4 Measuring trace concentrations With this background on the nature of errors, their effects on analytical results, and their relation to accuracy,the effects of eirors on measurements at low concentrations can be considered. Low is used here in a relative rather than absolute sense, since there are significant differences in sensitivity among methods. Errors in measurements at low concentrations are no different in principle from errors at higher concentration levels. However, at low concentrations, errors can also make it difficult to decide whether or not the determinand is present, which is usually not the case at higher concentrations. Random errors increase significantly as concentration is decreased. The effect of random errors on blank determinations, in particular, is an important consideration in the measurement of low concentrations. Concentrations can be considered low when they are comparable to the standard deviation of the blank.

2.5 The problem of detection 2.5.1 Random error of blarik responses

By definition, the blank is a sample with zero concentration of determinand. Heinrich Kaiser recognized the importance of the blank in determining the lowest concentration that can be distinguished from zero. He compared the problem of distinguishing a sample containing the determinand from a blank, with that of “searching for a ship in a stormy sea.” He asked “Is the higher prominence glimpsed really a ship or a larger wave than usual?’ The higher the waves, the more difficult it is to detect the presence of a ship. Conversely, it is easy to detect a ship when the sea is calm. Kaiser also argued that just as it is not the depth of the sea, but rather the height of the waves that determines whether we can see the ship, so “the cause of the uncertainty in the analytical value is not due to the size itself of the measure, but to the size of the fluctuations in it. A constant blank measure of whatever size can always be compensated’’ [3]. Others have also argued for an approach to the problem of detection based on the variability of blank responses. Possibly the first authors to introduce statistical concepts to the problem of determining the smallest concentration significantly different from zero were Sillars and Silver 141. Kaiser was apparently the first to use statistical concepts for a general definition of the limit of detection of spectrographic methods [5]. A later publication by Kaiser and Specker recommended the same approach for all analytical methods 161. A comprehensive review of the historical development of the concepts of detection is beyond the scope of this chapter. Reviews on the subject have been published by Kaiser 17, 81 and Currie [13]. A detailed mathematical treatment of the subject is given in a book by Liteanu and Rica [lo].

LIMITS OF DETECTION

43

There are two problems regarding detection decisions. The first is whether, after measurement, the result can be considered to indicate that the concentration of the sample is greater than zero. Currie has referred to this as a posteriori (i.e. after the fact) detection [9]. The second is what the concentration of the sample must be in order that the determinand will likely be detected. Currie referred to this as a priori (i.e. before the fact) detection [lo]. To understand these a priori and a posteriori decisions, it is necessary to understand the concepts of errors of the first and second kinds in statistics.

2.5.2 Errors of thefirst kind - the critical level (a posteriori detection) Kaiser also recognized that decisions regarding detection could only be solved through the use of statistics. Indeed, at low concentrations one may not be able to give a definitive answer to the question of whether or not the determinand is present. When performing trace analyses, an analytical result ( B )is obtained by subtracting the apparent concentration of the blank ( B ) from that of the sample (S) (i.e. R = S - B). If we compare individual sample and blank responses, it corresponds to the “paired comparison” situation. O’Haver stated that “In analytical procedures involving sample preparation, separation, or preconcetitratioti steps, it is almost always essential that a blank be carried through the entire procediire” [ 111. In fact, several blanks should be analyzed by the complete procedure in order to obtain the necessary degrees of freedom for a satisfactory estimate of within-batch variability. This needs to be emphasized because it is often ignored. In general, blanks and samples ntust be analyzed by the same procedure. Failure to do so can result in a biased blank correction. Of course, situations arise where it is impractical or not essential to analyze blanks and samples identically, but such situations generally require experimental confirmation” [ 121. In water analyses, the blank is pure water that is analyzed identically to samples. Reagents added during analysis may contain the determinand and give rise to an analytical response, or the response may arise from some indeterminate cause. Blank correction is needed so that the response present at zero concentration of determinand is not included in the result. In some procedures, blank determinations are made but their responses are instrumentally adjusted to zero, and sample responses are then read with respect to this adjusted zero. Provided that these blank determinations are made by exactly the same procedure as that used for samples, this is equivalent to the explicit subtraction of blank response from sample response described above [l]. However, O’Haver has emphasized that it is desireable that the blank signals be measured and recorded individually, rather than simply setting the instrument response to zero on the blank [ 1 11. In this way, excessively high blank values can

CLIFF J . KIRCHMER

44

be recognized. Currie has emphasized that insti.umenta1 zero adjustment, as for the null level or baseline, does not eliminate the need for this blank or baseline estimate error propagation [ 131. The limit of detection must refer to the entire analytical process. Instrumental detection limits may be useful in evaluating and comparing the detection capabilities of instruments, but will not be considered in this chapter, since they do not include effects of sample preparation and cleanup (e.g. digestion) on detection capabilities. Consider the case in which the sample has zero concentration of determinand, and has a response comparable to that of a blank. The analytical result is then equal to the difference between two blank responses, R = B, - B, and, assuming a normal distribution, the distribution of results corresponds to that represented in Fig. 2-1. It is clear that a claim of detection is not justified solely by a positive result since,

Critical Level = 2.330~ I (Shaded Area Denotes 5%

I x

-30:

-20'

-

0'

0

0'

DifferenceBetween Two Blanks

of All Results)

20'

30' c

Figure- 2-1. Illustration of the distribution of the differences between analvtical resuoiises for two blanks and the definition of the critical level -0-

when the sample does not contain the determinand, a positive result will be obtained 50% of the time. What must be the result to conclude that the sample concentration is greater than zero? It is apparent that there is no definitive answer to this question. Instead, the question must be stated in terms of probability, and specifically the probability of wrongly concluding that the determinand has been detected (i.e. the concentration is greater than zero) when in fact none is present. In statistics, this is refened to as an error of the first kind, and the probability of this error is equal to the portion of

LIMITS OF DETECTION

45

the total area under the distribution curve to the right of the analytical result chosen as corresponding to detection. Each point on the distribution curve corresponds to a probability (or percentage) point, with value of a (or 100 a). The acceptable probability of an error of the first kind should be selected according to the seriousness of the risk involved, which may involve a difficultjudgement call. Currie suggested a default value of 0.05 for a, corresponding to a 5% probability of a false positive, which is the risk level illustrated in Fig. 2-1. When n = 0.05, the result is equal to 1.645(&)oB or 2 . 3 3 0 ~(where oB is the within-batch standard deviation of the blank response) meaning that values greater than 2.330, are considered to indicate that the determinand has been detected, with 1 chance in 20 of being wrong. Note that this corresponds to a one-sided significance test. It assumes that oB is known (the degrees of freedom used to calculate standard deviation is large) and the blank responses are normally distributed. Figure 2.1 conveys visually what is meant by detection. In statistical terms, we are comparing a null hypothesis (Ho: The net sample concentration is equal to zero) with an alternative hypothesis ( H A : The net sample concentration is greater than zero). Hypothesis testing is equivalent to determining whether a sample result exceeds the positive upper 90% confidence limit for the expected results of a sample with zero concentration of determinand (i.e. a blank). Following the definition by Currie. this value of 2.33 oBis called the critical level. Three important characteristics of this definition are: (1) that a decision regarding a posteriori detection be based on experimental evidence of the magnitude of the variability of the blank response, (2) that blank correction of sample responses are used to obtain results, and (3) that one chooses the risk (5% for Currie’s definition) of concluding that the determinand has been detected when in fact the concentration of the determinand is zero. Due to the variability of the blank responses, a general conclusion of this analysis is that zero detection limits are unattainable. One may argue that for some analyses there is no variability of the blank responses, since the response of the blank is always zero. This is the case in which the discrinzination (defined as the smallest interval by which differences in analytical responses can be reliably distinguished) of an analytical system is coarse in relation to the size of the random errors of results. This does not mean that any concentration above zero can be detected or that the variability of the blank is not the key factor in determining the limit of detection, but rather that valid estimates of random error cannot be obtained if the discrimination is not sufficiently fine. Whenever the detection of very small concentrations is of interest, it is necessary to adjust the sensitivity so that the discrimination has negligible effect on the apparent size of random errors. Note that the decisioti regarding detection can be made based on a paired comparison of raw data responses for blank and sample. There is no need to convert response to concentration using a calibration curve or factor.

46

CLIFF J. KIRCHMER

2.5.3 El-i-0l-sof the second kind - the liniit of detection (a priori detection) To answer the question regarding the lowest concentration of sample that one can a yl-iol-ibe certain of detecting with an acceptable level of confidence, we need to consider errors of the second kind, i.e. false negatives, or falsely accepting the null hypothesis that the sample concentration is equal to zero. From Fig. 2- 1, it can be seen that the probability of a false negative is 50% if the concentration of the

F

Distribution of Difference Between

Mean, p = LD = Limit of Detection = 4.65 0 g Distribution of Difference Between a Sample and a Blank When the Sample Concentration is Equal lo the limit of Detectton

1p z

t

s

P

1

a

0

-b’

-20’

-0.

Figure 2-2. Illustration of the relation between the critical level and the limit of detection

sample is equal to the critical level. This means that the probability of detecting the determinand is also only 50%, equivalent to flipping a coin to decide if the determinand is present. Clearly, this is not an acceptable level of confidence for a priori detection. The acceptable probability should be selected according to the seriousness of the risk involved. Following the default definition by Cuirie, the probability of a false negative, 0, can be set at 5%. which, for normal symmetric distributions, makes the limit of detection L, equal to 4.65 uB,twice as large as the critical level [9]*The detection power is defined as 1-,B and coi-responds to the probability of detecting a sample of concentration L,, in this case 95%. The limit of detection and its relationship to the critical level is illustrated in Fig. 2-2. Table 2- 1 presents a Truth Table which describes the relationship between hypothesis testing ‘Currie used the expression “detection limit” rather than “limit of deteclion” for u priuri detection. These expressions are considered to be synonymous, but for uniformity “limit of detection” is preferentiallyused in this chapter.

LIMITS OF DETECTION

47

Table 2-1. Truth table - detection Possible true situations Analyte not

Analyte

present Ho: T=O

present HA: T=LD

Incorrect decision

Correct decision

False positive (Type I error)

1-P

Possible decisions Detected R >Lc

a:

Not detected R 5 LC

Correct decision 1-a

Incorrect decision False negative (Type I1 error)

P and component detection. Note that the probability of making the correct decision is 1 - a if the determinand is absent (H,,), and 1 - /3 if the determinand is present at a concentration equal to L, (HA), Finally, it should be emphasized that the limit of detection is basically a statement on the inherent detection capability, but is not used for making detection decisions. For detection decisions, the sample result is always compared directly with the critical level to decide if the determinand has been detected. This means that if the determinand concentration is greater than the critical level, it is reported as detected. But if the result of analysis is less than the critical level, it is reported as less than the limit of detection to take into account the possibility of an error of the second kind. For example, in the analysis of lead in water by ICP-AES, if the critical level, L,, is estimated to be 5 pg/l and the limit of detection, L, is estimated to be 10 p a , then a result of 7 p a would be reported as such (i.e. 7 p a ) . But if a result of 4 pg/l were obtained, it should be reported as less than 10 &I.This can be confusing if one is not familiar with the concept of errors of the second kind. It should be emphasized that other values for a and /3 could have been selected. Kaiser, for example chose a value of 0.0014 for both a and ,4 (rather than the 0.05 value chosen by Currie), corresponding to values of 3 oB and 6 mB, rather than the 2.33 m B and 4.65 bB values, for the critical level and limit of detection. He chose a value with a high degree of confidence for two reasons. First, it includes a reserve necessary to offset the uncertainty resulting from the practical determination

CLIFF J. KIRCHMER

48

of the standard deviation. Second, it provides some allowance in case the frequency distribution of measures departs from the normal [3]. Terms other than critical level and limit of detection have also been used to describe the same concepts. For example, Kaiser used the German expression “nachweisgrenze” (which has been translated as “limit of detection”) to include only a consideration of errors of the first kind (corresponding to Currie’s critical level) and the expression “garantiegrenze” (which has been translated as “limit of guarantee of purity”) to include consideration of both errors of the first and second kinds (coiresponding to Cunie’s detection limit). Roos [ 141 and later Wilson [ 151 used the expression criterion of detection with the same meaning as Cuirie’s critical level. Keith has recently proposed that the expression method detection limit (MDL) should be used when only errors of the 1st kind are considered, and the expression reliable detection limit be used when both errors of the 1st and 2nd kinds are considered [16]. It is important not to attach much importance to the different names used to describe detection. What is important is to know the probabilities for errors of the 1st kind (a)and 2nd kind (p). However, standardized nomenclature is crucial for facilitating communication among the international scientific community. Some authors have not recognized the existence of errors of the second kind in defining the limit of detection [17-181. This is basically a failure to distinguish between detection decisions and detection capabilities, as Currie has pointed out [ 131. False negatives occur whether their existerrce is recognized or not.

2 5 . 4 Limits to the use of the definitions of L, atid L, The definitions for L, and LD presented in Sections 2.5.2 and 2.5.3 are based on “paired comparisons,” in which each sample response is considered to be individually blank corrected. Currie has also described the case of a “well known blank,” in which the blank response has been estimated from a large number of observations and therefore its variability in calculating a sample result is negligible [9]. The variability of the difference between sample and blank response is then reduced by a factor of fi,giving values of 1.64 gB and 3.29 uB,for L, and L,, respectively. Hunt and Wilson [ 11have presented a general expression for L, based on n replicate blank responses and m replicate sample responses, in which the standard deviation of the difference between sample and blank responses ( o ~ -is~given ) by Eqn. (2.3): Cs-B = gB[(l/nz) -k (1/?%)]‘

(2-3)

This equation is valid only for very low concentrations, where standard deviation is independent of concentration. The equations derived for Lc and LD assumed that the population within-batch standard deviation of the blank is known and that standard deviation is independent of concentration (up to L,). However, in practice, only estimates of the standard

49

LIMITS OF DETECTION

deviation will be available. Currie has emphasized that the critical level, used to make detection decisions, can be estimated directly from the empirical estimate of sBand Students t as L, = ts(a = 0.05). The limit of detection, however, must include a confidence interval for the uncertainty in the estimate of sBand Cui-rie states that this can be derived from the x2 distribution as L, = 2 t s ( ( ~ ~ / sValues )~. fort and aUL (UL is the upper limit of 0)are given in Table 2-2 for a selected number of replicates. (Note that this expression represents a conservative upper limit for LD).

The theoretical definitions of critical level and limit of detection have been based on the following assumptions, which may not always hold: 1. The within-batch standard deviations of both the blank and samples containing

very small concentrations of the determinand are the same; 2. The expected value of the analytical response is not zero for finite concentrations of the determinand; 3. The sample and blank are not biased with respect to each other (that is, there are not interfering substances in the sample or the blank); 4. The blank (i.e. pure water in the case of water analyses) does not contain the determinand. If the blank does contain the determinand, then a special analysis must be done to determine the concentration in the blank, which must be included in the calculations of L, and L,.

If any one of the above assumptions is not true, then the critical level and limit of detection cannot be calculated using the equations given previously. However, adjustments can sometimes be made to the equations. Currie has presented an analysis which allows for corrections when assumptions (1) and (3) above are not met [2]. For example, as illustrated in Fig. 2-3, adjustments can be made to allow for differences in the standard deviation for blank and sample responses ( ( T ~ # ( and T~) for different values for errors of the 1st and 2nd kinds (i.e. (Y and /3 values). Table 2-2. LD estimation by replication: Student’s t and ( a / s ) - bounds vs. number of observations [ 131 ~~~~~

No. of replicates

~

5

10

13

20

120

00

Students’s t:

2.13

1.83

1.78

1.73

1.66

1.645

auL/s(a)

2.37

1.65

1.51

1.37

1.12

1.000

(a)

u n / s is at the 95th percentile (i.e.

1-sided 95% confidence interval)

50

CLIFF J . KIRCHMER

Figure 2-3. Illustration of the case in which the standard deviation for sample and blank responses differ and in which the values chosen for errors of the first kind (a)and the second kind (p) also differ. (Adapted with permission from Ref. 2. Copyright 1978 Wiley).

When systematic error cannot be assumed negligible, the critical level must be increased by amount A, and the limit of detection must be increased by an amount 2 Am, where An, is the assumed upper bound for the bias, i.e. L,, = L, + A ,,, and L,, = 2L,,

2 5.5 Regression theoiy approaches to the yrobleni of detection Several different approaches have been taken to account for the effect of uncertainty in the calibration curve or factor on the critical level and limit of detection. These approaches involve the application of regression theory to the responses from a set of calibration standards, to calculate values for the intercept, i, the slope, m, and the within-batch standard deviation of the blank, sB [19-221. The simplest approach is to estimate the uncertainty in the slope of the calibration curve, m. A graphical approach has been described by Long and Winefordner [21]. The uncertainty in m can be expressed as a confidence interval rn f tsm, where s, is the standard deviation of the slope and t is a t distribution value chosen for the desired confidence level, a, and the degrees of freedom, d f . The effect of the confidence interval can best be seen by referring to Fig. 2-4, which shows the graphical relationshipbetween the blank corrected analyticalresponse and concentration. To convert the estimate of the critical level, L,, in response units to concentration units, a calibration curve is needed. Figure 2-4 shows that uncertainty in the slope of the calibration curve leads to uncertainty in the estimate of the critical level in concentration units. If the uncertainty in the slope of the calibration curve

51

LIMITS OF DETECTION

Concentration

Upper estimate of LD (in concentration untis)

Figure 2-4. Illustration of the fact that uncertainty in the slope of the calibration curve leads to uncertainty in the estimate of the critical level, Lc. ( B is the mean blank response, A. is a constant, and SB is an estimate of the within-batchstandard deviation of the blank.)

is sufficiently large, the lower bound for m can be used to calculate and report an upper estimate of L, (or L,) in concentration units. Other approaches which are intended to take into account the uncertainty in the calibration curve or factor have been proposed. A “Propagation of Errors Approach,” which takes into account the error in the intercept, i, as well as the slope has been described by Long and Winefordner [21]. While there are instances when errors due to uncertainty in the calibration curve should be included in the estimate of the critical level and limit of detection, there are some problems with the regression theory approach as well. First, the estimates of the y-intercept and the standard deviation of the blank responses, which refer to zero concentration, are derived from calculations using some or all of the calibration standards, whose concentrations are greater than zero. Thus, the accuracy of these estimates depends on the accuracy with which the true relation between response and the determinand concentration is known, and on the accuracy with which the true relation between the standard deviations of responses and concentration is known [13. Second, the assumption in this approach is that the uncertainty in the calibration curve contributes to the uncertainty in determining whether or not the determinand

52

CLIFF J. KIRCHMER

has been detected. However, if the relation between response and concentration can be considered linear, the decision regarding detection can be made independently of the calibration curve, based on a comparison between the critical level and the difference between the sample response and the blank response, before the calibration curve is used to convert a result into concentration units. Of course, the calibration relation is necessary to estimate the limit of detection, which must be expressed in concentration units. The slope of the calibration curve does not affect the detection decision, only the accuracy of the concentration reported. Indeed, O’Haver [l 11 and others have argued that, because random error is so large, a concentration should not be reported at or near the critical level or the limit of detection, and that detection should just be a qualitative yes or no decision. However, this would result in loss of information, since a detection decision can always be made and the estimated concentration and its uncertainty can also be stated. The third problem with the emphasis on the uncertainty in calibration curves as being the determining factor in detection decisions is that, as R.V. Cheeseman and A.L. Wilson have demonstrated, the sample response, and not the calibration curve, is usually the primary source of random error in determining the result of analysis [ 121. This is because several standards can be used to reduce the uncertainty in the slope of the calibration curve to an acceptable level, while sample results are usually only based on a single analysis. In summary, the approach based on a comparison of sample and blank responses is more directly related to the problem of detection decisioiis than the approach based on regression theory, h d to require fewer assumptions for its validity, since the standard deviation used in the calculation of L, is estimated only from blanks.

2.6 Practical applications There have, unfortunately, been relatively few instances in which the theoiy of limit of detection based on the variability of blank responses has been applied in practice. Some methods have even been written to prohibit blank correction of sample responses, which of course makes a comparison of sample results to the critical level meaningless for deciding on whether the determinand has been detected. However, in the United Kingdom, several of the “Methods for the Examination of Waters and Associated Materials” prepared by the Standing Committee of Analysts of the Department of the Environment, have been evaluated by individual laboratories to determine the limit of detection based on the variability of the blank and using “paired comparisons” for blank correction. Published values for the limit of detection (a = /3 = 0.05) for several of these methods are listed in Table 2-3. The Standing Committee of Analysts has adopted a policy of including an estimate of the limit of detection (or the within-batch standard deviation of the blank, which is

LIMITS OF DETECTION

53

Table 2-3. Estimated limits of detection taken from “Methods for the Examination of Waters and Associated Materials” [22-261 Element

Estimated limit of detectioda)

Copper

Degrees of freedom

used in estimate(s) 5

Chromium

7-9

Phosphorus

35

Silicon

10

Aluminum

10

(’)

Note: Range for some elements is due to results in different laboratories

used to calculate the limit of detection) as one of the “Performance Characteristics of the Method” [23-271.

2.7 Reporting results at small concentrations Parallel to the problem of making decisions regarding a yriori or u posteriori detection, there is the problem of how to report results at small concentrations. An indication of this problem was given in Section 2.5.3, where it was concluded that results above the critical level should be reported as such, while results below the critical level should be reported as less than the limit of detection. Hunt and Wilson [ 11have suggested that all results at low concentrations simply be reported as the result plus or minus the confidence limits at a stated level of confidence (R f t,s), in order to avoid bias and information loss, especially when averages of low concentration results are calculated. Implementation of this recommendation would completely eliminate the use of the critical level and the limit of detection in reporting results. However, they could be provided separately as reference information for the user to aid in the interpretation of the reported results and associated confidence limits. In this regard, it should be remembered that negative results are possible, as is clear from Fig. 2-1. Sample and blank responses should always be greater than or equal to zero, but results, which are calculated as the difference between the sample and blank responses, can be negative. Indeed, for a sample of zero concentration of determinand, the results would be expected to be negative approximately 50% of the time. Investigators in water quality monitoring also have proposed that the limit of detection or the limit of quantitation, L,, not be used to censor data [28]. Like Hunt and Wilson, they believe that L, and L, aid in the interpretation of individual

CLIFF J. KIRCHMER

54

measurements, but they hinder statistical analysis of water quality data. Instead, one should report the results of all analyses plus an estimate of observation error. The concept of “not detected” is not altered because a confidence interval that overlaps zero is a valid statistical definition of “not detected.” lhere are also some who have proposed that numerical values not be reported at or near the limit of detection, since random error, as measured by the 95% confidence limits on a result whose concentration is equal to the limit of detection, is equal to approximately 60% at the 95% confidence level. With this level of eiror, one can argue that it is only necessary to report qualitative detecthion-detect decisions at or near the limit of detection. O’Haver has stated that “It should also be kept in mind that a concentration at the detection limit can only be detected, as the term “detection limit” implies and is not measured quantitatively” [ l l ] . Currie has defined a “determination limit” (other authors have referred to this as the limit of quantitation), at which a given procedure will be sufficiently precise to yield a satisfactory quantitative estimate [9]. He defined the determination limit as that concentration at which the relative standard deviation of measurement is 10%. Assuming that the standard deviation at this concentration is the same as the within-batch standard deviation of the blank, the determination limit, LQ,is equal to 14.1 uB in the case of paired observations and 10 gB in the case of a “well-known” blank. Thus, according to Cui-rie, the “working”expressions for L,, L,, and LQ can be summarized in Table 2-4, and Fig. 2-5 illustrates the three principal analytical regions [9]. An assumption is that the method of analysis is sufficiently precise so that relative standard deviations of 10% are achievable at the concentrations indicated. This is true for most spectroscopic methods of analysis, but for some analyses, a precision of 10%RSD may not be achievable at any concentration. Table 2-4. “Working” expressions for Lc, LD, L Q ( ~ ) LC

Paired observations

2.33 U B

4.65 CTB

14.1 U B

“Well-known”blank

1.64 U B

3.29 U B

10

UB

One can also describe an external or required limit of detection, L,, which determines the selection of the method to be used. This required limit of detection can be based on a regulatory requirement. Currie described L, as a regulatory limit, coiresponding to the external limit which drives the design of our measurement process [ 131. Wilson has recommended as a rule of thumb that the required limit of detection be at least lox less than the regulatory standard or criteria to be enforced. This is because, as explained previously, the random error at the limit of detection

55

LIMITS OF DETECTION

REQlON 111 D E T E R W I N A T I ONt UNRELIABLE DETECTION

DETECTION: OUALITATIVE ANALIBIS

a U A N T I T A T I V E ANALYSIS

I

I

I I I I

2.330~ 4

:

h'

I 1 1 1 0

Blank-Correcled

8 I

Response

I

:4.660~ 4

I

I

(Paired comparisons of sample and blank responses)

b

I I

4

I I

14.1

uB

I

, I

I

Lo 0

b

Figure 2-5. Illustrationof the three principal analytical regions (assumptionsare that the error of the first kind (a)and the error of the second kind (p) are both equal to 0.05 and that the standard deviation of results is independentof concentration in the range 0 - LQ

is very large relative to the concentration measured and, if the limit of detection is too close to the regulatory standard or criteria to be enforced, the results will not be suitable for making decisions as to whether a regulatory limit has been met. It is worthwhile noting that the critical level and limit of detection can be lowered by calculating results based on the analysis of more than one portion of a sample, since the standard error of the mean is equal to s/,/Z where n is the number of measurements used to calculate a result. In the absence of bias, therefore, repeated analyses of portions of a sample can be an effective way to lower the detection capability. The International Atomic Energy Agency recommendation is to make detection decisions by comparing the averages of at least three or four (but preferably nine or more) paired comparisons of sample and blank responses [29]. Repeated analyses can also make the assumption of a normal distributionof results more likely, because the Central Limit .Theorem states that averages derived from a sequence of mutually independent random variables having a common distribution tend toward normality, often rather quickly (by the time la = 3 or 4).

CLIFF J. KIRCHMER

56

2.8 Conclusions and recommendations

Decisions regarding detection must start with a recognition of the importance of the blank. The within-batch variability of the blank, as measured by standard deviation, is the most important factor in developing a theory of detection. It is important that blank responses be subtracted from sample responses before deciding whether sample results indicate that the determinand has been detected. Both errors of the first kind (false positives) and errors of the second kind (false negatives) must be taken into account when defining terms related to detection. For simple detection based on paired comparisons of sample and blank responses and an assumption of a normal, symmetrical distribution of within-batch blank responses and standard deviations of low level samples equal to those of the blank, Cui-rie defined the critical level as equal to 2.33 gB,correspondingto a 5% probability of false positives [9]. Decisions regarding detection are always made by comparing net (i.e. blankcorrected) sample response with the critical level. Similarly, Cui-rie defined the limit of detection as equal to 4.65 uB.coi-responding to 5% probability of false negatives as well as 5% probability of false positives 191. There is a 95% probability that the analysis of a sample with a concentration equal to the limit of detection will have a net sample response greater than the critical level. Different values for ei-rors of the first and second kinds can be selected. For example, 1% probabilities for both of these types of errors were recommended by Kaiser 131. Adjustments can be made to this basic theory to account for bias due to interference or calibration, random error in calibration, and increases in random error with concentration. When reporting data, particularly monitoring data, the critical level, limit of detection, or limit of quantitation should not be used to censor data. To avoid information loss and biased calculations of mean sample concentrations, all data should be reported, together with an estimate of the uncertainty in the results. The critical level should be provided separately as an aid in interpreting the reported results. References 1

Hunt, D.T.E.and Wilson, A.L.. The Chemical Analysis of Water. 2d ed.. The Royal Society of Chemistry,London, 1986.

2

Currie,L.A., in: TreatiseonAnalylicalChemislry,I.M. KolthoffandP.J. Elving, (Eds.), Vol. 1, 2nd ed., Wiley, New York, 1978, pp. 95-242.

3

Kaiser, H., lbo Papers on the Limit of Detection of a Complete Analytical Procedurc, A.C. Menzies, ed. Adam Hilger, Lld., London. (confains translations of Refs. 7.8).

4

Sillars, I.M. and Silver, R.S.,J. Soc. Chem. Ind., 63 (1944) 177.

5

Kaiser, H., Spectrochim. Acta. 3 (1947) 40.

6

Kaiser. H. and Specker, H., Z. Anal. Chem., 149 (1956) 46.

LIMITS OF DETECTION 7

Kaiser, H., Z. Anal. Chem., 209 (1965) 1.

8

Kaiser, H.. Z. Anal. Chem.. 216 (1966) 80.

9

Currie, L.A.. Anal. Chcm., 40 (1968) 586-593.

57

10 Liteanu, C. and Rica. I., StatisticalThcory and Mclhodologyof Trace Analysis.Ellis Horwood, Ltd., Chichester. 1980. 11 O’Haver. T.C., in: Trace Analysis - Spectroscopic Methods for Elements. J.D. Winefordner (Ed.), Wiley, New York, 1976, pp. 15-62. 12 Cheeseman, R.F. and Wilson, A.L., Manual on Analytical Quality Control for the Water Industry, Technical Report TR66. Water Research Centre. England, 1978. 13 Currie, L.A.. Chapter 1 in: Detection in Analytical Chemistry - Importance, Theory and Practice, ACS Symposium Series 361. L.A. Currie (Ed.). American Chemical Society,Washington, D.C., 1988,pp. 1-62. 14 Roos, J.B.. Analyst, 87 (1962) 832. 15

Wilson, A.L., Talanta, 20 (1973) 725-732.

16 .Keith. L.H.. Environmental Sampling and Analysis: A Practical Guide, Michigan, Lewis Publishers. Inc., 1991. 17

Glaser. J.A., Focrst, D.L., McKee. G.D.. Quave. S.A.. and Budde. W.L.. Env. Sci. & Tcchnol.. 15 (1981) 1426-1435.

18 “Nomenclature. symbols, units and their usage in spcclrochemical analysis - 11” International Union of Pure and Applied Chemistry. Analytical Chemistry Division, Commission on Spectrochemical and Other Optical Procedures for Analysis. Spectrochim. Acta B.. 33B (1978) 242. 19

Hubaux. A. and Vos, G.. Anal. Chem., 42 (1970) 849-855.

20

Gibbons. R.D., Jarke, EH., and Stoub. K.P., in: Waste Testing and Quality Assurance. D. Friedman (Ed.). Vol. 3, ASTM STP 1075, Philadelphia, 1991, pp. 377-390.

21

Long. G.L. and Winefordner, J.D.. Anal. Chem.. 55 (1983) 712A-724A.

22

Currie, L.A., Chapter 5 in: Trace Rcsidue Analysis - Chemometric Estimations of Sampling, Amount, and Error. ACS Symposium Series 284: D.A. Kurtz (Ed.). American Chemical Society. 1985. pp. 49-81.

23

Standing Committee of Analysts, Copper in Potable Waters by Atomic Absorption Spectrophotometry. 1980, Her Majesty’s Stationery Office, London. 1981.

24

Idem. Chromium in Raw and Polable Waters and Sewage Effluents, 1980, H.M.S.O.. London. 1981.

25

Idem, Phosphorus in Waters. Effluents and Sewages, 1980, H.M.S.O., London, 1981.

26

Idem. Silicon in Waters and EMuents, 1980, H.M.S.O., London, 1981.

27

Idem, Acid-Soluble Aluminum in Raw and Potable Waters by Spectrophotometry, 1979,

H.M.S.O.. London, 1980. 28

Porter, R.S.. Ward, R.C., and Bell, H.F., Env. Sci. & Techno]., 22 (1988) 856-861.

29

Currie, L.A. and Pam, R.M., Chapter 9 in: Deteclion in Analylical Chcrnislry - Imporlance, Theory. and Practice, ACS Symposiuin series 361. L.A. Curric (Ed.). American Chemical Socicty, Washington. D.C., 1988, pp. 171-193.


CHAPTER 3

Sampling and sample preparation

J.R.W. WOI?TEZ and J.E. SLOOF

Iitterfaculty Reactor Institute, Delft University of Techiiology,Mekelweg 15, 2629 JB Delft, The Netherlarids

Contents 3.1 3.2

3.3 3.4

3.5

Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes in trace element composition . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Element specific changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Sample specific changes . . . . . . . . . . . . . . . . . . . . . . . . . . . Pre-sampling considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aspects of sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Establishment of analytical control . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Sampling error in a test portion . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3 Uniformity of laboratory samples . . . . . . . . . . . . . . . . . . . . . . . 3.4.4 Uniformity of subsamples . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.5 Thegrosssample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 62 63 76 77

83 89 89 92 93 93 94

3.1 Introduction “Unless the complete history of any sample is known with certainty, the analyst is well advised not to spend his time in analyzing it.” This statement was made over 35 years ago [ 1301 and since then quoted by analysts who are concerned about the quality of a sample [40,137]. Nearly 20 years after Thiers made his remarks, a paper on sampling for chemical analysis concludes: “It is the capability of understanding and executing all phases of analysis that ultimately characterizes the true analytical chemist, even though he or she may possess special expertise in a particular separation or measurement technique” [72]. At present, this statement is still a basic principle for the trace element analyst, since the evolution in detection capability combined with the growing awareness of the effects of trace levels of pollutants

J.R.W. WOI’ITIEZ AND J.E.SLOOF

GO

on a national or global scale, asks for more analyses of more elements on a lower concentration level. Also, the questions asked to the analyst become more and more complicated. Interdisciplinary research in the medical, biochemical, environmental, geological or industrial field no longer demands data on just trace element contents, but needs information on interaction with other components, availability, transport with accessory kinetics and distinction between possibly occurring chemical forms. Unfortunately, some of the reactions of trace element analytical chemistry to this development (e.g. automation, computerization, modelling, lab-accreditation) tend to take away the view of the analyst from understanding and executing all phases of analysis and force him or her to “desk-top analytical chemistry.” It is the explicit task of the analyst to provide an interdisciplinary project with accurate, quantitative data. This starts with awareness of the possibility of appropriate sampling and sample preparation. Sampling and sample preparation strategies which parantee the chemical integrity of a trace element compound are extremely difficult and sometimes not (yet) possible to design. Especially when multi-element analysis in different but interlinked parts of an environmental or biological system is required, the knowledge of adequate sampling is mandatory. This statement, how superfluous it may sound, may be well illustrated by considering the complexity of biochemical and physical processes in the ocean and chemical forms of trace elements involved [109]. They are enumerated in Table 3-1. It is clear that simply analyzing seawater for its total elemental content is only partially meaningful in such complicated systems. Often, demands set by appropriate sampling and sample preparation directly contradict the demand of chemical integrity of a trace element compound. This may be illustrated by a practical example, concerning multi-trace element analysis, among which different inorganic iodine compounds, in seawater [152]. Here, preservation of the sample asks for acidification so as to prevent absorption of trace elements to the container wall. Addition of H+ however definitely disturbs the equilibrium between IO;, I- and I, [79], which is given by

51-

+ 10; + 3H,O

;t

31,

+ 60H-, K

= 1.0 x

The solution for this type of dilemma is to acknowledge that there is not just one sampling strategy for both analytical problems. Recognition, understanding and control of parameters which threaten good sampling, of course, is not beyond reach. Expert laboratories have demonstrated that high quality sampling and sample preparation is possible. Manufacturers of environmental and biological certified and standard reference materials (CRM’s and SRM’s) spend years of research on such items as homogeneity, preventing contamination, minimal sample size and tenability of their products. The attention which is paid to the understanding of all phases of the analysis here, is visualized by the contents of the International Symposium on Biological and Environmental Reference Materials (BERM), which has been held every two years since 1984 [e.g. 9, 101.

SAMPLING AND SAMPLE PREPARATION

61

Table 3-1. Physicocheniical fomis of trace elementsand major biogeochemical and physical processes in the ocean [ 1091

Physicocheniicalfornts of trace elentents in natural waters Tme solution (< 0.001 pm):

Collodial(O.OOl-O.1 pm):

Particulate (0.1-50 pm):

Simply hydrated ions Inorganic complex ions Organic chelates Molecules and polymeric species Ion pairs Mineral substances Hydrolysis and precipitation products Biopolymers and detritus Organic particles: Plankton Detritus Bacteria and microorganisms Inorganic particles: Mineral substances Precipitated and coagulated particles

Major biogeochemical and physical processes in the ocean Physical processes:

Influx by rivers and ice Transport by wind Influx by dissolved gases Wave action on sediments Transport by animals Efflux by animals and man Diffusion Turbulent mixing Volcanic action Upwelling of deep seawater Sedimentation

Chemical processes:

Chemical precipitation Sorption by sediment surface Redissolving from sediment

Biological processes:

Decompositionand respiration by marine animals Photosynthesis Sedimentation and decomposition by bacteria

Combined processes:

Halmirolysis Dissolution Scavenging

62

J.R.W. WOITTIEZ AND J.E. SLOOF

The items sampling, sample preparation, stability, quality control, application of appropriate trace element analytical techniques and statistical evaluation techniques are all addressed. One project deserves special mentioning in this context. The US EPA/NBS National Environmental Specimen Bank was founded in 1973 [98] with, among other things, the aims to “develop and evaluate protocols for contamination-free sampling of environmental specimens” and “develop and evaluate storage conditions which would permit the long-term storage of samples without change in pollutant concentrations” [99]. In this project, expert analysts protocolize, verify and control their trace element analytical techniques to a level that inadequacy in sampling and sample preparation can be detected. Subsequently, sampling and sample preparation protocols are improved until adequacy can be proven. Since long-term storage of samples and trend monitoring are prime goals of the project, this level of understanding of all phases of analytical chemistry has to be maintained during years [ 1611. In the specimen bank project trace element analytical chemistry is well incorporated in interdisciplinary research. In this chapter, theoretical and practical aspects of sampling and sample preparation, mainly for the biomedical and environmental field, are discussed. This includes causes (and precautions) of changes in trace elemental composition due to sample preparation (contamination, losses, inhomogeneity), sample pre-treatment (washing, drying, etc.) and sample decomposition (wet and dry ashing). Although preconcentration of trace elements may be considered as a part of sample preparation, it is treated separately in Chapter 4 of this book. Most of the knowledge on sampling and sample preparation presented here comes from research from a selected number of analytical laboratories and analysts. Without pretending to be even close to completeness, some names should be mentioned: Pioneer work in the biomedical field has been done by Versieck and Cornelis, Heydorn and Iyengar. Their work has been documented in a series of monographs and topical papers [39,41,51,61,139]. In the environmental field, the analytical group of NIST (formerly NBS), the US and German Specimen Bank Projects, the Max-Planck Institute (FRG) and those cooperating with reference material manufacturers (BCR, Belgium; NIST, USA; NRCC, Canada) have provided expert work. The importance of sampling and sample preparation of food items has been documented by the US Food and Drug Administration [46]. Important individual contributions have been made by Thiers in 1957 [130] and Zief and Mitchell in 1976 [162]. Sampling and sample preparation, focused on isotopic and radionuclide analysis, has been covered by the laboratories connected to the IAEA and Euratom [48].

3.2 Changes in trace element composition The very process of sampling and sample preparation bares the risk to change the trace element composition of the sample, resulting in systematic errors. The cause


63

can be a trace element specific change, such as contamination or losses and a sample specific change, such as segregation or change of moisture content. Since a sample inevitably contacts some container material and remains in the container for some time, all the processes are likely to happen during sampling and sample preparation. For the preparation of a uniform sample, suitable for elemental analysis, a sequence of actions with appropriate tools is obligatory (washing, crushing, milling, etc.) and thus contamination and losses again are bound to happen. In this paragraph some of the main causes of changes in trace element composition are discussed.

3.2.1 Elenierit specijic churiges 3.2.1.1 Contamination Contamination may arise from various distinctive sources. Air particulates, the analytical laboratory and the analyst, chemicals and equipment can be distinguished. Air-particulate matter-always plays a role during sampling outside the laboratory, unless the environment at the sampling spot is conditioned. Air particulates may carry major components of the earth’s crust (Si, Al, Fe, Ca, Mg, Mn, Ba, Sr) and the presence of seawater will be reflected in an aerosol contribution (Na, C1, Br, I). Heavy industrialization (V, Sn, Cd, Ni, Cr, Ag) and traffic (Pb, Pt) may also contribute. When deposited on a sample, elemental contamination will be a fact. The problem is manifest for the sampling of atmospheric deposition. Bulk precipitation consists of dry and wet deposition. Particulate matter may be separated by filtration, but leaching of trace elements into the solution cannot be prevented this way. Wet deposition can be obtained separately by so-called “wet-only samplers,” where the polymer sampling bucket is covered with a polymer cap that only opens duringrainfall[66]. Air particulate matter can be sampled separately by air filtration using impactors or filterpacks [25]. Similar problems may occur for sampling snow, hail, dew or fog. In biomonitoring, the dilemma whether to consider air-particulate matter and soil dust as an inherent part of the sample or not, always exists. An often used approach is applying some washing procedure, such as developed for human hair [l lo]. An obvious disadvantage of washing is the risk of both leaching of trace elements from the sample and contaminating it with the chemicals used for washing. In case of moss and lichens, often used for biomonitoring purposes, the biomonitor function is based on accumulation of trace elements into the cryptogams rather than on filtration of dust particles. Since this accumulation is more likely to occur from the aqueous or gaseous phase than directly from a solid phase (air dust), the washing step seems justified form a sampling point of view. Much attention has been paid to prevent contamination with air particulate matter in the laboratory. Several sources, apart from air from the outside world, have been identified: Particulates from the laboratory air supply (filters, vents), a concrete

J.R.W. WOITI'IEZ AND J.E. SLOOF

64

or painted wall, heating, furniture, ceiling, doors, sinks, etc. Also, the analyst is a source of air particulates through its clothes, shoes, hair, cosmetics, etc. The need for a clean laboratory environment has long been recognized. Key publications on the topic have been produced by Zief and Mitchell in 1976 [162], who systematically deal with all aspects of contamination control, and Moody in 1982 [92], who described the NBS Clean Laboratories. More recent communication in 1988, 1990 and 1991 [ 12,21,27,85] illustrate that the clean room concept, developed throughout the seventies and early eighties, is still state of the art. A clean environment can be classified according to the maximum number of particulates of a certain size per ft3, class 100 being the most stringent. Class 100 environments should be free of particles larger than 5 pm, not contain more than 10 particles larger than 2 pm per fe and not more than 100 particles larger than 0.5 pm per ft3 [92]. For a class 1000 environment these numbers are 10, 100 and 1000 respectively. This division in classes not only refers to particle sizes, but intrinsically also to the total mass of airborn particulates. Class 1000 environments are acceptable for non-laminar-flow clean labs, while class 100 is the requirement for laminar flow environments. It should be noted, that not only the number of particles is important with respect to contamination, but the composition is just as vital. This may be illustrated by Fig. 3- 1, on the relative size of common air contaminants [96]. The basic principles of the air-household for a clean laboratory are four-fold: (1) The inner laboratory

10 :

8 % E v)

F

5

Qm

v)

c

2

v)

0.01

-

I

I

I

I

a' I

I

Data from Murphy, 1 9 7 6

Figure 3-1. Particle size distribution of some common air contaminants

I

I

I


65

atmosphere is in overpressure compared to the outer environment. This usually demands an air supply system which is independent of the main laboratory air supply and may thus be costly. (2) The air which enters the laboratory has to be purified from airparticulates. To this aim, the high efficiencyparticulate air filter (HEPA) can be used, which has a minimal scavenging capacity of 99.99% for 0.3 pm particles. (3) In order to minimize air turbulence within the laboratory, vertical laminar-flow air inlet is the best solution. (4) The total laboratory volume of air should be changed sufficiently frequently so as to allow the removal of particles generated in the lab. Again, since a frequency of several times per minute will be needed, the principle asks for a separate, costly air supply system. The NBS Clean Laboratories described by Moody [92] meet these criteria. Many laboratories, however, compromise on the laminar laboratory air flow. Instead, a class > 1000 non-laminar laboratory environment is created, in which class 100 laminar flow clean boxes are situated. Open sample preparation work is performed in these boxes. This situation has been described for the Max-Planck Institute Clean Laboratory by Tschopel et al. [ 1321. In their recent book on quantitative trace analysis of biological materials, McKenzie and Smythe [85] warn against too high an expectation of class 100 clean laboratories. They state: “It in no way overcomes contamination by gasses, poor quality reagents, faulty sampling, careless techniques, problems of sample preparation, digestion methods, etc.” This is true of course, when all these parts of an analytical procedure are not considered in their coherent context. Since this coherence is the leading thread running through this chapter,we feel another threat for a well equipped, clean laboratory: To realize the needed time (and money) for the challenging task to construct a clean lab is one thing, to bring in the required devotion and discipline for the unthankful task to preserve its cleanness is another.

As already pointed out, the analytical laboratoiy and the analyst should be considered as important sources of contamination. This refers mainly, but not exclusively, to airborne contamination. Boyer and Horwitz [141presented a practical checklist of necessary actions to change an ordinary laboratory into a trace element laboratory. This list, with some additions, is reproduced in Table 3-2. In general, poor hygienic conditions such as rusty surfaces, peeling off paint, remnants of metal spatula, tweezers, etc. and chemicals in glass or metallic containers, dirty lab coats are uncontrolled sources of contamination. The most important parameter in the successful exploitation of a trace element laboratory is the discipline of the analyst in charge. c The analyst itself may be an unexpected source of contamination. Human-fingers are a notorious source of Na, C1 but surprisingly also of pg amounts of lead [96]. Human sweat, hair, skin flakes, and the analysts’ clothing are established sources of contamination. Often overlooked, the main pathway of contamination originating from the analyst is by cross-contaminating, i.e. transfer of contamination from

J.R.W. WOITI'IEZ AND J.E. SLOOF

66

Table 3-2. Necessary actions to change an ordinary laboratory into a trace element laboratory [ 141

Replace all rusted and corroded hot plates, heating units, ashing ovens, etc. with ceramic-top hot plates and ceramic-lined ovens. Replace rusting ring stands, clamps, racks, hood and window panels. metal cabinets, etc. or strip and paint with epoxy paint. Remove all unnecessary shelving, partitions, furniture and other dustcollection items from the laboratory. Install filters on all incoming air sources (air conditioning, heating, etc.) Class loo00is advisable. Install a laminar-flow hood, provided with HEPA filters, for cleaning of equipment by boiling in acids or for open dry or wet digestion. Install a laminar-flow box where cntical work has to be done. Coat ordinary bench tops with epoxy

paint and cover them with adhesive backed Teflon or polyethylene sheeting.

Remove all metal glass cylindersfrom the laboratory lo an adjacent lab or separate cylinder room. Run a Teflon or polyethylenedistribution line from each gas source to the laboratory requiring the gas. Paint walls in the analysis room with two component epoxy paint or cover the walls with plastic (e.g. PVC) insertions. Cover floors with one-piece vinyl flooring. Place sticky mats near the entrance. Limit the accessibility of personnel into the lab. Construct a barrier in the entrance of the lab which discourages quickly slipping in and out. Oblige the wearing of special dustpoor clothes and change of shoes. Preferably, hair should be covered. Leave the trace element lab as empty as possible. The more equipment present, the more people have to be in the lab and the more difficult the clean conditions are to be maintained.

Replace all metal parts of lab furniture or equipment with plastics when possible, or cover with epoxy paint.

source to source via the analyst. In radiochemical laboratories, this phenomenon is well known since for health physics reasons, uncontrolled appearance of a few Bequerel, corresponding to picogram amounts or less of radionuclides, are considered to be radioactive contamination. A radiochemical analyst, working with an open radioactive source thus has to avoid the transfer of picogram amounts of a certain radionuclide from the source to his working environment or to himself. Inadequate manual operation of micropipettes and tips, spilling drops of radioactive solution, contaminating wiping tissues and touching contaminated glassware,acid hoods, etc. have been found to be some of the main routes of radioactive contamination of the analyst. Trace element contamination via the analyst, sometimes considered to be


67

cross-contamination, follows exactly the same pathways as radioactive contamination. It goes without saying that the prevention of the effects of both manifestations of the same contamination principle are identical. Some are explicited here: (a) Two sources of different concentration should never be brought close together. Standards and sample should be handled separated, and stored separated; (b) Re-use of chemical equipment such as pipettes, beakers, spatula, etc. should be avoided. Only disposables have to be used and to be disposed after use; (c) Non-chalked disposable gloves, preferably polyethylene, have to be used and they should be disposed after each manipulation; (d) Disposable protective clothing, preferably of a non-fluffing polymer, should be used and should be disposed after use. Of most importance, again, is the attitude of the analyst towards contamination. Murphy states: “And, finally, the analyst must ‘think blank’ that is he must be aware as to the effect on the blank of every step of the procedure. He must ask himself “If I do this, what is the effect on the blank?” and avoid those operations which tend to increase the blank or whose effect is not known, whenever possible” [96].

Chemicals arid equipnzent are the third main source of contamination. In sampling, chemicals are used for preservation or washing. Mineral acids, often in high excess, are used in the decomposition of geological and biological material. Acids are also used for the cleaning of polymer laboratory equipment and for stabilization of aqueous samples. Water is consumed in enormous quantities in every trace element laboratory. It is used for rinsing, cleaning and dilution. Organic solvents in trace element analytical chemistry are used for cleaning, solvent extraction or, nowadays, HPLC separation of metal-ligand compounds. The need for the highest obtainable degree of purity of these chemicals has long been perceived. Organic solvents are commonly purified by fractional distillation [162]. Already in 1955, Thiers describes the need of purification of water, hydrochloric acid, nitric acid, sulfuric acid, perchloric acid and ammonia. His conclusions are that ion exchange produces far better quality water than just distillation, but the purest water is obtained by ion exchange followed by distillation from fused quartz. For HCl and NH, Thiers recommends the synthesis of the chemicals from gaseous products and pure water rather than purification of the aqueous solutions. Such prepared HC1 was reported to contain a few ngmkg-’ of transition and heavy metals, compared to pg.kg-’ amounts for reagent grade material. Considering the state of the art of trace element analysis of nearly 40 years ago, this may be considered an extraordinary achievement. Nitric acid, used widely in wet ashing procedures, was identified as the major chemical contamination problem by Thiers. He recommends repeated distillation of the azeotrope (65% HNO,, 35% H,O) from a quartz still.

68


A few years later Irving and Cox [49] describe the simple procedure of isopiestic (or isothermal) distillation to prepare upto 10 N HC1 or NH,OH within 3 days, starting from double distilled water and reagent grade chemicals. This technique combines the volatility of gaseous acids and their solubility in water at room temperature. In his 1972 review, Tcilg [131] also emphasizes the advantages of the use of the gaseous phase at low temperature, so as to minimize contamination of chemical reagents. In 1972 Hamilton et al. [37] mention the need for a continuous recycling system of pure water, based on filtration, distillation, ion exchange and again, filtration. It is the better alternative for storage in PVC tanks, which leads to leaching. For ultra trace element analysis, Hamilton et al. find nitric, sulfuric and perchloric acid to lack sufficient purity, while HC1 (obtained by isopiestic distillation) and HF (triple distillation in Teflon) are qualified as acceptably pure. In the early seventies, the first papers describing the purification of acids by sub-boiling appear. Pure quartz sub-boiling units are described for purification of HCI, HNO,, HClO,, HZSO, and H 2 0 and all-Teflon units for the purification of HE The two bottle Teflon still, designed by Mattinson [82], resulted in Pb blank values in 48% HF down to the 2-5 ng.kg-' levels. Kuehner [74] reported a combined impurity for 16-18 elements of 6.2 pg-kg-' in sub-boiled HC1, of 16 pg-kg-' in HClO,, of 2.3 pgakg-' in HNO,, of 28 pgekg-' in H2S0,, of 17 pug-kg-' in HF and of 0.5 pg-kg-' in H20, while reagent grade and commercial pure acids displayed orders of magnitude more impurities. This was achieved at the NBS laboratories by sub-boiling purification in Quartz and Silica stills and NBS home-design Teflon units, under clean conditions. In their 1976 monograph, Zief and Mitchell [ 1621 summarize the existing techniques of purification of acids and water. They mention sub-boiling distillation, isopiestic distillation and absorption of volatile gases in purified water as the main mechanisms. They follow Hamilton's suggestion to use ion-exchange of distilled water for the productionof pure water. In 1982, Moody and Beary reviewed 10 years of experience at NBS in the production of pure acids [91]. No principle changes are reported as compared to the pilot work at NBS of Kuehner et al. in 1972 [74], with the exception that the need for a specialized class 100 laboratory for distilling acids has been recognized. The NBS clean acid lab has been completed in 1981. The main feature of the lab is that corrosion of the environment by acidic fumes is accepted, understood and controlled; the complete laboratory is constructed from polymer or polyurethane-painted aluminum, the environment is class 100 and the air flow balanced subtly to minimize the action of acidic fumes in corroding the laboratory. Figure 3-2 shows the schematic diagram of the quartz double sub-boiling distillation unit, used for the purification of H2S04 at the NBS laboratories since 1976. In 1982, Michell reviews the past decade as far as purification techniques for trace element analysis are concerned [88,89]. He discusses a modification by Oehme and Lund [102], of the nowadays widely used Millipore Milli-Q system for

69

SAMPLING AND SAMPLE PREPARATION FEED

BomE

COLD FINGER

8 . 1.. ~I.~ ...1.

COLD FINGER

.I.l+....~~ I...

I.

I..I.I..I.I..III(I(..~I..I.

Figure 3-2. Schematic diagram of apparatus for sub-boiling distillation1911

water purification. The data for Pb, Cd and Cu presented by Oehme and Lund in the water from the modified Milli-Q system are yet one order of magnitude higher than the levels obtained one decade earlier by sub-boiling distillation [74]. The results given by Moody and Beary [91] probably still represent the state of the art in clean acids and water. Figure 3-3 visualizes the results. Recent developments in the purification of chemicals, relevant for trace element analysis, are the use of preparative HPLC for organic solvents [106], improvements in the commercially available water purification systems down to < 0.5 pg.kg-' total dissolved metals [24] and the interest in anionic impurities in mineral acids [95]. Sampling and sample preparation with minimal contamination demands proper equipment. The history of precautions against contamination via equipment is more or less similar to that of chemicals. The general trend since the sixties has been the change from glassware, porcelain and stainless steel to wherever possible the use of polymers, quartz and pure metals. Again, pioneer research has been done in a few laboratories, mostly the same who experimented with clean environments and purification of chemicals (NIST, Max Planck Institute). Already in 1955, Thiers strongly recommended the use of polyethylene as a container material for aqueous solutions of trace elements, since it is orders of magnitude purer than Pyrex glass, polystyrene, methacrylate and Tygon. In 1968, Robertson [ 1081 produces an impressive amount of data for 10 trace elements in a wide variety of container materials and structural materials. Robertson concludes, that polythylene, Teflon and Plexiglas tubing and synthetic quartz are the purest materials. Rubber, structural nylon, polyvinylchloride, borosilicate glass, wiping tissue, adhesive tape and membrane filters all contain unacceptable high


70

=

HCI

. I WO,

HCIO,

H,SO,

I+

500

4

Y

.B.

300

20

200

+.

u

3

U

100

0

Pb

TI

Ba

Te

Sn

Cd Ap Element

Sr

Se

Zn

CP Ni

Figure 3-3a. Average impurity concentration in doubly distilled acids (ng/kg)

6000

4800

3600

2400

1200

0

Fe

Ct

C8

K

Mg

Al

Element

Figure 3-3b. Average impurity concentration in doubly distilled acids (nglkg)

N8


71

amounts of trace elements. By the early and mid seventies, the exclusive use of Teflon, polyethylene and synthetic quartz were established to be the materials of choice for sample containment [37,38,65,96,107,131]. In 1976, Zief and Mitchell review the state of the art in sample containment material [162]. Polytetrafluoroethylene (Teflon PTFE and Tefzel ETFE), fluorinated ethylenepropylene (Teflon FEP), polychlorotriRuoroethylene (Kel-F), conventional polyethylene (CPE), synthetic quartz, pyrolytic carbon, high purity platinum and aluminium are mentioned as clean materials from a trace element point of view. The choice of these materials does not appear to have changed drastically in the last decades. Tschdpel et al. [132,133] draw some attention on the use of glassy carbon for simple laboratory ware such as beakers, while Hoffmann [44] et al. report on contamination by different polymer foils and quartz in the trace element analysis of purified water. They conclude that cleaning procedures for foils and quartz can be appropriate, but airborne contamination can only be prevented by experimenting in a clean box. In an extensive study on cleaning of polymer containers, Moody and Lindstrom [90] conclude that the various available polyfluorocarbons (PTFE, ETFE, FEP, PFA [perfluoroalkoxy]) and conventional polyethylene, have been found to be the least contaminating materials for storage, provided they have been cleaned properly. These authors recommend a combined HCl/HNO,/H,O cleaning procedure, which is reproduced in Table 3-3. A comparison of cleaning methods for polyethylene, prior to the determination of trace elements in freshwater samples, leads to a much simpler procedure 1751. These authors recommend a 48 h soak with 10% HNO, for both preliminary and routine cleaning to allow the collection and storage of freshwater prior to analysis of Zn, Cd, Pb and Cu. The knowledge on clean sampling-, storage- and sample preparation materials has led to many specific applications. Iyengar et al. [50] developed the brittle fracture technique for biological material, nowadays part of the well-known cryogenic homogenization procedure. A suprasil quartz knife and nylon tweezers and forceps were used to cut liver samples, which were then milled in an all Teflon mill with Teflon and quartz balls. The technique was later sophisticated by Zeisler et al. [170], who exclusively used titanium knives and Teflon PTFE, PFA and FEP containers, sheets and cryogenic homogenization equipment for preparing human liver samples for analysis. In a series of contributions, Iyengar and colleagues addressed the topic of contamination during sampling of several selected clinical specimens [52,54,56,58,111,112]. The success of the use of polyfluorocarboncompounds for containers and equipment and pure metals like Al, Ti or Pt for sampling and sample preparation can be deduced from the successful operation of the NBS EPA National Environmental Specimen Bank for human liver [ 1461, the successful preparation of a mixed human diet reference material [59,166], a codfish candidate reference material [71,103] and a bovine serum reference material [ 1671. All these materials are prepared in kg


12

Table 3-3. Suggested method for cleaning plastic containers [901

(a) Fill the container with a 1 + 1 H20/HCI (AR grade) mixture. (b) Allow to stand for one week at room temperature. Teflon should be heated to 80" C. (c) Enipty the container and rinse with distilled water. (d) Fill the container with a 1 + 1 mixture of HzO/HNO3 (AR grade).

(e) Allow to stand for one week at room temperature. Teflon should be heated to 80"C. (f) Empty the container and rinse with distilled water. (g) Fill with the purest available water.

(h) Allow to stand for several weeks. Change the water periodically to ensure continued

cleaning. (i) Rinse with the purest available water and allow to dry in a particle and fume-poor

environment.

amounts from fresh gross samples under clean conditions (air, laboratory, chemicals and equipment) and successfully certified for trace elements on or below the pg.kg-' level. An even more illustrative demonstration of the success of polymers in trace element analytical chemistry is the application of polyfluorocarbon materials in all types of pressurized wet ashing, including modern micro-wave digestion. The classical stainless steel housing is usually combined with PTFE digestion vessels, while microwave digestion is peiformed in PFA [67]. Critical reviews of Maienthal [Sl], and more recently Gillain [33], learn that the situation is identical for the analysis of trace elements in seawater and its components. Glassware, stainless steel, coating with silicone oil, tygon tubing in pumps, membrane filtration setups, etc. have all shown to generate serious contamination problems and are replaced whenever possible, by polyethylene, polyfluorocarbon and synthetic quartz. A rather new development in the large scale use of polymers is in High Performance Liquid Chromatography (HPLC). The need for less oxygen permeability, higher temperature resistance and improved thread integrity compared to polyfluorocarbon tubing and fittings has caused the introduction of polyetheretherketone, PEEK, into HPLC devices. A comparison of different polymers with regards to solvent compatibility and thread integrity suggests better characteristics for PEEK with the exception of chemical resistance to concentrated inorganic acids [ 1641.


73

It should be noted that, although polyfluorocarbons are relatively clean towards contamination by leaching, the material is not inert to mechanical decomposition. Large amounts of F, stemming from PTFE particles, were found in human liver and total human diet samples which were cryogenically crushed using Teflon PTFE plungers [lsl]. Here, not just the (lack of) transfer of trace element from the polymer to the sample is important for contamination control, but also the trace element content of the plastic itself. In 1987, Versieck et al. [ 1411 published a paper which took away the confusion on natural levels of trace elements in human serum or plasma. In this paper, he describes the preparation of a “second generation reference material,”being human serum to be certified for some of the so-called “difficult” trace elements such as Cr, Co, Mo, Mn, Al, Ni, Hg and As, present in serum around or below the pgskg-’ level. Figure 34 gives the trace element composition of Versieck’s serum compared to updated

=

I I1yeng8r.r

Versiook’r

mrnm

ref oroooo

A1 Cr

ME U

E!

Ei

co Ni A8

Mo C1

cs HS 0.0 0

0.40

0.s 0

1.20

1.60

2.00

Content In ng/mL

Figure 3-4. Comparison of Versieck’s serum to Iyengar’s reference values for serum

reference values for trace elements in human serum [60,142]. Versieck describes the successful large-scale application of sampling human serum by clean techniques, developed throughout the years. The critical part is the use of polyethylene or Teflon intraveneous catheters for venepunctures and Spectrosil, Suprasil or Teflon containers to collect blood. Some of the more recent publications [ 135-1401 amply illustrate, that only this type of utmost care in sampling and sample preparation under extremely clean conditions generates unbiased results for trace elements in serum. It also means, that numerous data published in the literature on normal or pathological values of trace elements in human serum can be considered as analytically inadequate. This statement was confirmed by an attempt to establish

74

J.R.W. WOITIlEZ AND J.E. SLOOF

trace element reference values from literature data for six different human clinical samples [60]. For the elements F, I, Ni, Al, B, Br, Cs, Li, Rb, U and V, it was not possible to reach meaningful median values. 3.2.1.2 Losses Element specific losses during sampling and sample preparation can be divided into losses due to evupoi*utioii,ubsorptioii und (co)precipitutioa. It is obvious that a major source of element specific losses in the analytical procedure takes place during chemical manipulation in the preparation step directly prior to detection (preconcentration, extraction, electroplating and stripping, chromatographic separation, hydride generation, etc.). Only those techniques using a yield determination for every element in each sample (IDMS and RNAA) can escape this source of negative systematic enor. Further discussion of this item is beyond the scope of this chapter. It is disputed in detail elsewhere [ 150,1561. Losses of trace elements via evuporutiori is most likely to occur during the ashing step. A discussion on methods of ashing is given in Section 3.5. Elements may volatilize as oxides, hydrides, halides, organometal compounds, etc. Those techniques most vulnerable to losses are low pressure wet ashing, such as variations of the H,SO,/H,O, (Kjeldahl) and the HNO,/HClO, (Bethge) procedures and open high temperature dry ashing. Losses of Hg, As, Ge, Sn, Se and Sb as halides, Cr, Os, Ru as oxides or halogenated oxides have been reported during wet ashing. In dry ashing, at elevated temperatures, also Pb, Cd, Cu, Co, Ni, V, Fe and Zn may be lost, mainly due to volatilization of the halides. High pressure wet ashing, including microwave destruction and recently developed dry ashing techniques, such as Trace-0-Mat and Low Temperature Asher, appear to overcome this problem [ 1531. Another part of the analytical procedure where the risk for losses by evaporation is obvious, is drying of a sample or evaporation to dryness of an aqueous solution or wet-ashed residue. Volatile chlorides of selected elements are bound to be lost from chloride rich environments (HC1, HCIO,) at elevated temperatures [ 1331. Iyengar [51,53] reports on tissue specific losses between 2 and 15% for Hg, Se, I, Sb, Sn, Ce, Mn and Sc during oven-drying at temperatures of 80, 105 and 120"C. Lyophilization, or freeze-drying appears to be the better approach to prevent losses during dehydration. However, results are strongly dependent on the sample matrix studied and the chemical form(s) in which the element is present in the sample. Iyengar et al. [51,53,55] report no losses for Sb, Co, I, Hg, Se, Zn, Hg, Cs, Ce, Mn, Sc, Ag, Cr(V1) and Sn after freeze-drying of 10 different rat tissues. Only Cr(II1) is lost for about 7% from skin tissue. Hg is lost for upto 50% through evaporation from acidified aqueous solutions simply by storing the solution sealed in polyethylene at room temperature for 5 weeks [41]. Hg has been reported to be lost during freeze-drying for upto 10% as methylmercury and inorganic mercury from blood [76], for 8-71% as methyl mercury and inorganic mercury from biological


75

material and sediments [77] and for 100% from aqueous solution [16]. Also Se is reported to be lost during lyophilization, depending on the sample matrix and chemical form [20,31,32,41]. There is consensus in literature, that there is not one selected drying procedure that a priori can be considered to be free of losses. Furthermore, the extent of loss is definitely matrix, element and chemical form-dependent. The best way to check on possible losses due to drying, is to analyze the trace element of interest before and after the drying procedure. The use of stable or radioactive tracers is only allowed when the tracer has been metabolized by the sample under study, i.e. all the chemical forms in which the trace element occurs in the sample should be proportionally represented by the added tracer. The extent of absorption of a trace element by a container wall is dependent on pH, composition and ionic strength of salts, trace element under study, its chemical form(s), its concentration, contact time, ratio surface of container over volume of solution, temperature, type of container material and cleaning procedure. Again, there is not one specific procedure which can claim to prevent losses through absorption, but there are guidelines to be followed to minimize the effect. These guidelines are constructed and summarizedhere, based on several review and topical papers [20,26,41,81,83,93,112,119,126,127,131-133,162,163]: (a) The shorter the contact time, the better. This means fresh diluted solutions such as animal blood, urine, milk or seawater, fresh water and rainwater should preferably not be stored, but analyzed on the spot. When required, the trace element(s) of interest should be separated and transferred to a solid chemical form (pre-separated immediately);

(b) If storage is inevitable, than in thoroughly cleaned (cf. cquipmertt) polyfluorocarbons or conventional polyethylene. Laboratory glassware should be avoided for contamination reasons. The ratio inner container surface over sample volume should be as low as possible; (c) If a sample solution is collected in the storage container, it should (if possible) be stored frozen or at least refrigerated. Kinetics of absorption equilibria are slowed down by decreasing temperature; (d) If refrigeration is not possible, the solution has to be acidified to at least pH 1. This has to be done with either sub-boiling distilled HC1 or HNO,. Some researchers advise pre-equilibration of the container with the sample matrix prior to real sample storage, so as to occupy absorption sites on the container wall; (e) The time span between sample storage and analysis has to be minimized.

76


There are many comments thinkable against this type of procedure. Freezing and defreezing may change the sample composition of biological material (denaturation of proteins) or seawater (changing salinity). Cleaning procedures may etch the container’s surface and increase the number of absorption sites. Acidification may transform a trace elemental compound into an insoluble chloride or volatile hydride. It may also disturb the equilibrium between the amount of trace element bound to suspended matter and that in true solution or act as a denaturating agent. The laboratory may not have the capacity in containers, personnel or detection devices to avoid storage of samples. Finally, it may simply not be possible to avoid sample containment, transport and storage, e.g. as for in hospitalized patients or a midoceanic monitoring campaign. Basically, these limitations should all be considered in the pre-sampling plan.

(Co)precipitation can be considered as a special form of absorption. Colloidal elemental hydroxides, phosphates, sulfates or organic material or suspended particles, etc. present in the sample or formed after the addition of a preservative or acid or after defrosting, may (co)precipitate or sedimentate during sample storage. The only way out for a quantitative trace element analysis for such a sample is complete mineralization of the total sample.

3.22 Saniple speciJic changes Some of the sample speciJic changes are trivial. One could consider spillage, sputtering during ashing, formation of a precipitate during storage, etc. as trivial changes; it needs no further comments. Segregation of particles of different particle size often occurs in geological or freeze-dried and homogenized biological samples during long-term storage. Re-homogenization by mechanical shaking is mandatory here [141. The problem has been elegantly handled by BCR, which provides some of its certified reference materials in bottles containing a Teflon ball. The bottle has to be shaken manually for 2 min before it is allowed to be opened [7].Disintegration of biological samples via enzymatic or bacteriological action can be avoided by immediate (freeze)drying and sterilization [93]. Iyengar [56] has made a detailed study on sample changes for human or animal clinical specimens and mentions, e.g. cell-swelling, haemolysis, medication, sub-clinical conditions, humidity, freezing (and defrosting) as threatening for the integrity of the sample. Drying and mineralization are by far the most drastic actions which cause sample specific changes. Although it may seem a trivial discussion, since drying and ashing are meant to change the sample, a few notes can be made. In a solid, but humid material it is best to determine the moisture content of a sample on a separate aliquot. When this is not possible, as in the case of a small, unique sample, the method of preference for drying is lyophilization. A critical point here is that the freeze-dried material may be extraordinary hygroscopic. Pauwels et al. [ 1031 states that only a


77

conditioned atmospheric environment with low residual humidity allowed control of the moisture content of lyophilized codfish powder. Earlier, a rapid uptake of moisture from humid air by lyophilized human serum and NBS SRM 1577 “Bovine Liver” has been reported [ 1471. Within 100 min, 90 mg of dried serum gained 20% of its weight by moisture uptake from air with a relative humidity of 82%. The very contact of the sample with laboratory air at the moment the underpressure in the freeze-dryer is annulled means absorption of water and can only be controlled and standardized by air, which is conditioned in humidity. Another change in sample matrix caused by oven-drying at higher temperatures, may be via chemical reaction. Decomposition,oxidation or volatilisation of organic compounds such as ethers can be detected by the analyst’seye and nose. Sometimes sample changes are less easy to discover. In an attempt to convert a slurry of CaS04.2H20(dihydrate) in water to dry gypsum and subsequently determine natural radioactivity, a polyethylene bottle containing the slurry was dried in an oven at 70”C for 48 h. The Ca determination in the dry gypsum, however, revealed the presence of CaS04-1/2H,O (hemihydrate) rather than the dihydrate, inducing a systematic error in the determination of the water content of the slurry [157]. The conversion of calcium sulfate dihydrate to hemihydrate is known to occur at 125’ C, but apparently also happens, though slower, at lower temperature. As already mentioned, addition of chemicals for preservation purposes is to be considered a drastic change of the sample composition. It may lead to unexpected consequences. Addition of acid to a sample induces a higher degree of protonation of inorganic and organic anions, leading to changes in volatility (e.g. HI, HE HCl compared to the corresponding sodium salts) or solubility of organic complexing agents. Adjustment of the pH of a solution by addition of HC1 or NH40H may lead to precipitation of chlorides or hydroxides. Leaching of suspended material and destruction of the quartinary structure of proteins in biomedical fluids has already been mentioned.

3.3 Pre-sampling considerations Most laboratories active in the field of trace element analysis and operating on a routine base, agree to analyze samples “as received.” If the analyst is not involved (or consulted) in the sampling procedure, it means that the responsibility for the interpretation of the analytical result is denied by the analyst. If sampling has taken place in cooperation with and in accordance to the guidelines of the analyst, “as received” analysis may be an acceptable practical procedure. An alternative approach is, that the analyst, or well trained coworkers, perform the sampling themselves. In this case, the sampling step should be part of a sampling protocol, based on knowledge of and experience in the required trace element analysis. The need to design a protocol is explicitly stressed by several authors, [26,40,72,1123.

78


In theory, the construction of a sampling protocol should be done prior to execution of any part of the analytical procedure. This implies that a pre-sampling plan, being a critical a priori evaluation of the sense of the analytical contribution in a multidisciplinaryresearch project, has to be assembled. In practice, a pre-sampling plan may consist of a checklist of questions as suggested in Table 3-4. This list asks for the active participation of the analyst in a project before samples actually reach the analytical laboratory. The need for an accurate definition of the actual research topic (question 1) is vital. It may look like an obvious statement, but research aims, clearly and explicitly explained in advance, are rare. An elucidating paper by Cornelis [ 181 entitled: “Trace elements in hair; failure of a mission” illustrates the limited significance of elemental analysis of human hair and clearly demonstrates how much analytical effort is based on insufficient definition of the research goals. The exclusive derivation of the analytical task in the project (question 2) is of no less importance. An example here may be the distinction between wet and dry atmospheric deposition for a proper interpretation of trace element patterns in rain water [66]. Sometimes, the analytical task cannot be performed because analytical science is not yet ready for this type of problem (question 3). This has long been the case for the analysis of trace elements in human serum [60] and is now the case for the analysis of metal-ligand complexes in environmental and biomedical systems 11551. Elemental analysis on trace (< 100 pgakg-’) and ultra-trace level (< 1 ,ug.kg-’) requires special expertise, experience and equipment (question 4). Clean room facilities, for example, are slowly being introduced in trace element laboratories for a number of reasons [85], while their need in obtaining accurate results in environmental and biomedical analysis is firmly established [93]. The question of sample definition (question 5) is often recognized but seldom appreciated. Current terms in literature like liver biopsy (with or without arteries, veins and blood?), ecosystem, litterlayer (including or excluding animal life), atmospheric deposition (dry, wet, gaseous?) ask for a detailed, specific definition. Many studies on the kinetics of metal-uptake by microorganisms lack comparability (and thus verifiability) by the nature of the equipment, since the results for uptake-rate constants are dependent on substrate, microorganism and experimental conditions [94,122]. The possibility of drawing this well defined sample (question 5) is an extension of the sample definition problem. Often, practical circumstances or ethical considerations force the analyst to compromise, as in the case of human biopsy samples. The topic of sample representativity (questions 6 and 7) has been addressed in the literature 147,723. It will be discussed in further detail in the next paragraph. Knowledge about the distribution of the element in the sample (uniform or homogeneous) and thus on the necessary sample size is requisite. Question 8 refers to the well-known physical principle that no measurement on a system can be performed without disturbing the system. h trace element analysis, the risk of contamination and losses due to sampling and sample transport has been fully recognized and thoroughly


79

Table 3-4. Pre-sanipling plan in checklist format N

Question

Remarks

Has the interdisciplinary research item been adequately fomiulated?

If not, sampling and analysis will be a waste of time. Refomiulate.

Has the derived analytical task been exclusively defined?

If not, sampling and analysis will be a waste of time.

Can this task be executed considering the "state of the an" of trace element analytical chemistry?

If not, sampling and analysis will be a waste of time, unless pioneer analytical work is budgeted.

Does the laboratory have the necessary expertise. experience and infrastructure to execute the analytical task?

If not, the analyst should be assisted by expert laboratories. If time and money for this assistance are not properly budgeted, sampling and analysis will be a waste of time.

Can the object of sampling be exclusively defined and is it possible to draw this sample?

If not, sampling and analysis will be a waste of time. An alternative is to redefine the analytical task (question 2).

Is it necessary to consider the saniple to be representative for a bulk

If so, infomiation about the uniformity of the analyte in the sample and of the sample in the population has to be known or gained.

sample or a population

Is is possible to take a representative sample?

If not, conclusions on the final analysis will be restricted to the specific single sample drawn. Refomiulate the analytical task (question 2).

Can the integrity of the sample be guaranteed during sampling and sample transport?

If not, analysis will be a waste of time. Reformulate the analytical task (question 2).

80

J.R.W. WOlTTIEZ AND J.E. SLOOF

documented [ 1621. In the earlier mentioned specimen bank project, all samples are frozen to LN, temperature immediately after sampling to block all chemical and biological activity in the sample (proteolysis, cell division, infection) [29]. In the field of trace element speciation research, not much is known yet on the effects of sampling on vulnerable chemical equilibria. The pre-sampling plan can also be used to evaluate the sense of published analytical results a posteriori. Table 3-4a illustrates the results of application of the checklist to an investigation on the occurrence of protein-bound trace elements in human serum. The work was done by one of the authors [ 1471. The table illustrates that in an early phase of the project, some essential questions on the design of the experiments were not sufficiently taken into account. The definition of protein fractions and the qualitative and quantitative analysis of (at least some characteristic) proteins in these fractions has not adequately taken place. Ultimately, this has led to a very modest impact of the interpretation of the results. The essential weakness has been that the interdisciplinary character of this work has been severely underestimated. Table 3-4b shows the results of the application of the checklist to a study on the preparation of codfish candidate reference material to be certified for selected trace elements [103]. The aims of the study were explicitly made clear: to prepare a reference material with a trace element composition as close to the original natural situation as possible. Materials (seafish muscle), equipment (only titanium, Teflon and KEL-F), elements (Pb, Cd, Hg,Fe, Zn) and analytical techniques (solid sampling Zeeman AAS) were selected and tested on their suitability in advance. The factors which might jeopardize a successful completion of the work (moisture content, contamination, losses, minimal sample size, bacterial infection, etc.) were recognized, analyzed and their influence brought under control. The study was successful, since the codfish material has been subjected to a BCR certification campaign [ 1651. In speciation chemistry, the need of a sample protocol has to be even further stressed. Mutatis mutandis, the considerations on total elemental analysis are equally valid for inorganic or organic metal compounds. To understand the behaviour of trace elements in biomedical, environmental or geological compartments, it is necessary to gain insight on the chemical form(s) in which they exist in the compartment. An early example of speciation (formerly called “chemistry”) is the understanding of the behaviour of N in soils by dividing it into a nitrateand an ammonia problem. This was successful, since NH; and NO; are not easily chemically converted into each other and are thus both stable with respect to sampling [ 1231. In general, stability of the compound is the extra complicating factor in speciation analysis. Consider a compartment containing the simple elemental compound ExLywhere E is the element. The compouiid may be inorganic, as in 10, or organic, e.g. CH,HgCI. The type of chemical bonding may be ionic (Sr3(POJ2 in

81


Table 3-4a. Evaluation of a pre-sanipling plan in checklist fomiat, applied to the detemiination of protein-bound trace elements in human serum [147]. The project comprised serum sampling, protein separation by gel-filtration and neutron activation analysis (NAA) of the separated fractions N

Question

Remarks

Has the interdisciplinary research item been adequately formulated?

The approach was purely analytical. The choice to analyze serum was for practical reasons. No serious consideration of other clinical specimen has taken place.


No. Quantitative protein analysis, study of the influence of buffers, etc. have been ignored.

Can this task be executed considering the No. The applicabilityof the combination of protein separation and “state of the art” of trace element analytical chemistry? trace element analysis for human serum had to be investigated. NAA was sufficiently under control Does the laboratory have the necessary expertise, experience and infrastructure to Clean-room facilities were available. execute the analytical task? Contaniination-free sampling of serum was developed. For gel-filtration assistance was obtained.

Can the object of sampling be exclusively Whole blood and serum are well defined and is it possible to draw this defined. Serum protein fractions are sample? defined according to fractionation technique.

Is it necessary to consider the sample to be representative for a bulk sample or a population.

Yes. A blood sample is supposed to be representative for the individual. The representativity of the individuals for a population was not the item.

ISit possible to take a uniform sample?

Yes. Venepuncture with a Teflon needle had proven to generate reproducible results.

Can the integrity of the saniple be guaranteed during sampling and sample transport?

No. The effects of Lt v i m chemistry on the nietal-protein equilibrium have been subjected to limited study.

Conclusion: Results cannot be interpreted quantitatively. Very modest impact.


82

Table 3-4b. Evaluation of a pre-sampling plan in checklist format, applied to the preparation of codfish candidate reference material to be certified for Pb, Cd, Hg,Fe and Zn [ 1031. The project comprised collection of codfish base material and transforniation into a candidate reference material. Homogeneity studies were done by solid sanipling Zeeman AAS. ~~

N

~~~

~

Question

Remarks

Has the interdisciplinary research item has adequately formulated?

Yes. A fresh seafish muscle material had to be prepared and certified for Pb,Cd, Hg, Fe and Zn.


Yes. A priori,the parameters particle size distribution, moisture content, stability, contamination. losses, heterogeneity and microorganisms were perceived.

Can this task be executed considering the “state of the art” of trace element analytical chemistry?

Partially. Special cryogenic grinding equipment in Teflon had to be developed.

Does the laboratory have the necessary expertise, experience and infrastructure to execute the analytical task?

Yes. The study was executed at CBNM, Belgium. This laboratory is experienced in clean room work and accurate trace element analysis.

Can the object of sampling be exclusively defined and is it possible to draw this sample?

Yes. The sample is codfish muscle, to be obtained by filleting and removal of bones. Sampling is done by fishing.

Is it necessary to consider the sample to representative for a bulk saniple or a population.

No. Some 200 kg of codfish was caught. After preparation, this defines the entire population of candidate reference material codfish.

Is it possible to take a unifomi saniple?

Yes. The study focused on saniple preparation techniques to allow representative subsanipling of nigs.

Can the integrity of the sample be guaranteed during sampling and sample transport?

Yes, as far as trace element concentrations are concerned. No information available on vulnerable chemical fornis of trace elements.

Conclusion: Results can be interpreted quantitatively. Adequacy of sanipling and saniple preparation procedure has been proven. Material has been subjected to certificationanalyses.


83

bone) or covalent (e.g. tetraalkyltin derivatives), but many compounds in nature are some metal-ligand complex form. If the elemental compound decomposes slowly and reacts slowly with chemicals from the environment (including the laboratory equipment), compared to the time needed for sampling and analysis, the speciation analysis may be meaningful. This situation may be valid for covalently bonded organic-metal compounds. In case of ionic species or metal-ligand complexes, the compound can be expected to be in equilibrium with its environment. In a simplified dissociation equilibrium E,L, F! xE + yL, either inorganic or organic, the knowledge of the equilibrium constant li,the formation rate constant b, and the dissociation rate constant b2 are requisite. Also, not only contamination and losses of the element, but also of the complexing agent has to be taken into account. If other ligands are present, or (un)deliberately added in the laboratory, complexation of the element with this ligand has to be anticipated. Also, competition with other elements are likely. It is clear that the utmost precautions have to be taken during the sampling and the sample preparation step. Accuracy of the sampling procedure should be checked by analysis in different phases of this procedure. However, the determination of ExL, demands some kind of separation, which by its nature is a distortion of the equilibrium between E, L and ExLy.If the reaction rate constants are high compared to the time needed for analysis, the problem ends in a vicious circle. If not, a structured sampling protocol can only be designed when all interactions during the analysis are fully understood and controlled. 3.4 Aspects of sampling

This paragraph discusses aspects of sampling and subsampling. It is again based on the sampling plan concept, described by Kratochvil[72] and Taylor. The need for a proper pre-sampling plan, a basic part of the sampling plan, has been pointed out extensively. The sampling plan basically deals with the technical aspects of sampling and visualizes the expected fate of the trace element(s) during sampling and sample preparation steps. It should show the rational set of decisions, taken from the moment samples are taken until the actual trace element analysis starts. Table 35 gives a list of possible questions to be asked in order to construct a thorough sampling plan, in check-list format. The application of the list is illustrated by its application to trace element analysis of human serum by Neutron Activation Analysis. Grosso modo, a sampling plan, covers activities outside the laboratory, sampling, and activities inside the laboratory to prepare the sample for analysis, sample prepamion. The leading thread running through the sampling plan is size reduction of the sample, while maintaining its original composition. Table 3-6a shows a generalized process of size reduction of the sample population to the test portion actually analyzed and gives definitions of sample terms [72]. To illustrate the practical meaning of the terms used, Table 3-6b gives the application of the transformation scheme for the example of the preparation of a codfish candidate

J.R.W. WOIlTIEZ AND J.E. SLOOF

84

Table 3-5. Sampling plan in check-list format Stage of sampling plan

Example: trace elements in human serum [ 1471

1. Sampling What is sampled?

Human blood.

When is sampled?

Patients should be fasting.

Where is sampled?

Hospital.

How is sampled?

Venepuncture in the left ami. Patient in upright position.

Special equipment needed?

Yes. Teflon intraveneous catheter.

Who samples?

Qualified instructed nurse. Analyst present.

How many samples?

Two samples per patient. The first used for clinical analysis, the second for trace elements.

Which size per sample?

The first 3 ml, the second 10 nil.

How are samples coded?

Patients’ registration number with extension.

Is a composite saniple needed?

No.

Are samples directly contained for transport?

Yes.

Which containers are used?

25 ml Teflon tubes with PE snap-cap.

Can contamination and losses at containment be avoided?

The use of just Teflon minimizes contaniination.

Iti situ analysis necessary?

No.

111situ sample cleaning needed?

No.

Geographical and meteorological data needed?

No.

Which data are recorded and how?

Regular personal hospitalizationdata.

2. Subsampling and (re)packing

III siru subsampling needed?

Yes. Serum needs to be separated from red- and white blood cells.

85


Table 3-5.(Continued) Stage of sampling plan

Iii

situ sub(samp1ing) repacking needed?

Example: trace elements in human serum [ 1471 Yes. Separated serum needs to be contained.

Which containers are used?

15 nil Teflon containers with cap.

Are containers clean?

Yes. Cleaned by HNO3 and H20 cleaning procedure.

Can contamination and losses be avoided?

Using just Teflon materials minimizes contamination.

Are containers coded?

Yes. Both test-tubes and containers are re-used.

In sitic chemical preservation needed?

No.

Can contamination and losses be avoided?

Not applicable.

1ii sitic physical

No.

preservation needed?

Special equipment needed?

No.


For details see [ 1471.

3. 'Ransport, and sample preparation Dead-line for sample transport?

Yes. Serum needs to be transported to the lab as soon as possible.

Storage mom available?

Yes, but not applicable,

Immediate preparation needed?

Yes. Weighing and lyophilization.

Special conditions in lab needed?

Yes. Clean room absolutely necessary.

Is sample cleaning needed?

No.

Is sample division needed?

No.

Is sample reduction needed?

Lyophilization.

Is drying needed?

Ibid.

Is crushing needed?

No.

Is milling needed?

No.

Is sieving needed?

No.

J.R.W. WOITIIEZ AND J.E. SLOOF

86

Table 3-5. (Continued) Stage of sampling plan

Example: trace elements in human serum [ I471

Is mixing needed?

Yes. The dried powder (around 1 g) is mixed with a Teflon bar in the Teflon

container.

Can contamination.losses and change in saniple composition be avoided in all these steps?

All materials are Teflon. The critical step

is lyophilization. Losses during evacuation and contamination during aeration may occur.

Is immediate analysis needed?

No.

Is the laboratory sample stable?

If stored locked at -25 C for several weeks.


For details see [ 1471.

O

reference material. In this example, 200 kg codfish is gradually transformed to hundreds of bottles, each containing 7 g of fish powder with known moisture content and particle size distribution. The sampling plan begins with the understanding of sampling errors. Following Heydorn’s definition, a sample is homogeneoics when the trace element is evenly distributed in the sample and thus the analytical result is independent of the sample size. The term represenfufivssample refers to homogeneity [72]. Homogeneity is usually only the case for the matrix components of a sample and only to be expected for minor or trace elements in a well-defined solution. A sample is uniform for some trace element when it is randomly distributed through the sample and thus the analytical result is a function of the sample size. Providers of certified reference materials (CRM)have to indicate the users of their material that drawing a sample of prescribed minimal size from a bottle induces a known inherent analytical ei-ror due to non-uniformity. They have to guarantee that this non-uniformity error is negligible on the sample size level of a bottle, i.e. the uniform distribution has approached the even distribution for bottle sample sizes. This guarantee is vital, since it means that all bottles can be regarded identical with respect to the trace elements certified. From an analytical point of view, the discussion on sampling errors can best be constructed starting from the analytical measurement up, ending with the interpretation of results for the target population. The sequence of actions the analyst has to undertake is the establishment of analytical control, estimation of the sampling error by analyzing a test portion from the laboratory sample (test portion error),

87


Table 3-6a. Transfoniiationof a population to a sample to be analyzed through sample size reduction Sample size

Definition

Assumptions and remarks

Actions

Target population

Population to which the interpretation of the analytical result is intended to be extended to.

To be addressed when restricted in size and characteristics. Otherwise hard or impossible to define.

Restraint in definition.

Part of the target population which is sampled.

Parent population is representative for or identical to target population.

Careful sampling obliged to relate to target population.

May be representative, random1y selected, composite or systematic.

Size reduction required with maintenance of chemical integrity of trace elements of interest.

Aliquots or aliquands of the gross sample.

The homogenized gross sample is uniform for the elements of interest. No contamination or losses occurred.

Random sampling and analysis has to take place in all stages of sample preparation.

Aliquots or aliquands of the subsaniple.

The homogenized subsaniple is uniform for the elements of interest. No contaniination or losses occurred.

Random sampling has taken place to confirm uniformity.

Aliquots or aliquands of the laboratory sample, used for elemental analysis.

The homogenized laboratory sample is uniform for the elements of interest on the weight level of the test portion.

Sampling error has to be established as well as the number of samples to be analyzed.

1

Parent population

1 Gross samples Actual samples in some kind of relation with the parent population. 1.

Subsamples

J.

Laboratory samples

1

Test portions


88

Table 3-6b. Sample size reduction for BCR candidate reference material codfish Sample size

Remarks

Assumptions

Actions

Muscle of the population of codfish in the North Sea.

None. The final material does not pretend to be representative for the target population.

None.

Muscle of 200 kg of codfish, caught in the North Sea.

None. Parent population and target population are identical.

None.

No Pb, Cd, Hg. Fe and Zn has entered or left the material other than with loss of sample.

Jaw crushing, ball milling, freeze drying, sieving and mixing with minimal losses or contamination.

1232 bottles of each 7 g codfish muscle powder.

The unifomi and known distributionof Pb, Cd, Hg, Fe and 2% in the 9.3 kg powder remains unchanged by bottling.

Random sampling and analysis has taken place in all stages of saniple preparation.

Identical to subsample.

cf. subsample.

cf. subsample.

At least 100nig froni

Sampling error inherent to unifomiity in 100 mg sample is < 5% for each element.

Sampling error has been established and found identical for one bottle and a composite of twenty bottles.

~~

~~~

Target population

1 Parent population

1 Gross samples 72 batches of 1 kg of codfish flesh, gradually J. transformed to 9.3 kg of dry powder.

Subsamples

1

Laboratory samples

.1 Test portions

a bottle to be used

for elemental analysis.


89

estimation of the sampling error by preparing different laboratory samples from a subsample (laboratory sampling error), estimation of the sampling error caused by taking different subsamples from a gross sample (subsampling error) and defining the meaning of the gross sample with respect to the target population (gross sampling error). 3.4.1 Establishment of analytical control The first task of the analyst is to establish the sampling error by analyzing a test portion from a laboratory sample. To determine this error, his analytical technique has to be in statistical control. For most trace element analytical techniques, statistical control depends not only on elemental content or elemental mass (sample size x content), but especially on the type of sample. This means statistical control has to be proven by analysis of a certified reference material with an identical or similar matrix. This is usually referred to as matrix matching. In practice, it is often impossible to apply perfect matrix matching, and thus proof of statistical control has to be based on a compromise. One technique, neutron activation analysis (in its instrumental and radiochemical mode) is less sensitive to matrix effects and can be brought to statistical control by considering an a yriori analyticalerror [17,40,147]. Table 3-7 outlines the statistical considerations and an application to demonstrate control of precision, based on the concept of predicted or internal and observed or external variances (s; and sz). If a test quantity F, defined as the ratio of the external variance over the internal variance exceeds a critical value, and the material analyzed is guaranteed to be homogeneous,the analytical technique is out of control. It is shown in Table 3-7 that statistical control is difficult to achieve for multi-trace elements problems.

3.4.2 Sampling error iit a test yortiori Once statistical control has been demonstrated for some element in some sample matrix, the sampling error caused by drawing a test portion from the laboratory sample can be estimated. There is no use in taking a portion of the laboratory sample, if the distribution of the element is not at least expected to be random. This means that a reasonable amount of homogenization (mixing) must have been performed. Consider the analysis of some trace element in a laboratory sample, from which a number of test portions have to be drawn. The first questions to be answered are: How many test portions have to be drawn to be able to establish a mean value with a certain level of confidence and what is the minimal sample size? A practical approach is the undertaking of a pilot experiment [3,72]. From this pilot experiment a provisional average (zavg)and external standard deviation (s,) are determined. If a certain acceptable standard deviation in zavgdue to sampling analysis, savg, is set, the number of analyses necessary to obtain zaVg f savg with a certain level of confidence (usually Q = 0.05) is given by N = (t - S , / S , ~ ) ~ .

90

J.R.W. W O I T I E Z AND J.E. SLOOF

Table 3-7. Statistical considerations on analytical control of elemental analysis by instrumental neutron activation [ 1471

In principle, the sources of statistical errors in a neutron activation analysis of a sample can be quantified. Thus, an interiiul relative variance s:(T) in the final counting result can be defined as: s

m = s:o7,T) + s:w

4-

s: Na', and SO:-

> C1-;

2. Among the ions of equal charge, the smaller the ionic radii of hydrated ions, the higher is its adsorptivity, then, (monovalent) Tl+ > Ag' > Cs' > Rb+ > K+ e HNf > Na+ > H30+ > Li', (divalent) Ra2+ > Ba2' > Pb2+ > Sr2+> Ca2+ > Ni2+ 2 Cd2+ 2 Cu2+2 Co2+2 Zn2' 2 Mg2+ 2 U02+ 2 Be2+,and (trivalent) Ac3+ > La3+ 2 Ce3' > RE (light to heavy) 2 Y3+ 2 Lu3+2 Sc3+ 1 A13+. With monovalent anion, I- > NO; > Br- > C1- > OH- > F-. On the other hand, weakly acidic cation exchanger and weakly basic anion exchanger strongly adsorb H30+and OH-, respectively. Formation of metal complexes, especially negatively charged complex anion, e.g. halides complexes or 8-quinolinol-5-sulfonicacid chelates of transitional metals, is much favorable to ion-exchange separation of metal ions by the use of strongly basic anion-exchange resins. Table 4-6. Selectivity coefficient of ions Resin

K#

Dowex 50 Monovalent cation H3O Na Li 0.6

K$, Dowex 1

1.0

1.2

NH4

K

Rb

Cs

Ag

TI

1.2

1.5

2.2

2.0

8.7

8.6

Monovalent anion OH H2PO4 HCO3 C1

CN Br

NO3

HS04

I

0.09 0.09

1.6

3.8

4.1

8.7

F

0.25

0.32

1.0

2.8

4.4.4 Practical colunin operation In the usual method, the solution is percolated through a column packed with an ion-exchange resin at a definite flow rate. During column operation, the solution is continuously exposed to new exchange resins, thus displacing the equilibrium in the required direction. This continuous exchange process will ensure an increase in selectivity between the different ions of relatively low selectivity coefficients. For simple separations, it is usual to choose conditions of the ions so that selectivity coefficients are high, but the weight distribution coefficients are less 30. This minimizes the volume of eluate for an ion, which is directly proportional to the distribution coefficient.

SEPARATION AND PRECONCENTRATIONOF TRACE ELEMENTS

133

Since commercial-grade resins contain various kinds of impurities, such as soluble organic compounds, iron and calcium derived from manufacturing processes, they should be purified by washing with acid, alkali, and methanol before use. On the other hand, prepurified resins are also available from Bio-Rad (Dowex resins) or Malinkrodt Chem. Works (Amberlite resins). These resins need only washing in distilled water before use for sufficient time to swell. These treatments are called “conditioning of resins.” For analytical purposes, resin particle sizes of 100 to 200 mesh are generally recommended. In ion-exchange separation of trace metals, the column length used is usually 10 to 20 times the diameter which is in the range of 6 to 10 mm. A resin bed 10 mm in diameter and 150 mm long is a good compromise. The purified resins are packed in a column by pouring as a water slurry. Packing should be as uniform as possible so a narrow sieve range of particle size is important. The resin must be in the column wet, and air does not enter in the resin since this would cause channeling which considerably reduces the exchange efficiency. 4.4.5 Preconcentration

Preconcentration techniques have been mainly applied to water samples such as seawater, river and lake waters, and other natural water samples. In this field, the chelating resins are much preferable than ordinary ion-exchange resins, because the former reveals higher selectivity to transition metals, but not to alkali and alkaline earth metals. A variety of chelating resins have been synthesized and used for separation and preconcentration of trace heavy metals from natural waters. A typical chelating resin, Chelex- 100 in which the iminodiacetate chelating group is incorporated to a PS-DVB resin, exhibits the ability of selective retention for many transition metals from saline media [40]. However, arguments about the quantitative preconcentration of trace metals with the resin from seawater have continued for years. Recently, it was reported that the lower metal chelating efficiency of the resin in seawater than in fresh water should be caused by the complicated speciation of heavy metals in seawater media and by the high concentration of magnesium and calcium present which act as competitors to heavy metals, and a column containing 2 g of resin in the magnesium form with a flow rate of 4 ml min-’ was suitable for preconcentration of Cd, Co, Cu, Mn, Ni, Pb and Zn in seawater after adjusting to pH 6.5 [41,42]. High exchange velocity and capacity, high selectivity, physical and chemical stability, and reusability are usually required for the chelating resins. To date, many efforts have been made to improve the specificity of the resin and techniques of the application. The functional groups capable of forming complexes and introducing into the polymer by chemical modification of the matrix have been reviewed by Suzuki et al. [43] and Kantipuli et al. [MI.

134

KIKUO TERADA

4.4.6 Ion chromatography Ion chromatography (IC) was first introduced in 1975 and since that time, the technique has grown in usage at a phenomenal rate. The reason for this is clear: IC offers the only simple, reliable and sensitive means for the simultaneous separation and determination of inorganic (and organic) ions in complex mixtures. The IC now embraces a very wide range of separation and detection methods, many of which bear little resemblance to the initial concept of ion-exchange separation coupled with conductivity detection [45,46]. The sample to be separated is introduced into the flowing eluent stream by means of an injection device inserted into the flow-path prior to the column. The detector usually contains a low volume (e.g. 4 pl) cell through which the eluent flows. The components of an ion chromatograph are shown schematically in Fig. 4-9.

Figure4-9. Components of ion chromatograph. (1) Eluent, (2) pump, (3) sample injection, (4) separation column, (5) suppressor column. (6) detector, (7) data system

The main factor which differentiates the ion-exchange materials used in IC from the conventional ion-exchangers is their ion-exchange capacity. Ion-exchange separations in IC are generally performed on ion-exchangers with low ion-exchange capacity, typically in the range 10-100 peq g-I. This characteristic can be attributed chiefly to the fact that IC was developed originally for use with conductivity detection, which introduces a preference for eluents of low background conductance. Though the detection methods currently available make it possible to use columns of much higher ion-exchange capacity, the majority of separations continue to be performed on low capacity materials. Agglomerated ion-exchange resins contain an internal core particle, to which is attached a monolayer of small-diameter particles which carry the functional groups comprising the fixed ions of the ion-exchanger. Provided the outer layer of functionalized particles is very thin, the agglomerated resin exhibits excellent

135


chromatographic performance due to the very short diffusion paths available to solute ions during the ion-exchange process. The central core particle is generally PS-DVB of moderate cross-linking, with a particle size in the range 10-30 pm. Schematic illustrations of agglomerated anion-exchange resin and analogous cationexchange resin using electrostatic binding are in Fig. 4-10(a) and (b), respectively.

Aminaled latex particles

Sulfonrted latex particles

i

(b)

(a)

Figure 4-10. Schematic representation of agglomerated(a) anion- and (b) cation-exchangers

fl (Li-Csl

Li' I

0

r L

8

12 16 20

Time (min)

(4

0

I

I

L

8

1'2

Time (rnin)

6;

-

0

2

L

6

Time (min) (b)

Figure4-11. npical chromatograms obtained with (a) surface sulfonated resins, and (b) PS-DVB based surface aminated exchanger

136

KIKUO TERADA

The suppressor is a device inserted between the chromatographic coluinn and a conductivity detector. The function of the suppressor is to reduce the background conductance of the eluent. Suppressors operate through one of the following mechanisms:

1. The most common mode of suppressor operation is one in which the eluent cations are replaced by hydrogen ions. This mode of suppression is used for anion-exchange eluents and the pertinent a reaction is represented in the following equation for an eluent made up of the sodium salt of the weak acid, HE. Suppressor - H ' + Na'E-e Suppressor - Na+ + HE The background conductance of the eluent is therefore reduced because the relatively strongly conducting ions Na' and E- are replaced by the weakly conducting species HE; 2. The opposite mechanism to that described above involves the replacement of eluent anions with hydroxide ions from the suppressor. The hydroxide introduced into the eluent may then be used to neutralize acidic eluents, or to precipitate metal ions from the eluent represented in the following equations: Suppressor - OH2Suppressor - OH-

+ H' A-= Suppressor - A- + H,O + Mz++ 2NO; e 2Suppressor - NO; + M(OH),,,,

In some cases, it is possible for both the eluent anion and cation to be removed, usually with the aid of a precipitation reaction. For example, a cation-exchange eluent formed from AgNO, can be suppressed by exchange of the eluent NO; ions for C1- using a suppressor in the C1- form. Alternatively, an anion-exchange eluent formed from NaI can be suppressed by exchange of the eluent Na' ions by Ag' using a suppressor in the Ag+ form. The conductance of an eluent can be suppressed through the use of a suitable complexation reaction which converts the eluent components into non-conducting or weakly conducting components represented as follows: Suppressor - CU'+

2 ~ + +EDTA~ e Suppressor - (I!?)'

+

+ Cu-EDTA

Typical chromatograms obtained with surface sulfonated resins and PS-DVPbased surface-aminated anion exchanger are illustrated in Figs. 4-1 1(a) and (b), respectively.

SEPARATION AND PRECONCENTRATION OF TRACE ELEMENTS

137

4.5 Separation and preconcentration by sorption 4.5.1 Introduction

The preconcentration techniques based on sorption seem to be convenient, rapid and capable of attaining high concentration factors. The sorption phenomena used for preconcentration generally include adsorption, absorption, chemical adsorption, and capillary condensation of gaseous components or dissolved substances on solid or liquid adsorbents. The efficiency of sorption at equilibrium is usually described by the distribution coefficient which is represented as a ratio of the total amount of an element in unit quantity of the sorbent (g or ml) and the total amount of the element in unit quantity of the solution or carrier gas (ml). The selectivity of sorption is given by a ratio of the distribution coefficients of two elements interested and is expressed as the selectivity coefficient. The recent development of sorption techniques for preconcentration of trace elements have been summarized in several books [3-51. 4-52 Activated carbon

It has been found that various trace metals are effectively retained on activated carbon in the presence of some complexing agents such as ethylxanthate, diethyldithiocarbamate, ammonium pyrrolidinecarbodithioate,dithizone, 8-quinolinol, and xylenol orange. Moreover, single metal ions such as mercury (11),methyl mercury, and iron (111) are also adsorbed from hydrochloric acid solution. Table 4-7 shows some examples of preconcentration with activated carbon. Generally, preconcentration of trace metals with activated carbon is carried out by one of the following two procedures:

1. The sample solution added with a suitable complexing agent is passed through a thin layer of the sorbent (50 to 150 mg) supported on a filter paper; 2. After shaking the sample solution containing a certain amount of the sorbent (50 to 150 mg) and complexing agent, the solution is filtered through a filter paper.

The metal collected on the sorbent is readily leached out with hot nitric acid for atomic absorption or induced plasma emission spectrometry, while the sorbentloaded filter can be directly submitted to the X-ray fluorescence method or neutron activation analysis.

138

KIKUO TERADA

Table 4-7. Sorption of trace metals on activated carbon Matrices

Trace metals

Complexing agents

Determination method

Water

Ag. As, Ca, Cd, Ce, Co, Cu, Dy, Fe, La, Mg, Mn, Nb, Nd, Ni, Pb, Pr, Sb, Sc, Sn, U, V, Y,Zn

8-quinolinol

SSMS, XRF

Water

Ba, Co. Cs,Eu, Mn, Zn

APCD, DDTC. PAN, 8-quinolinol

XRF

Water

Hg, Methyl mercury

-

AAS

Water

Hg (halide)

-

AAS

Water

U

L-ascorbic acid

INAA

HN03, water Ag, Bi, Cd. Cu,Hg, Pb. Zn

Dithizone

AAS

Mn, MnO2, Bi, Cd, Co, Cu, Fe, In, Ni, Mn salts Pb, TI, Zn

Ethyl xanthate

AAS

Al

Cd, Co, Cu, Ni, Pb

Thioacetamide

AAS

Ag, TIN03

Bi, Co, Cu,Fe, In, Pb

Xylenol orange

AAS

Cr salts

Ag, Bi, Cd.Co,Cu.In, Ni,

HAHDTC

AAS

DDTC

AAS

Pb, TI, Zn Se

Cd, Co, Cu,Fe, Ni, Pb, Zn

APCD: ammonium pyrmlidinecarbodithioate, DDTC diethyldithiocarbamate,HAHDTC: hexamethyleneammonium hexaethylenedithiocarbaniate, PAN: 1-(2-pyridylazo)-2naphtol, INAA: instrumental neutron activation analysis, SSMS: spark-source inass spectrometry, XRF: X-ray fluorescence analysis

45.3 Poroirs polymers 4.5.3.1 Styrene-divinylbenzene copolymers

Macroreticular polystyrene-divinylbenzene copolymers (Amberlite XAD- 1, XAD-2, and XAD-4) and more hydrophobic methyl metacrylate-based copolymers (Amberlite XAD-7 and XAD-8) have been studied for the preconcentration of trace metals as their complexes from sea- and surface-waters. Chromium (VI) ions formed a Cr (111)-diphenylcarbazone complex in aqueous solution and the complex was effectively adsorbed on XAD-2 resin in the presence of sodium chloride [47]. A XAD-4 resin was employed as a collector of silver, cadmium, cobalt, copper,


139

iron, mercury, nickel, lead, and zinc in seawater after complexation with sodium bis(2-hydroxyethy1)dithiocarbamate [48]. On the other hand, the resins impregnated with liquid extractants or complexing agents are extensively used for concentration of trace metals. Gold was concentrated on XAD-4 resin impregnated with p-dimethylaminobenzilidenerhodanine (DMABR) [49], and In(II1) and Pb(I1) were collected on XAD-2 resin impregnated with pyrocatechol violet [50]. 4.5.3.2 Polyurethane foams Braun and co-workers have extensively investigated polyurethane (PU) foam sorbents regarding the preconcentration and separation of trace metals [51]. The selective sorption and separation of Fe, Co, Hg, Zn, and In, Rh, and Ir, and Pd by unloaded PU foam were successful in acidic aqueous thiocyanate media. The foam columns are prepared by placing the foam cylinders in glass columns and applying gentle pressure with a glass rod. The columns are then filled with water under vacuum. A sample solution is allowed to percolate through the foam column at different flow rates. Various reagent-loaded open-cell PU foams have been prepared and used for preconcentration and separation of traces of inorganic and organic species of metals under static and chromatographic conditions. Since according to neutron activation analysis, ether-type PU foams contain relatively high amounts of metal contaminants, great care will be needed to use them for preconcentrating trace metals. 4.5.3.3 Poly(chlorotrifluoroethy1ene)(PCTFE) resin PCTFE resin which has been widely used as a reversed-phase chromatography column packing, was found to efficiently adsorb some soluble metal complexes in trace concentration [52]. A commercially available PCTFE, Neoflon molding powder was used for preconcentration of Cd, Cu, Fe(III), Mn(II) and Zn as their 8-quinolinol (Ox) and 8-quinolinol sulfonate (SOX)complexes, and Bismuthiol I1 (Bis-11)complexes of Cd,Cu and Ag [53]. In the case of Cu(I1)-Sox complex anion, quantitative adsorption was achieved in the presence of tetrabutylammonium cation (TBA+) as a counter ion. In addition, adsorption increased with increases in TBA+ concentration. Therefore, it is seen that an ion-associated complex can be sorbed on the resin by a hydrophobic interaction. Preconcentration of Cu(I1)-Ox complex on the resin has been successfully applied to the determination of oxine-Cu which is spread on golf links as a fungicide, in environmental water sample [54].

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45.4 Conipfex-forniirtgadsorbeti rs 4.5.4.1 Reagent-immobilized silica gel or glass beads Hill [ 5 5 ] has synthesized 8-quinolinol immobilized silica via silylation, amidization, diazotization and coupling reactions. The major use of this new chelating adsorbent has been in the preconcentration of trace metal ions from dilute aqueous samples or relatively simple separations of pairs of metal ions [56,57]. SG-8HOQ was used for the preconcentration of Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn from seawater prior to their determination by GFAAS. Some other kinds of complex-formingfunctional groups such as P-diketone [ 5 8 ] , 2,3-dihydroxy-benzoyI and 3,4,5-trihydroxy-benzoyl-amide [59] and Chelex-100, [60] have been immobilized on silica gel and studied for their sorption ability and application to preconcentration of traces of metals. 4.5.4.2 Reagent-loaded silica gel or glass beads

Silica gel seems to be excellent support material for sorbent of trace metals owing to its porosity and large surface, hydrophilic property, resistance to acids, resistance to heat, lack of swelling in various solvents, and mechanical strength. In preparation of reagent-immobilized silica gel, the silylation of the surface of the gel results in loss of its hydrophilic nature so that fast retention of metal ions cannot be attained. Therefore, several complex-formingsorbents have been prepared by simple loading water-insoluble complexing agents on silica gel [6 11. 2-Mercaptobenzothiazole-loadedsilica gel (MBT/SG) is prepared by impregnating SG with tetrahydrofurane (THF) containing MBT,drying the impregnated gel under reduced pressure, washing with pure water to remove free reagent, and drying again under reduced pressure [62]. The amount of MBT retained on SG was found to be 114 f 6 mg and retention capacity for silver was 8 1.5 pmol g-'. By the use of a MBT/SG column (10 mm i.d.), Ag, Cd,Cu(II), Hg(II), Pb, and Zn are quantitatively retained at flow rates 15, 17, 21, 23, and 21 ml min-', respectively. The metals retained on the loaded SG column are readily eluted with suitable eluents, and the metals were determined by AAS or ICP-AES. The thionalide/SG shows high selectivity for Pd(II), Sb(II1) and Bi(II1) ions at pH below 1.0. Arsenic (111) is retained at pH 2 6.5, but As(V) and organic arsenic compounds are not adsorbed; hence sequentialconcentration of As(II1) and the other arsenic species is capable. The sorption behavior of several metal ions by p-dimethylaminobenzilidenerhodanine loaded SG (DMABWSG) revealed selective adsorbability for Ag, Au (111), and Pd (11) ions at pH below 1.0, but no adsorption occurred for Cd, Cu, Fe (111), Pb and Zn at this pH. Therefore, the separation of noble metals from the latter metals is possible.


14 1

455 Natitrnl polymers

4.5.5.1 Celluloses The modified celluloses which contained 1-(2-hydroxyphenylazo)-2-naphthol (Hyphan) or 4-(2-pyridylazo)-resorcin (PAR) was used for concentrating Cd, Co, Cr, Cu, Fe, Hg, Mo, Ni, Pb, U, V, and Zn in natural waters [63]. The exchange capacity of these resins equals 0.5 mmol g-'. After passing 1 to 5 liters of water sample through a column of the sorbent, the retained trace elements were eluted with 1 M hydrochloric acid varying concentration factor between 20 and 100. Four types of dithiocarbamate-celluloses were prepared [64] by treating cellulose with p-toluene-sulfonyl chloride in pyridine with subsequent reaction of tosyl cellulose and amines (aniline, benzylamine, n-butylamine, and piperazine) in dimethylformamide and then by treating the aminocelluloses obtained with carbon disulfide in a NH3-CH,OH medium. The last cellulose showed good adsorption characteristics with relatively large capacity for Ag, Cr (VI), Cu, Hg (11). Pb, Se (IV) ranging from 9.5 to 70 mg g-' of the modified cellulose.

4.5.5.2 Chitin and chitosan Since the natural polymer chitosan, which is readily obtained by deacetylation of chitin, has been utilized as an adsorbent for collection of metal ions [65], this polymer has received attention regarding its use for the removal of toxic heavy metals or radionuclides, as well as for the recovery of useful metals from industrial sewage. On the other hand, only a few investigations on chitin regarding concentration heavy metals have been reported. Chitin has been used for the rapid concentration of Ni, Cu and Cd ions as maleonitriledithiolateanionic complexes, and also of Fe, Cu and Cr (VI)as their colored complexes with 1,lO-phenanthroline, neocuproine, and diphenylcarbazone,respectively. Recently, dithiocarbamatefunction has been introduced into an aminosaccharide chain of chitosan (DTC-chitosan) [66]. The adsorbability of DTC-chitosan has been studied for several metal ions in the pH range from 1 to 12. However, the DTC-chitosan has a tendency to gelatinize in an acidic medium so that it can hardly be used for column operation. Hence, dithiocarbamate-chitin has been prepared by mixing chitin powder with carbon disulfide, 2-propanol, and tetramethylammonium hydroxide in benzene. DTC-chitin obtained does not gelatinize even in 6 M nitric acid and shows effective adsorbability for transition metal ions from an acidic solution. Silver, copper and cadmium ions are quantitatively retained on DTCchitin from 1 M or higher acidic solution, nickel and cobalt at pH 2-3,and iron at pH 4 1671. Gold and palladium are quantitatively retained on DTC-chitin at 6 M hydrochloric acid medium [68]. The metals retained on the column are eluted with

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dilute nitric acid containing 2-395 thiourea except for palladium which is strongly retained on the polymer and eluted with 1.6-fold diluted inverse aqua regia.

References 1 Bonner, M.A. and Kahn, M., Behavior of carrier-free tracers, in: Radioactivity Applied to Chemistry, A.C. Wahl and N.A. Bonner (Eds.),Wiley, New York, 1951,p. 102.

2 Minzewski, J., Chwastowska, J. and Dybczynski, R., Separation and Preconcentration Methods in Inorganic Trace Analysis, Ellis Horwood, Chichester, 1982, 3 Mizuike, A., Enrichment Techniques for Inorganic Trace Analysis, Springer-Verlag, Berlin, 1983. 4 Zolotov, Yu. A. and Kuz’min, N.M., Preconcentration of Trace Elements, Elsevier, Amsterdam, 1990.

5 Alfassi, Z.B., Preconcentration by Coprecipitation of Trace ELements, in: Preconcentration Techniques for Trace Elements, Z.B. Alfassi and C.M. Wai (Eds.), CRC Press, Boca Raton, FL, 1992,p.33. 6 Hahn. 0.. Applied Radiochemistry, Cornell University Press, Ithaca, NY, 1936.

7 Caramella Crepsi, V., Genova, N., Meloni, S. and Oddone, M., J. Radioanal. Nucl. Chem., Art., 114 (1987)303. 8 Goshkov, V.V.. Org. Reagenty Anal. Khim., Tezisy Dokl. Vses., 2 (1976)63.

9 Vanderstappen, M.G. and Van Ccieken, R.E., Talanta, 25 (1978)653. 10 Genova, M., Caramella Crepsi, V. and Meloni, S., Radiochem. Radioanal. Lett., 58 (1983)271.

11 Tappnieyer, W.P.and Pickett, EE.. Anal. Chem., 34 (1962)1709. 12 Sebba, F.. Ion Flotation, Elsevier, Amsterdam, 1962. 13 Fukuda, K. and Mizuike, A., Bunseki Kagaku, 17 (1966)319. 14 Mizuike, A. and Hiraide, M., Pure and Applied Chem., 54 (1982)1556.

15 Hiraide, M. and Mizuike, A., Kagaku, 31 (1976)57. 16 Hiraide, M.and Mizuike, A., Bunseki Kagaku. 26 (1977)47. 17 Hiraide, M.,Ito, T., Baba, M., Kawaguchi, H. and Miuzuike, A., Anal. Chem., 52 (1980)804. 18 Zolotov, Yu. A., Boodnya, V.A. and Zagmzina, A.N.. CRC Crit. Rev. Anal. Chem., 14 (1982) 93. 19 Federson, C.J., Science, 241 (1988)536. 20 Bunzli, J.G. and Wassnef, D.. Coord. Chem. Rev., 60 (1984)191. 21 Bartsch, R.A., Solv. Extr. Ion Exch., 7 (1989)829. 22 De Jong, G.J. and Brinkman, U.A.T., Anal. Chim. Acta, 98 (1978)243. 23 Healy, T.V., J. Inorg. Nucl. Chem., 19 (1961)314. 24 Cox, E.C. and Davis, M.W., Sep. Sci. Technol., 8 (1973)205. 25 Patil, S.K., Ramasrishna, V.V. and Prakas, B.H., Sep. Sci. Technol., 15 (1980)131.


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26 Manchanda, V.K. and Chang, C.A., Anal. Chem., 58 (1986)2269. 27 Manchanda. V.K. and Chang. C.A.. Anal. Chem., 59 (813) 1987. 28 McDowell, W.J.. Scp. Sci.Technol.. 23 (1988) 1251. 29 Tang, J. and Wai. C.M., Anal. Chem.. 58 (1986)3233. 30 Naliamura, S. and Suzuki, N.. Polyhedron, 5 (1986) 1805. 31 Zhivopistsev, V.P., Ponosov, I.N., and Selezneva. I.A., Zh. Anal. Khim., 18 (1963) 1432. 32 Petrov, B.I. and Galinova, K.G., Zh. Anal. Khim., 33 (1978) 1481. 33 Murata, K. and Ikeda, S., Bunseki Kagaku, 18 (1969) 1137. 34 Kawamoto, H. and U w a , H., Chem. Lett.. No. 3 (1973) 259. 35 Calmon, J.E. and Hale, D.K., Ion Exchange, A Laboratory Manual, ButterwoRhs.London, 1957. 36 Samuelson, O., Ion Exchange Separations in Analytical Chemistry, Wiley, New York, 1963. 37 Rieman, W.,111 and Walton, H.F.. Ion Exchange in Analytical Chemistry, Pergamon Press, Oxford, 1970. 38 Marinsky, J.A. and Marcus. Y.. Ion Exchange and Solvent Extraction. Vol. 5, Deliker, New York, 1973. 39 Marcus, Y. and Kertes, AS.. Ion Exchange of Metal Complexes, Wiley, New York, 1969. 40 Riley, J.P. and Taylor. D., Anal. Chim. Acta, 40 (1968) 479. 41 Pai, S.-C., Whung, P.-Y., and Pai, R.-L., Anal. Chim. Acta, 211 (1988) 257. 42 Pai, S.-C.. Anal. Chim. Acta. 211 (1988b) 271. 43 Suzuki, T.M. and Yokoyama, T., Bunselii, (1984) 736. 44 Kantipuli, C., Katragadda, S.,Chow, A., and Gesser, H.D., Talanta, 37 (1990) 491. 45 Walton, HP.. Ionexchange chromatography. in: Chromatography, Fundamentals and Applications of Chromatographic and Electrophoretic Methods, Part A: Fundamentals and Techniques, E. Heftman (Ed.), Elsevier, Amsterdam, 1983. 46 Haddad, P.A. and Jackson, RE., Ion chromatography, principles and applications, J. Chromatogr. Library, Vo1.46, Elsevier, Amsterdam, 1990. 47 Osaki, S., Osaki, T., and Talrashima. T., Talanta, 30 (1983) 683. 48 King, J.. and Fritz. J.S., Anal. Chem., 57 (1985) 1016. 49 Chien, Chung-C., and Chang. Fu-C.. J. Chin. Chem. SOC.,Ser. 11.30 (1983) 243. 50 Brajter, K., Olbrych-Sleszynska, E., and Staskiewucz,M., manta, 35 (1988) 65. 51 Braun, T., Navratil, J.D., and Farag, A.B., Polyurethan Foam Sorbents in Separation Science

and Technology, CRC Press Boca Raton, FL, 1986. 52 Akita, C., Matsumoto, K., and Terada, K., Anal. Sci., 3 (1987) 473.

53 Yamaguchi, T., Zhang Liping, Matsumoto, K., and Terada, K., Anal. Sci., 8 (1992) 851. 54 Yamada. N., Tomita, B., Chaya, K., and Mumkami, N., Eisei Kagaku, 38 (1992) 188. 55 Hill, J.M.. J. Chromatogr.. 76 (1973) 455.

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56 Sugawafa, K.F., Weetall, H.H. and Schucker, G.D., Anal. Chem., 48 (1974)489. 57 Jezorek, J.R. and Freiser, H., Anal. Chem., 51 (1979)366.

58 Seshadri, T. and Kettrup, A., Fresenius Z. Anal. Chem., 296 (1979)247. 59 Seshadri, T.,Dietz, G. and Haupt, H.-J., Fresenius Z.Anal. Chem., 319 (1984)403. 60 Ryan, D.K. and Weber, J.H., Talanta, 32 (1985)859. 61 Terada, K., Anal. Sci., 7 (1991)187. 62 Kubota, M.. Matsumoto, K. and Terada, K., Anal. Sci., 3 (1987)45. 63 Lieser, K.H. and Gleitsmann, B.. Fresenius Z. Anal. Chem., 313 (1982)203. 64 Imai, S.,Muroi, M. and Hamaguchi, A., Anal. Chim. Acta, 113 (1980)139. 65 Muzzarelli, R.A.A., Chitin, Pergamon Press, Oxford, 1978,p. 139. 66 Muzzarelli, R.A.A., Tanfani, F., Mariotti, S., and Emanuelli, M., Carbohydr. Res., 104 (1982) 235. 67 Hase, A., Kawabata, T. and Terada, K., Anal. Sci., 6 (1990)747. 68 Terada, K. and Kawamiira, H., Anal. Sci., 7,Suppl. (1992)71.

CHAPTER 5

Determination of trace elements by atomic absorption spectrometry

ITHAMAR Z . PELLY

Department of Geology and Mineralogy. Ben Gurion University of the Negev. Beer.Sheva. Israel

Contents 5.1 5.2

5.3 5.4 5.5

5.6 5.7

5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quality of results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Standards and chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Samplepreparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Major components of the instrument . . . . . . . . . . . . . . . . . . . . . . . . . . Radiationsources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wavelength selection system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atomization by flame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrothermal atomization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydride generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrumental background corrections . . . . . . . . . . . . . . . . . . . . . . . . . . ModiEers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

146 147 148 150 152 154 155 156 159 164 172 173 178 180 183 187 188

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5.1 Introduction

Atomic absorption spectrometry is a method for elemental analysis in solution (mainly). It is very sensitive, can detect different elements (67) and can detect elements in the range of a few ppm or less (atomization by flame) or in the range of a few ppb or less (electrothermal atomization). In most cases it is not important in what molecular form the metal exists (measuring the total concentration, in all the molecular forms in the sample). The analysis of the metal can be carried out in the presence of many other elements without, usually, having to separate the analyzed element from other elements in the sample, an advantage which makes the process simpler and saves a lot of time and errors. Atomic Absorption Spectrometer (AAS) is one of the first modem commercial instruments for trace element analysis. Since its appearance it became the “working horse” of analytical laboratories. Later, other methods, namely X-Ray Fluorescence (XRF) and Inductively Coupled Plasma (ICP) spectrometry threatened its position and became relatively widespread. Still, it seems that to this day atomic absorption spectrometry in all its forms (flame, electrothermal, hydride generation and Zeeman) remains the most commonly used methods. When one scans the early literature dealing with atomic absorption analysis (as well as that dealing with the introduction of almost all other new major analytical instruments) one gets the impression that it was considered to be the magic that will solve all elemental analysis problems. From now on all one has to do is to reduce the sample to the form needed for introduction to the instrument, press the botton and collect the results. With the passage of time, when experience was gained and the problems were identified, the limitations of the method were realized and its usefulness, compared with other major methods, was determined. As a result, when one wants to buy a major instrument for elemental analysis, one has to consider (in addition to the price of the instrument) what he needs the instrument for (mostly) i.e. analysis of major elements, analysis of trace elements (and at what level - ppm or ppb), samples with a generally constant matrix (for instance water or alloys) or not, number of samples per unit time, operational expenses, etc. A word of caution. It is a common practice to talk about the detection limit and the sensitivity of the instrument, hut one should realize that these are not the only factors determining the quality of the results (precision and accuracy). This is determined equally, or even to a higher extent, by the treatment the sample received before it was analyzed by the instrument, i.e. by sampling, homogenization or splitting, weighing, dissolution or melting, etc. Extreme care should be taken in executing these operations in order to fully utilize the capabilities of the instrument. This chapter was written aiming at a reader who has only a basic knowledge of the technique (some basic undergraduate course) and no real experience in using the method. Theory, the instrument and methods will be described mainly paying

ATOMIC ABSORPTION SPECTROMETRY

147

attention to the implications to experimental work, so that a reader who wants to use AAS for analytical work would save a part of the necessary trial and error. If one is interested in enlarging his knowledge of the physics and theory relevant to the various sections, one should turn to the appropriate textbooks or journal articles (see Section 5.2). There are hundreds of specific applications of atomic absorption analysis and several thousands of references to articles in the literature concerning these applications. When one wants to carry out an analysis (or to develop a new method), unless one has specific instructions for the analysis, the best advice is to start by a thorough scan of the literature (or compiled references, see Section 5.2). This should be done either to find an existing method or to find similar cases and based on this to decide whether to use the method or to develop a new method (knowing now what the specific problems are and how they were solved by others) without wasting time to reinvent the wheel. One should remember that many analytical methods are subject to strict analytical procedures resulting from legislation (implemented by governments or the European Common Market, etc.) or recommended by various organizations. If an analytical method is required for compliance with regulations, then the relevant guidelines and procedures should be obtained and followed.

5.2 Literature Atomic absorption spectrometry is a well established analytical method, consequently, one can find a vast amount of literature dealing with various aspects of AAS. The theory of the method and a description of the various parts of the spectrometer can be found in textbooks dealing with instrumental analysis methods, for instance [1-4], or books wholly dedicated to AAS [5-71. A short description of the theory, in addition to description of important experimental aspects, can be found in manufacturers’ books [8-101 obtained with the instrument. These books, usually referred to as “cookbooks,” describe the detailed experimental conditions for the analysis of each element that can be determined by AAS in water or other materials, e.g. geological materials, biological materials, blood and serum, urine, beer, cement, alloys, food products, plastic materials, etc. Perkin-Elmer Corp. used to include a bibliography of articles dealing with AAS that were published in various scientific journals and update them from time to time. In recent years the updating of bibliography is published every 6 months in the journal Atomic Spectroscopy (published up to 1980 under the name Atomic Absorption Newsletter) together with new original articles concerning AAS methods. Sometimes, bibliographies concerned with a special subject are published in this journal, for example, a bibliography for the years 1973-1989 on the subject of “Chemical Modification in Electrothermal Atomization Atomic Absorption Spectrometry,” published in 1991.

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Varian Co. publishes articles dealing with new AAS instrumental techniques and new analytical methods for analysis of elements in different environments or i n different matrices, in a series called Varian Instruments at Work-Atomic Absorption. Analytical Chemistry has a section called Analytical Chemistry Reviews or R pages (the letter R follows the page number) which contains a chapter in which progress in AAS in the preceding two years is reviewed (there also are chapters with reviews of other analytical methods). Every alternate year the application reviews are published according to instrumentation methods (theories, instrumentation and applications). In the years in between, they are reviewed according to research fields, i.e. different methods used in the discussed field, reported in the preceding two years. Articles dealing with AAS (both theoretical and experimental aspects) can also be found in several journals, for instance, Applied Spectroscopy, Arialytica Chiniica Acta, Talanta, Fresenius’ Journal of Analytical Chemist!y (formerly Fresertius’ Zeitschrift fur Artalytische Chemie, including articles also in English), Journal of Analytical Atontic Spectrometry, Progress in Analytical Spectrontetiy whose name was changed to Spectrochintica Acta R and Aiiulytical Chemistry. In addition, paragraphs dealing with AAS methods can be found in the experimental sections of articles in other journals dedicated to other branches of science. 5.3 Quality of results

One can often see that an unexperienced analyst who wants to buy an analytical instrument is concerned with one thing only - what is the lowest concentration of the determined element the instrument can detect. One should realize that there are many factors that determine the quality of the results. One should consider factors such as the intensity of radiation from the radiation source, the fuel-oxidant composition of the flame, the matrix of the sample, the treatment the sample received before the determination by the instrument, etc. All these things determine the sensitivity, the detection limit and the reliability of the results. Detection limit- a term that shows the lowest concentration that can be measured, under favorable conditions, with a certain degree of reliability. This depends on the signal to noise ratio, the higher the noise the higher should be the signal in order to be distinguished as something above the noise - an analytical signal. One can find different definitions in the literature. Here we refer to it as the concentration in solution of an element (in pg/ml) which can be detected with a 95% certainty, i.e. a reading equal to twice the standard deviation of a series of at least ten determinations near or at blank level. When one compares numbers published by different manufacturers, one should check what their definition is and how was the experiment conducted. One can measure the signal of a solution concentrated 5-fold and divide the result by 5. This means that the experiment was


149

carried out under much more favorable conditions and had it been carried out with the right concentration, maybe nothing above noise level would have been detected. Sensitivity - a term that shows the change in absorption due to a change in concentration or, in other words, the slope of the calibration curve (absorbance against concentration). It is defined as the concentration of an element in aqueous solution (in pg./ml) which absorbs 1% (absorption) of the incident radiation. In absorbance units this means 0.0044. The sensitivity is useful for checking if the determination conditions are optimal and result in the same sensitivity described in the instrument book. One can measure the sensitivity as the concentration that results in 0.44 absorbance and divide the result by 100 (provided all this is in the linear range). For instance if for an element a concentration of 1 mg/ml is needed for 0.44 absorbance, the sensitivity is 0.01 mg/ml (or 10 pg/ml). One can use the sensitivity to determine the needed working range. If the sensitivity is 10 pg/ml than the absorbance of 1 mg/ml is 0.44 and of 2 mg/ml is 0.88 (again, provided all this is in the linear range). If one analyzes a complex material and does not obtain the absorbance expected from the sensitivity, one should prepare a pure solution of the analyzed element and check to see if the operating conditions are optimal and the published sensitivity is obtained. If the answer is positive, than the reason for the low sensitivity is probably interferencedue to the matrix and this should be dealt with. The values of the elements’ detection limits and sensitivities for the specific instrument one uses are printed in the instrument’s book or in the analytical methods book (“cookbook”) of the instrument. Usually one does not measure the sensitivity as defined. The analytical methods books or the operation programs of computerized instruments inform the user what is the concentration needed to give an absorbance of 0.2. One can calculate the expected absorbance for the concentration range of the analytes (or of the standards for the calibration curve) and check if it is really obtained. Some “cookbooks” do not state the sensitivity but the optimum working range (the concentration range that will yield 0.2-1.0 absorbance). The relevant terms for graphite furnace work are listed in Section 5.12 Detection limits and sensitivities are the results of the instrument design, the quality of its parts, and of the operational parameters. All these do not ensure a “true” result. This also depends on other factors such as how representative the sample is, grinding, splitting, weighing, dissolution, transfer by pipets, etc. The “exactness” of the results is measured by precision and accuracy. Precision - a term measuring the reproducibility of the measurements. The precision really measures how much the results differ from each other. There is a possibility that the precision is good, i.e. all the measurements of the same sample are almost identical, but, the result can be totally wrong (for instance, if the samples were weighed using a balance that has a constant error of 10%). How can one

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check the precision of the method to be used (or rather of the analytical system including how much the sample is representative, the effect of the matrix of the sample, handling of the sample, the instrument, calibration curves, the operator and his personal errors, etc.?) In atomic absorption-analysis the precision may (and should) be checked in several ways. A method to estimate the precision of a whole analytical procedure is running one unknown (after grinding and mixing) 5 times, including weighing and dissolution or melting and calculating the standard deviation. Another method is analyzing duplicates of some samples (if possible one should assign sample names and order in such a way that the operator will not know that he is running the duplicate, to avoid bias). Another method (helping in checking the change in instrument readings with time) is reading one of the solutions every few samples. Accurucy - a term that expresses the “correctness” of the measurement, or, how close a measurement is to the “true” value. Well, what is the true value and how can one determine it? After all, to know the true value is the purpose of the analysis, which means it is not known. What can the obtained measurements be compared with? The mean of several measurements of the same sample is considered to be the true result. In atomic absorption analysis, one should always check the results obtainable by the analytical procedure to be used, by analyzing certified standards exactly the way he intends to analyze his samples and comparing the results with those stated for the standards (for the types of relevant standards see Section 5.4). When a complex material is analyzed and good results are not obtained, one should prepare pure solutions of the analyzed element and check if the instrument operates under optimal conditions and the desired sensitivity and detection limit are obtained. If not, something is wrong with the instrument or the operation mode. If everything is right, then the complex material is responsible for the bad results and one should look for ways to lower the material’s interferences.

5.4 Standards and chemicals One type of standards used in atomic absorption spectrometry is chemicals (solids or solutions) used for calibration curves. It cannot be overstated that atomic absorption is a comparative technique. Consequently, the accuracy of quantitative measurements depends on the accuracy of the standards employed. If (in addition to having the right concentration) standard solutions and samples are not matched with respect to physical and chemical properties, the comparative analytical response in the flame may be different (see Section 5.14) destroying the basis for comparison. If matching cannot be obtained, the standard additions method should be used. These calibration standards used to be very pure chemical compounds (e.g. CaCO,, KCl, etc.) or metals (e.g. Ni, Cu as wires or powder) which were weighed, dissolved and diluted to obtain (master) standard solutions of known concentrations.


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One has to be careful with metals (powders, wires or foils) as once many of them are exposed to air (by opening their containers) their surface is oxidized quite easily. The next time they are to be used, if they can still be used at all, they have to be etched or “purified” in another way before use. The various “cookbooks” describe, for each element, in addition to the operational analytical parameters, also a method to prepare a standard solution. The master standard solutions, which are prepared for long time use, have to contain a high concentration of the element (usually the solutions are made to contain 1000 mg/L metal) and these master solutions are diluted to prepare the low concentration solutions needed for the calibration curves. These dilute solutions can be used the same day or for a maximum of a few days, and then fresh solutions must be prepared. Nowadays prepared elemental master standard solutions (usually 1000 mgA, or, an ampule containing a precisely defined quantity of the element, to be diluted to a specified volume) spectroscopically pure for atomic absorption analysis, can be bought from various commercial sources. There are standards for aqueous matrices or oil dissolved standards to match samples in an oil matrix, etc. In addition to being easier to use, they prevent oxidation problems and “technician errors” of weighing and dilutions. These may cause severe problems as they may be discovered (if at all) after aresearch program or a set of analyses was finished (not to say reported). Using certified standards (see the following) to check the method may help in catching such errors. Quite often such prepared standards should be preferred for calibrations. Special attention must be paid to the blank solution which is a most important standard (if not the most). This solution should contain the appropriate amounts of all the components of the sample solution, except for the analyte itself. These components can include, for example, the same water used for dilutions, acids used for dissolution, flux used for melting, organic material added to the sample (also to match physical properties), ionization suppressor, etc. All these materials may contain trace amounts of the analyzed element which should be subtracted from each measurement. Another type of standards includes materials of known composition, for checking analytical procedures. There are several kinds of this type of standards to fit the various fields using AAS, e.g. biological standards, geochemical standards. The concentrations of the elements in some standards can be determined beforehand, by preparing mixtures with the desired concentrations. For example, standardsof serum containing Cu ions, Norm - in the range of normal serum Cu concentrations and Path - in the range of pathological concentrations, or, standards containing desired concentrations of elements (organometallic compounds) in an organic oil matrix, for the analysis of metal wear in engines’ lubrication oils. A different method had to be used for geochemical standards, e.g. those prepared by the U.S. Geological Survey. These are rocks (such as granites, basalts, syanites) of which very large amounts were taken, ground very finely, mixed very thoroughly and sent to many

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laboratories for analysis by different methods. The mean of all the results (for the same element) was taken as the true result (which was changed as time passed and new results kept coming from additional laboratories or by using more sophisticated methods and instruments). Samples of the first batches are not available any longer, however, new standards of rocks or ores (e.g. different phosphates) are available from many sources, in different countries, from both institutional and commercial sources. The compiled results - working values - could be found in different sources (for instance, [111 for values of some U.S. Geological Survey standards or 1121 for a discussion of subjects such as the determination of usable values, verification of values, statistical problems and values for about 75 various rock and mineral standards obtainable from different sources). Since 1977 a special journal, Geostundurds Newsletter, is dedicated to reference samples, values and analysis methods, in addition to which a new compilation of working values is published every few years (for the last one, 272 geostandards, see [ 131). These “international” standards are quite expensive, so that “home” standards are frequently used, in addition to the certified commercial standards. A rock sample is finely ground, thoroughly mixed and analyzed (the method is checked with a certified standard). This “home” standard is then used whenever a standard is needed when a new method is developed. Only for the last check, after debugging of the method, is the certified standard used. The purity of reagents is one of the things that determine the accuracy of the analysis, especially in the case of trace element analysis and all reagents (and distilled water!) should be of the highest purity availahle (though, if one analyzes elements not in trace levels, there is no gain in using expensive spectroscopically pure acids for dissolution). These include acids for dissolution, fluxes for melting, chemicals used as ionization suppressors, releasing agents or chemical modifiers, etc. One should not forget the level of cleaning needed for labware and the laboratory itself (e.g. no dust). Special care must he attached to the quality of the water used, for dilutions and preparation of reagents, especially when analyzing trace levels. In most cases, distilled water or ordinary deionized water can be used, but sometimes the element to be analyzed has to be removed from the water by a better procedure. 5.5 Sample preparation

Sample preparation is one of the critical factors determining the quality of the analysis results. Any error in preparation will be expressed in the analysis results. Most samples cannot be analyzed as obtained and need some treatment before analysis. Samples have to be reduced to solution in order to be presented to the spectrometer and aspirated into the flame. Even if the sample is originally a solution, pretreatment may be needed, e.g. dilution, preconcentration, addition of ionization


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suppressor, extraction. In graphite furnace, sometimes analysis of solid samples is possible (e.g. parts of plants, animal tissue) in graphite cups, as drying and ashing stages may take care of the matrix. Some geological materials can be analyzed this way if the analyzed element can easily be volatilized. The problem in such cases (and that is why solid state analysis is almost not used) is that the part of the sample actually used for analysis cannot be considered to be representative of the whole sample. If in order for a rock sample to be representative, one thousand grams are taken and ground very finely, how representative will a few milligrams used in the spectrometer be. When one wants to analyze geological samples, the first step is to collect fresh (uneroded) representative samples in the field. The next step is grinding the rock sample or minerals separated from the rock using one of the several mineral separating processes. There are several grinding techniques and the grinders are made of different materials. During grinding of the rocks, a small amount of contamination comprising several elements will be transferred to the sample, depending on the material the grinder is made of and the hardness of the sample. In the analysis of trace elements this can be a problem and the grinder should be chosen also with this aspect in mind. The following is a brief listing of the more common sample preparation techniques. The reader should turn to other sources for experimental details and for other techniques. The sample may be reduced to solution in several ways (depending on the sample), such as dissolution (in organic or inorganic solvents, in an acid or in a combination of acids - including HF to decompose silicates and volatize silicon as a fluoride) or melting (with sodium carbonate, sodium hydroxide, sodium peroxide or lithium metaborate, etc. as a flux, and then acidifing). Dissolution can be carried out in a beaker (glass or teflon), in a teflon bomb (inside a metallic jacket), using a flame, an electric oven or a microwave oven (in a closed teflon bomb without a metallic jacket). Melting is usually carried out in Pt crucibles, Pt - 5 % Au crucibles (not wetted by silicate melts) or Zr crucibles (with NaOH flux silicates can be melted at relatively low temperatures without losing relatively volatile trace elements). For geological samples, sometimes two “openings” are needed, one for major elements and one for trace elements or, one in which silicon is volatilized (getting rid of an interfering matrix) and one for silicon analysis. Pretreatment of samples may include the use of chelating resins (ion exchange) and/or extraction, either to remove interfering elements or to separate the wanted elements. These methods can also be used for preconcentration of the analyzed elements from dilute solutions. Flow injection analysis system (FIAS) can be used for analysis in a closed system to diminish contamination, for on-line dilution, concentration, introducing reagents and for working with microliter samples.

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5.6 Theory

Only the bare minimum of theory will be given in this section. Readers interested in more thorough theoretical treatment of the theory and of some subjects discussed in the following sections should refer to the proper textbooks (Section 5.2). Under normal conditions, atoms are in their lowest possible energy state, the ground state. The technique is based on the absorption of energy by valence electrons of ground state atoms, by which the electrons are raised to a higher energy level (excited state). Transitions that start of end in ground state levels are resonant transitions. There are allowed transitions and forbidden transitions. In the process of absorption, atoms change from a low energy state to a higher energy state. There are also transitions between excited states (non resonant) but these are not used in atomic absorption, thus restricting the number of transitions (wavelengths in the ultraviolet and visible range) that can be used for each element. Different atoms need different energies to be excited. This energy can be supplied in many forms like flame and heating by electric current. In atomic absorption spectrometry, the energy used is radiation from a specially built light source, emitting radiation in wavelengths that the analyzed atom can absorb. The total amount of radiation absorbed depends (among other factors) on how many atoms there are to absorb it. In atomic absorption analysis the analyzed solution is aspirated into the spectrometer, turned into an aerosol and passed into a flame where the sample is dissociated into ground state atoms. Radiation from a lamp, at a wavelength that can excite the analyzed atom, is passed through the flame where it is absorbed by the analyte atoms. The radiation is measured before absorption and after absorption and the amount absorbed is proportional to the concentration of the analyte. Beer-Lambert law (a combination of Beer's law and Lambert's law) mathematically describes the absorbance of light passing through a sample solution (in atomic absorption - the atom population in the flame) as a function of the length of the optical path through the sample (length of the flame) and the concentration of the absorbing species (ground state atoms).

It = 1,e-e'c A = logIo/It = d c where A is the absorbance (or optical density), I, the incident radiation power, It the transmitted radiation power, c is absorptivity (absorption coefficient at the wavelength used for the analysis), 1 the length of the absorption path, and c the concentration of absorbing atoms. This equation means that when analyzing the same type of atom (e.g. Cu) in samples of unknown concentrations and standard solutions of known concentrations, where the absorptivity remains the same and the absorption path (length of flame) remains the same, the absorbance will be a linear function of the Cu concentration. It should be linear but it is not always


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linear throughout all the range of concentrations. Hence, one cannot rely upon mathematical calculations and analytical measurements are made using calibration curves (though work is essentially limited to the range where the calibration lines are not too curved). There is no need to know the values of E and 1 as atomic absorption is a comparative technique. One measures a set of standards of known concentrations, prepares an absorbance vs. concentration line and for each absorbance reading of samples with unknown concentrations, finds the respective concentration (in modern spectrometers this is done by the instrument’s computer). Atomic absorption spectrometers read the amount of incident light without the sample and with the sample (after absorption), compare them, and show the results in absorbance units. 5.7 Major components of the instrument

The atomic absorption spectrometer includes many components and may contain parts which are unique to a special model of a certain manufacturer, but all spectrometers include some major basic components (whose shape, mode of operation, efficiency, etc. may change from one manufacturer to another):

I. Rdiution source; Usually a hollow cathode lamp which emits light of the special wavelength needed to excite the analyzed atoms; 2. An opticd system to direct the light from the radiation source through the ground state atoms in the atomizer to the monochromator and on to the detector;

3. Groundstute atom reservoir. There are two types: (a) a device for atomization by flame, including a nebulizer which aspirates the analyzed solution and passes it to the spray chamber where it is turned into a fine aerosol and passed onto the burner where the solvent is evaporated and the analyzed compound is dissociated into ground state atoms; and (b) a device for electrothermal atomization (known also as graphite furnace, graphite tube atomizer, carbon rod atomizer or flameless atomization) which includes a specially coated graphite tube (or cup) which acts as a sample holder and is heated by an electric current passing through two carbon electrodes, thus evaporating the solvent and dissociating the analywd compound into ground state atoms;

4. Monochromator. A grating that selects the desired wavelength and passes it onto the detector;

5. Detector. A photomultiplier that translates the residual intensity of the light from the hollow cathode lamp, before and after absorption by the analyzed atoms in the flame, into an electric current and amplifies the current;

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6. Signal mrriipulation device. In modern instruments - a computer that translates the response of the detector into analytical results, calculates concentrations (compared to a calibration curve) and standard deviations (in addition to acting as a system control - selecting the desired wavelength, controlling fuel-oxidant ratios and flow rates, etc.);

7. Reudout device. In older instrument, meters, recorders, or digital readout devices were used. In modern instruments, the computer screen and printer are used. In addition to differences in the components, instruments and computers of different manufacturers differ in software - the amount of automation, special “tricks,” data handling and report managing. 5.8 Radiation sources In order to measure the absorption under conditions that will guarantee a high sensitivity, a radiation with a very narrow wavelength band, at the absorption peak, is needed. Atomic ahsorption lines are very narrow. If a relatively wide bandwidth reaches the reading device (photomultiplier), the narrow absorption line will only be a small fraction of it, resulting in a reduced sensitivity and Beer-Lambert’s law will not be obeyed. This will result in a reduced correlation with concentration, i.e. a non linear calibration curve. One could wonder why for a special single wavelength (or several individual wavelengths) a radiation source (a line source) is needed, as the simplest way would seem to be the isolation of a single wavelength form a continuum source (emitting light over a wide wavelength range) by using a monochromator. The problem is that the width of the band separated by the monochromator is wider than the width of the atomic absorption peaks (which in the flame is determined by temperature and pressure). Contrary to this, emission from a line source (operating at much lower temperatures and pressures) has a bandwidth considerably narrower than the matching absorption peak. A monochromator and a slit are used, nevertheless, in addition to the line source, in order to select the needed wavelength region from all the wavelengths that the radiation line source emits. The best source for a single sharp wavelength would be a laser source. No commercial instrument is built with a laser. The most commonly used source is the hollow cathode discharge tube or the hollow cathode lamp (HCL) as it is usually called. The HCL emits the resonative wavelength which will be absorbed by the analyte, so the HCL is specific to the metal to be analyzed (i.e. a Fe lamp for analyzing iron, a Ni lamp for analyzing nickel, etc.). The HCL is formed of a glass cylinder, one end of which is a quartz (or Pyrex, i.e. a material which does not absorb the needed emission line) window and to the other (closed) end an anode


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GLASS SHIELD 1’

/

HOLLOW CATHODE /‘ QUARTg WINDOW

, J

I

I

\

\

ANODE

Figure 5-1. A scheinatic diagram of n hollow cathode lamp

and a cathode are sealed (see Fig. 5-1). The tube is filled with a noble gas (argon or neon) at a pressure of a few torr. The anode is made of tungsten. The narrow hollow cylindrical cathode is made of, or lined with, the metal (or alloy) of the element to be analyzed and whose spectrum the lamp is to emit. For convenience and in order to save the cost of lamps, some cathodes are made of a combination of a few metals (usually 2 or 3), a possibility only where the lines used for analysis do not overlap. These multi-elemental lamps may result in the need for a better monochromator (more expensive) and may shorten the lamp’s life. Nevertheless, some multi-element lamps are very common. A potential difference is applied between the electrodes causing a current between them. Electrons leave the cathode, collide with atoms of the inert gas and ionize them. These positive ions hit the cathode and as they have enough kinetic energy, they eject atoms from the surface (sputtering). Some of these metallic atoms are in excited states (probably by collisions with the inert gas atoms or ions) and when they return to the ground state they emit the characteristic radiation. The cylindrical shape of the cathode causes the radiation to be located within the tube. When one looks at an HCL, one sees a narrow black ring around the glass cylinder. This is a very thin metal layer, evaporated onto the walls during the manufacturing process, to serve as a getter (to trap gas remains and create a better vacuum). In addition, while most of the metallic atoms return to the cathode (because of the cylindrical shape of the cathode) a part is deposited on the surface of the glass walls. A warm-up period of a few minutes is needed to stabilize lamp intensity before use. What lamp current should be used? There are several factors to be considered. If the lamp current is too small, the analytical signal will be low and require excessive amplification. In addition, the amplification will cause higher noise. Noise is inversely proportional to the square root of the current. This means that increasing current 10-fold (e.g. from 10 to 100 mA) will decrease the noise 3-fold. One may think that one could obtain better results by increasing lamp current. However, by increasing lamp current too much, the lamp’s life is shortened considerably (proportional to the square of current change, i.e. an increase from 6 to 12 mA will

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shorten lamp life 4-fold). A second factor to be considered is that above a certain current, in most cases, increasing lamp intensity will actually lower sensitivity (lower absorbance for the same current). This means that there is a reason to increase lamp current only near the detection limit, where noise may be too high. Increasing lamp current can lower sensitivity for two reasons. One is Doppler broadening of emission lines width with increasing currents. Atoms emitting the radiation in the lamp, move randomly relative to the observer (in this case the monochromator); thus the apparent emitted wavelength will be higher or lower according to the direction, and instead of a single line, a wider band will be obtained. The other affect is self absorption or self reversal. A higher current will form more ground state atoms in the cloud and these will absorb a part of the radiation from the emitting atoms. The absorption is greater at the wavelengths in the center of the broadened emission line (which, as a result, will look like a saddle) and hence a lowering of the sensitivity. The amount of this effect depends on the element and the lamp’s construction. The right current to be used for a specific lamp depends on the above-mentioned considerations and on the specifics of the lamp’s construction. Unless there is a special need to change lamp current, it is best to use the current recommended by the manufacturer in the “cookbook”, in the certificate arriving with the lamp, or the default value in the computer controlled instruments. For Varian Co. lamps, this is usually 5-10 mA and for Perkin-Elmer Corp. lamps, 10-20 mA, respectively. Modern instruments have a turret for 4 or 8 lamps. The turret can be rotated manually or automatically (in computer controlled instruments). This has two benefits: (1) one can warm-up the next lamps to be used while still using the current lamp and save time; and (2) in computer-controlled instruments, this enables automatic consecutive analysis of many elements according to a prearranged program. The instrument recognizes the lamps, turns them on for warm-up (with the recommended or other pre-determined current), turns the turret to bring the needed lamp to the right position in the optical system, and turns off the lamps. A word of caution - this automatic analysis can be run without the operator being present, for instance, at night, gaining working hours, or during the day, saving operator time. While this can work well with a graphite furnace, it may be dangerous to leave a flame burning without supervision! A different type of radiation source, sometimes used in atomic absorption analysis, is the electrodeless discharge lamp (EDL). These lamps are energized by a microwave or radio frequency radiation. The lamp is a quartz tube containing a small amount of the metal (or its salt) of interest and filled with inert gas (at low pressure) which is ionized and accelerated by the field until it has enough energy to excite the metal atoms. These lamps emit a much more intense radiation than the HCLs. They were claimed to be more sensitive than the HCLs but less reliable.


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EDLs for several metals are commercially available. These EDLs need a special control unit. A different type of lamp used in atomic absorption analysis is the deuterium lamp, which emits a continuous radiation in the ultraviolet region. This lamp is used for background correction - correction of interferences caused by absorption (or scattering) of particles other than the analyzed element. As the output of radiation in the visible zone is low, this lamp is used for elements whose analytical liens have wavelengths below 350 nm. The use of this lamp for background correction will be described in Section 5.14. The construction of a deuterium lamp is different from that of the HCL. It is filled with deuterium, has a metal anode, the cathode is an electron emitting themionic cathode and there is a restrictive aperture between them, causing an area of high excitation and high light emission. This lamp is used simultaneously with the element’s HCL. In former instruments, a very careful tedious alignment of the two was needed, otherwise, an erroneous correction resulted as the gaseous system (flame or arc) is very unhomogeneous. In modern instruments, this alignment is very simpel. In computer-controlled instruments, the current for the deuterium lamp and the ratio of intensities of the two lamps are automatically controlled.

5.9 Wavelength selection system The first step of isolating a specific wavelength is taken by using a line source, i.e. a hollow cathode lamp emitting only a small number of wavelengths. A wavelength selection system (a monochromator) is needed to isolate a very narrow emission line from all the emitted wavelengths and produce a beam of radiation of high spectral purity, the wavelength of which can be varied at will, over a wide range. The main components of such a system are: an entrance slit, a collimating mirror (causing the beam to travel as parallel rays), a dispersive device (separating the polychromatic light into the monochromatic components), a focusing mirror and an exit slit (Fig. 5-2). There are several types of monochromator system arrangements (referred to in the literature as mounts, e.g. [23: the Littrow, Ebert and Czerny-Turner. The schematic diagram of the last one is shown in Fig. 5-2. They differ from one another in compactness, number of mirrors used, the use of prisms and focus arrangements, GtC. The light emitted from the exit slit contains unwanted radiation scattered from different internal parts of the instrument (to avoid this, some parts are painted black) and from dust particles inside the instrument. In order to avoid this (and damage), the whole monochromator is a sealed unit with quartz windows for entrance and exit of light.

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SLIT

1 MIRROR

GRATING

ENTRANCE SLIT

T

Figure 5-2. A schematic diagram of a monochromator (Czerny-Turner type)

A slit consists of two exactly parallel metal jaws, carefully machined to have very sharp edges. In some instruments there are several slits with a fixed aperture and the operator has to use the one which is best for the analyzed element. In other instruments the distance between jaws can be varied as needed. There are two types: one in which one jaw is fixed and the other moves (which means that the center of the slit moves with the change of width). In the second type (more common) both jaws move relative to each other and the center remains fixed. The physical distance between the jaws is called the mechanical slit width. Another term used is the spectral slit width (or spectral bandwidth) which is the range of wavelengths (polychromatic light) that passes through the slit for a given monochromator setting (or, the span of monochromator settings needed to move the image of the entrance slit across the exit slit [seethe following]). The entrance slit acts virtually as a light source to the system (passing on the light arriving from the hollow cathode lamp). If a lens or a mirror (which is usually used in the commercial instruments) is placed after the slit in such a way that its distance from the entrance slit equals its focal length, the light arriving from the slit will be collimated - passed on as parallel rays. If a second lens or mirror is placed before the exit slit in such a way that its distance from the slit equals its focal length, the parallel rays arriving from the first mirror will be focused on the focal plane the surface containing the exit slit. The rectangular image of the first slit - a bright line, hence the term “emission line,” will be formed on the surface containing the second slit. The system is arranged in such a way that the image will be formed on the exit slit and the light passed on from the slit to the detector. Usually the exit slit is adjusted to the dimensions of the entrance slit. The dispersive unit (in former instruments, a prism and in modern instruments a grating) is located between these two slit-mirror systems. The grating resolves the light into separate wavelengths so that several rectangular images of the entrance slit, one for each wavelength, will be


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formed on the focal surface of the second mirror, the surface which contains the exit slit. By rotating the grating, the image formed by the desired wavelength is brought to overlap the exit slit and will be passed on to the detector. By rotating the grating, any one of the separated wavelengths can be selected, by moving the slit image it forms, so that it overlaps the exit slit. In computerized instruments, the grating is rotated automatically so that the prescribed wavelength reaches the slit and after that, a careful scan (called peaking), in small increments, is carried out to locate the exact point of the emission line’s peak. One of the reasons a grating is preferred to a prism is that the spectrum produced by a grating is linear (i.e. it has a linear dispersion, an equal length for the same range of wavelengths, i.e. and equal length for a 100nm range throughout the whole uv and visible spectrum) while that of a prism is not. As a result, in order to obtain a linear change of wavelengths reaching the detector, a grating has to be rotated in a linear way while the prism should be rotated in a non linear way, which is much more complicated and expensive. If the monochromator passes on the desired wavelength A, the image should exactly fill the exit slit. If the slit is wide, part of the adjacent images will also pass the slit. The choice of the slit width is acompromise. A wide slit is needed to increase the light throughput to the monochromator, to increase signal-to-noise ratio. On the other hand, good separation of wavelengths will be obtained with narrow slits. If the separation is not good, the calibration line will become curved (Section 5.8). The optimal width of the slit (for each element) will be determined by the existence of close lines and their distance from the desired line. In the “cookbooks” the width for each element is recommended, usually between 0.1-1.0 nm. In computerized instruments, a default value of a recommended slit width is automatically used (it can be changed by the operator if desired and should be changed if warranted). If monochromatic light passes through a narrow slit, the slit acts as a light source and the light, after passing through the slit, instead of forming the image of the slit, will form a diffraction pattern which consists of a central bright band bordered by alternating dark and bright bands of decreasing intensity and quite diffused borders. As the number of slits is increased, interference occurs and the central and subsidiary bright bands are split into several sharp bright lines with most of the intensity in the central lines. The greater the number of slits, the sharper, narrower and brighter are the illuminated lines. A grating is essentially an array of a great number of slits. The first gratings consisted of fine wires stretched across a frame, acting as a series of slits. Today a grating is a series of parallel equidistant straight lines (grooves; for the uv and visible range, usually 1200-2000/mm, but the number may be lower or higher) cut into a plane surface (coated with highly reflective aluminum) each of which acts as a slit a light source. Each groove in the grating gives rise to a diffracted beam and these

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diffracted beams then interfere with one another to produce the final pattern. There are two types of gratings, transmission gratings (where light falls on one side of the grating and is emitted as separated wavelengths on the other side) and reflecting grating (where incidence and dispersion occur at the same side of the grating, the lines are actually ruled on a mirror). In modern instruments, reflection gratings are used. The dependence of grating dispersion on interference means that there are angles in which the diffracted rays are destroyed (dark lines) and angles in which the rays are reinforced. The condition for reinforcement is described by the grating equation IZA = &in i f sin 8) where A is the wavelength of the radiation, d is the distance between grooves which is the reciprocal of the number of grooves per unit distance, a the angle between the normal to the surface and the line of incidence of the light beam, 8 is the angle between the normal to the surface and the line of dispersion of wavelength A, and n.is a whole number called the order. Incident light may be diffracted on either side of the normal. When i and 0 are on the same side of the normal, the + sign applies and when they are on opposite sides, the - sign applies. Zero order, 11 = 0, corresponds to the direct reflection ray (reflection from a mirror) which will occur at an angle of -i, i.e. an angle equal to i on the other side of the normal to the surface. It does not depend on the wavelength and all wavelengths in the incident beam will be included in the zero order ray. For different wavelengths, the angle of dispersion 8 is different. This means that a beam of polychromatic light will be dispersed into the monochromatic components because each wavelength will be dispersed at a different angle. A beam of polychromatic light falling upon the grating will be dispersed into a series of spectra located symmetrically on each side of the zero order position, the closest spectrum is the first order, the next the second order, etc. Each order consists of a whole spectrum, in which the shorter wavelengths are at the smaller angles. The equation shows that there will be overlapping lines, for instance, wavelengths 4000 A of the first order, 2000 A of the second order and 1000 A of the fourth order will be dispersed at the same angle, as they have the same value of n X . Master gratings are prepared by ruling the lines on a hard polished surface with the aid of a properly shaped diamond tool (having elaborate controls to ensure the necessary precision). From this master replica, gratings are prepared by applying a layer of parting agent to the master, a layer on which a film of aluminum is deposited. To this film a glass or quartz base is attached with a suitable cement and the replica is separated from the master. In modern instruments the so-called holographic grating is used. The grating is obtained by using laser technology to form the grooved surface. Much better line shapes, line dimensions, a much higher groove density and bigger gratings can be formed, thus causing purer spectral lines, at a relatively low cost. The quality of a monochromator is measured by the purity of the radiation it

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passes on, in other words, by the ability to resolve adjacent wavelengths and by the intensity of the radiation passed on. Resolution is the smallest wavelength interval that a grating is capable of isolating. Resolving power is a measure of the ability to separate one wavelength from another, nearly identical, wavelength, or, the ability to distinguish between two adjacent slit images having a small wavelength difference. Some books do not distinguish between these two terms. The resolving power R of a grating can be determined in two ways: (1) by the equation R = n N where n is the order and N is the total number of grooves. It is not the density of lines which determines R but their total number, i.e. a 5 cm, 1200 lines/cm grating will have in the 3rd order the same resolving power as a 10 cm, 1800 lines/cm grating in the first order. One should note that it is not the total numbers of grooves on the grating which counts, but rather, the total number of grooves upon which light falls by the optic system, a difference which may give room to false claims of high R. The dependence upon N enables working with bigger, coarse gratings having a lower groove density (easier to manufacture and cheaper). (2) By the equation R = A A where is the mean wavelength of the two lines to be resolved and A A is the difference in wavelength of the two lines that can just be distinguished as two lines. These two equations enable calculating the resolving power, and hence the combination of order - number of grooves needed to separate two lines (e.g. to just separate the sodium doublet lines 5890 and 5896 A - the resolving power needed is 5893/6 = 982 which is only about 1% of obtainable values) or, to calculate if a given n. and N can resolve two given wavelengths. Dispersion is the separation of polychromatic light into the monochromatic components. The effectiveness can be described in different terms: (1) angular dispersion - the change in angle of dispersion from the grating - 8 - per unit wavelength, i.e. do/ dA (radians/A). The greater it is the better the separation; (2) linear dispersion - the lateral separation of the slit images per unit wavelength, i.e. dZ/ dX (mm/&. The greater it is, the better the separation; (3) reciprocal linear dispersion dA/ d l (hmrn). The smaller it is (less images/mm), the better the separation. The last two definitions depend on the focal length (the distance between the mirror and the exit slit) of the monochromator, the longer it is, the better the separation. The reciprocal linear dispersion of a grating is almost constant throughout the uv visible range. Attention should be paid to the fact that there is an overlap of wavelengths so that 1000 A in the second order falls exactly in the same place as 2000 A of the first order and 1000A of the third order falls where 3000 A of the first order falls. If for resolving power considerations one has to use third order 1000 A,one should take care to eliminate first order 3000 A, second order 1500 fourth order 750 A, etc. This can be accomplished by putting, in front of the grating, a filter that will pass on

x/

A,

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to the grating, only wavelengths close to the desired one, or, a prism that will pass on only the desired (or a part of it) order (order sorter). One of the measures of a monochromator’s quality is the energy throughput or the amount of radiation passed on to the detector. For the same grating, the higher the order the better the dispersion, but the lower the intensity of the radiation, with the greatest intensity being in the first order. This can be changed by changing the shape of the grooves in such a way that the intensity is concentrated mainly in a predetermined angle. Instead of the grooves being symmetric, they are grooved or blazed in such a way that they have the form of steps having broad faces from which reflection occurs and narrow faces which are unused (Fig. 5-3). There is an GRATING

I NORMAL

Figure 5-3. A schematic diagram of an echelette grating

angle p between the broad faces and the surface of the grating, or, in other words, between the normal to the surface and the normal to the broad face. This type of grating is called an echelette grating and p is the blaze angle. This angle will determine where the intensity will be concentrated. The diffracted energy will be concentrated around the diffraction angle for which the ordinary law of reflection from the individual faces is most nearly obeyed. By optimization of the blaze angle, as much as 90%of the available energy of a particular wavelength can be made to fall in a single order. The wavelength for which ordinary reflection and first order diffraction coincide is called the blaze wavelength A,. Most light is concentrated in the first few orders and centered about A, in the first order, &/2 in the second, etc. The monochromator can be blazed at two different wavelengths to provide high energy at both the uv and visible ranges.

5.10 Atomization by flame One of the methods of atomization used in atomic absorption analysis is atomization by flame (the other being electrothermal atomization). The purpose of the


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atomization system is to generate uncombined analyte atoms, which in turn will be excited by radiation having the characteristic wavelength A. The system includes (Fig. 5-4) a nebulizer, a spray chamber (and a liquid trap), a burner, and a flame.

CAPILLARY

(

//A

GLASS BEAD

IDANT WASTE

Figure 5-4. A schematic diagram of a nehulizer-spray chamber-burner system (with a flaw spoiler and a glass bead)

This system is very important in determining the sensitivity, detection limit, precision and accuracy of the determinations. There are momentary variations in flame characteristics so that conditions are not exactly the same during the analysis and the stability of the excitation system (nebulizer-burner) is one of the major parameters determining the quality of results. At the beginning of the analysis, several adjustments should he done (see the following) to raise sensitivity, but the aim is not to obtain maximum signal or maximum sensitivity as such, but to obtain the highest possible sensitivity while remaining under stable conditions (and acceptable signal-to-noise ratio). The nebulizer sucks up the sample solution to the spray chamber where it is broken into a very fine aerosol which is passed on to the flame (ignition by a pushbutton) where it is heated to a temperature high enough to evaporate the solvent and then to dissociate the compound into its constituent atoms. The light beam from the HCL excites the atoms in the flame and the amount of the absorbed light is measured. A delay time (aspirating the solution to the flame without reading the absorbance) of a few seconds before measuring each sample is necessary. First, to clean the system from the remains of the former solution, and second, it takes time from the beginning of aspiration to obtain the stabilization of the highest part of the

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absorption peak. The instrument reads a great number of readings per second. Two or three second-long measurements are usually taken to obtain the mean and the relative standard deviation of the sample’s concentration. Ordinary nebulizers are fixed nebulizers (fixed uptake rate) but there are also ad-justable nebulizers (adjustable uptake rate). They are recommended for work with organic solvents, as these also act as fueIs. With a fixed nebulizer, there will be access fuel during aspiration and the acetylene flow rate has to be lowered. The result is that when aspiration stops there will be too little fuel and the flame may go off. The adjustable nebulizer solves this problem. The oxidant flows through the nebulizer, passes through the venturi and draws solution (from the sample vessel, through a plastic capillary tubing) into the venturi as a mist of fine droplets which is then mixed with the incoming fuel and passed on to the burner. There is an optimum size for an analyte droplet which will allow it to be evaporated completely and atomized during the short time the droplet is in the flame. This is considered to be in the range of 2-10 p. Small droplets will be carried to the flame while the big drops will be drained away to the liquid trap. The higher the part of the solution used, the higher the sensitivity and the detection limit. The amount utilized will depend on the distribution of drop size, i.e. the percentage of droplets having the right size. The aim is to increase the percentage of droplets having the right size and this is done by breaking the bigger droplets into smaller ones. In some instruments the spray from the nebulizer hits flaw spoilers (or baffles) in the form of a propeller (not rotating) in the spray chamber, and are broken into smaller droplets. A better way seems to he the glass bead (or impact bead) used in other instruments. This is a few cm long, bent, thin glass rod with a bead at its end (with a special coating), in the spray chamber opposite the nebulizer. The spray hits the head and is shattered into fine droplets. The absorbance critically depends on the distance of the bead from the nebulizer venturi (reaches a maximum and then drops) and this depends upon the physical properties of the solution. The distance of the glass bead has to be adjusted at the beginning of the analysis, while aspirating a standard solution of the analyzed element, until maximum stable signal is obtained. Nowadays, there are instruments that use both types of spoilers. Though the amount of droplets of the right size in the spray can almost be doubled, only about 10%of the solution is utilized while the rest is drained. There were total consumption burners in which most of the sample was passed on to the flame, but the energy in the flame was not high enough to evaporate and dissociate this amount of solution during the short residence time in the flame, resulting in low sensitivity and numerous interferences. In modern pneumatic nebulizers only about 10% of the solution reaches the flame, but this is done under conditions which favor stability. Burners are made of special metals that can resist corrosion by acidic or basic solutions reaching the burner at high temperatures. There used to be a plastic material film on the inner part of the burner, to avoid corrosion, but this was


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discontinued as it was found to cause a memory effect. Solution droplets adhered to the film and later renebulized and passed on to the flame while the next solution was aspirated. There are special burners for air - acetylene and for nitrous oxide - acetylene flames and they should not he mixed as explosions may occur. The burner's design depends on parameters such as gas flow rates, burning rate, fueloxidant type, efficient heat dissipation so that the burner will not become red hot (some burners have cooling fins). The burner's slot dimensions depend on the fueloxidant for which it is intended. The burning velocity is the speed of propagation of the flame front in the gas mixture. A certain correlation must be maintained between burning velocity and the combustible mixture flow velocity. Higher flow velocities can cause the flame front to blow away while lower ones can cause a flashback (the flame propagates within the burner) which can lead to explosion. Safety precautions in the instrument's manual should be followed. In modern instruments, several safety precautions are built in the system, for example, a burner interlock inhibits ignition if a burner is not fitted to the spray chamber or a burner is fitted which is not suitable for the gas mixture selected; a pressure relief bung at the rear of the spray chamber is designed to minimize the effects of a flashback; an interlock designed to inhibit ignition if the pressure relief bung is not correctly fitted and to shut down the flame if the bung is ejected as a result of a flashback; a liquid trap provides a gas-tight seal between the spray chamber and the atmosphere and allows excess solution to be drained from the spray chamber, fitted with a lever sensor which is designed to inhibit ignition if the liquid trap is not filled to the correct level and to shut down the flame if during operation the level falls below the required minimum. Several gases were used as oxidants and fuels. The most common were:

1. Air-propane; relatively low temperature. It was used mainly for excitation of alkali metals (which can be ionized in hotter flames). Severe chemical interference can occur and noise level is relatively high. Used today only for flame emission analysis (flame photometer) of the alkali metals; 2. Air-hydrogen; relatively low temperature. A reducing medium fit for arsenic, selenium and tin, for which this flame is mainly used. Severe chemical interference can occur;

3. Air-acetylene; compressed air is used (from a lab central supply, a cylinder or a lab compressor). A relatively hot flame (about 2300" C); causes partial ionization of alkali and alkaline-earth elements; used for most elements; 4. Nitrous oxide-acetylene: the hottest flame used (about 3000OC); used for refractory elements and to reduce chemical interference; may cause severe ionization and an ionization suppressor may have to be added.

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Because of safety, cost, usefulness and convenience, only the last two flames are used in modern commercial instruments. The quality of the gases to be used depends upon the purity of the gases available locally. In some places commercial acetylene is good enough and in some places extra pure acetylene should be used. The compressed air may contain water and oil and the system should contain a trap to remove them from the gas. A compressor which does not need lubrication and does not emit oil vapors should be used. In all flames, temperature profiles and flame characteristics are determined by fuel-oxidant ratios. There are fuel lean or oxidizing flame (the hottest), fuel-rich or reducing flame (the coolest) and chemically balanced or stoichiometric flame (about midway between the first two). They can be identified by the height and color of the inner part of the flame (not a cone as in ordinary lab burners but a long inner part parallel to the length of the burner). One can find in the "cookbook" (in the analysis instructions for each element) what type of lame should be used (according to the element, the matrix, etc.) what interferences can be expected and what to do in order to overcome them. The properties of the analyzed atom, its molecular association and the final products it may form in the flame will determine what type of flame has to be used (as recommended in the "cookbook" or by the default parameters of computerized instruments). There are several types of atoms according to their behaviour in flame:

1. Easily atomized atoms (such as potassium, sodium, lead) that can be analyzed in air-acetylene flame. There are only a few interferences and the fuel-oxidant ratios have no great influence;

2. Atoms that can be determined in both air-acetylene and nitrous oxideacetylene flames, but not as effectively. For instance, if a solution of Ca nitrate is analyzed, it will decompose in both flames intoCaO, but a much higher proportion of this oxide will dissociate into atoms in the nitrous oxide flame and a higher signal will be obtained. The analysis can be carried out also in air-acetylene flame but many interferences may occur and the fuel-oxidant ratio is important;

3. Refractory elements which almost will not be atomized by a cool flame and have to be atomized in a nitrous oxide flame; 4. Elements for which not only temperature but also the chemical nature of the flame determines the dissociation so that a specific fuel-oxidant ratio is needed. For instance, Mo in a stoichiometric air-acetylene flame will remain as an oxide but, in a fuel-rich flame, though it is cooler, it will be reduced to the free atom. Similarly, a strongly reducing nitrous oxide-acetylene flame is needed to effectively atomize silicon;


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5 . Atoms that react with other species in the sample, In the presence of aluminum, silicon and phosphate, atoms like magnesium, calcium, strontium and barium may form the respective compounds and will only slightly dissociate into atoms. The use of the hot nitrous oxide-acetylene flame will cause higher dissociation and better results will be obtained. Another common method to overcome such a problem is the use of releasing agents. If a large excess of strontium is added to a silicate containing calcium (to be analyzed) nitrate solution, most of the silicate will combine with the strontium. Lanthanum is a very common releasing agent. Another common method is matching the calibration standard solutions with the samples, i.e. adding the interfering element to the standards (in about the same concentrations), thus avoid high results for the standards and low for the samples.

For atomic absorption, ground state atoms which can be excited by radiation are needed. There is an optimum flame temperature for vaporization and dissociation. If the flame temperature is too high, the atoms may receive enough energy for ionization to occur (which, depending upon the atom, temperature and concentration, may reach the range of 80%). The electron will leave the atom completely and will not take part in the excitation by radiation process. This may be a serious problem for elements with low ionization energies. The method to overcome this problem is to add to the analyzed solution another element, such as Na, K or Cs, which is ionized more easily (or about the same) at a very high concentration compared with that of the analyzed element. For instance, for the analysis of trace amounts of Ca (in the range 1-4 pLg/ml) in the nitrous oxide-acetylene flame, the solution is made to contain 2000 &ml potassium, which will cause signal enhancement due to suppression of ionization. In the analysis of traces of strontium, the sensitivity may be highly increased by the addition of an ionization suppressor-potassium ions (with the same amount of potassium the sensitivity may be highly increased by raising the temperature, using nitrous oxide-acetylene instead of air acetylene flame). The ionization suppressor has to be added to the unknown solutions as well as to the calibration standard solutions. The effectiveness of the suppressor may change with its concentration and the right amount has to be determined experimentally (absorption vs. concentration, till a plateau is found) or taken from a “cookbook”. One has to be careful not to cause harm by this addition. The added potassium chloride contains, for example, some Na (depends on the purity of the material used, let’s say 0.01%). If one analyzes a very low concentration of Na, this may cause an error. In this case a very pure potassium chloride powder or solution has to be used. Raising the temperature should (due to increased evaporation and dissociation) increase the total number of atoms in the flame and hence the signal. On the other hand, the intensity may be lowered because of increased excitation (not by HCL but by flame), ionization and Doppler broadening. Because of these opposed

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effects, each element has an optimal absorption as a function of location in the flame (e.g. absorption increasing with height, decreasing with height or increasing to a maximum and then decreasing with height). Maximum signal will be obtained if the light beam from the HCL passes in the flame, through the maximum population zone which is not identical for all elements. For any given combination of element, matrix, sample aspiration rate, flame type and burner, the population density of absorbingatoms will vary throughout the flame in a manner characteristic of that combination. At the beginning of the analysis, several adjustments are made while aspirating a standard solution of the analyzed element. The height of the burner relative to the light beam has to be adjusted (care should be taken not to raise the burner into the light beam) for each element until maximum stable signal is obtained. The burner position has to be adjusted rotationally, so that the length of the burner is parallel to the light beam (so that the whole length of the flame will be utilized) until maximum stable signal is obtained. The burner position has to be adjusted horizontally (backward and forward) to align the center of the burner slot with respect to the light beam, until maximum stable signal is obtained. For each element there are several optimum working ranges (a linear calibration curve) at different wavelengths, e.g. for Cu, 2-8 &ml at 324.7 nm, 6-24 pg/ml at 327.4 nm, 70-280 pg/ml at 222.6 nm. One has to dilute the Cu solutions to obtain concentrations in the working range to be used. In many cases, when the concentration is higher than the optimum range, it is worthwhile (instead of diluting the samples) to rotate the burner at a certain angle. The distance that the light beam travels in the flame is shortened, the beam meets less atoms, thus lowering the sensitivity, resulting in the same absorbance as if the concentration is lower. The angle of rotation can be adjusted to reach absorbance values in one of the optimum working ranges for lower concentrationsand the analysis is carried out at that angle. In order to be able to do this, one has to know the true range of concentrations (e.g. by diluting and measuring the lowest and highest concentrations) and to prepare calibration standards that will fit that range when they are measured at the same rotation angle. Attention must be paid to the fact that when the whole length of the burner slot is used, the interferences at the two edges are small compared to the optical path length in the flame. Using a rotated burner, the optical path length is shortened and the interferences at the two edges are comparatively high. Changes in sample flow rate can change the number of atoms in the flame and can change the sensitivity. One may think that if more solution reaches the flame the more dissociated atoms there are, however, the additional solution is not effectively evaporated and atomized because the solution cools the flame, thus lowering the signal. The system tends to be saturated and the curve describing absorption as a function of intake rate rises and then flattens at rates higher than about 6 ml/min. At the beginning of the analysis the nebulizer has to be adjusted (while aspirating a


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standard solution of the analyzed element) to an optimum flow rate until maximum stable signal is obtained. Changing oxidant-fuel ratios will change the flame temperature profiles (hence the location of maximum population) and characteristics (oxidizing, reducing or stoichiometric). At the beginning of the analysis of each element, the ratio has to be adjusted (while aspirating a standard solution of the analyzed element) so as to obtain maximum stable signal. In computerized instruments there are (for each element) default values for flow rates of air, acetylene and N,O so as to obtain a high sensitivity and the right amounts are delivered by a control unit. One can change these values when needed, for example to compensate for matrix effects, simply by typing the new values to be used. A difference in physical properties (such as surface tension or viscosity) between unknown solutions and calibration standards will result in different flow rates (for the same instrument setting) and hence, in different signals for equal concentrations. Thus one has to match (as much as possible) these properties. If because of the dissolving procedure the solutions have a relatively high acid concentration, the calibration standards should contain the same amount of the, same acid. If the aqueous solution contains a certain amount of inorganic salts or of an organic material, the same amount should be added to the standards. For instance, for the analysis of trace amounts of Cu in blood serum, the Cu is leached into a 12.5% trichloroacetic acid. The standards should also contain 12.5% TCA [ 141. The analyte atoms in the flame do not know that they are in an atomic absorption spectrometer and in addition to the absorptionof energy from the HCL, they also emit their characteristic radiation at the resonance line (flame emission) and this (together with radiation from other atoms and flame luminosity, at this same wavelength so that separation by a monochromator cannot be achieved) may compensate for a part of the absorbance at this same wavelength. The problem is solved by modulation of the system. One way is to modulate the output of the source (the power to the source is modulated) so that its intensity fluctuates at a constant frequency. Another way is by use of a chopper - a disk of which alternate quadrants are removed. Rotation of the disc causes the beam to be chopped at the desired frequency. In both cases the HCL radiation is modulated but the flame emitted radiation is not. Only the modulated light is amplified and read. Caution! Some safety precautions should be strictly enforced. A flame should never be left unattended. Keep all compressed gas cylinders outside the laboratory in a cool, ventilated area. The fumes and vapors produced in the analysis can be toxic and should be extracted from the instrument by an efficient exhaust system. Always use a liquid trap filled to the correct level to avoid explosions. A special type of burner is needed for each type of flame. An air-acetylene burner should never be used for a nitrous oxide-acetylene flame. Acetylene can react with copper,

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or copper alloys, to produce acetylides which may explode. Hence, no copper (or brass) pipes should be used with this fuel and special pressure regulators, not containing copper, should be used. Do not use oxygen-acetylene flames. Acetylene cylinders should not be used completely as the acetylene is dissolved in acetone and when the acetylene pressure in the cylinder is low, acetone will also be evaporated, will enter the burner and may explode. Always stop using the cylinder when some pressure still remains (for the exact amount check the manufacturer’s instructions). Nitrous oxide can cause spontaneous combustion of oil, and care must be taken to clean connections and the pressure regulator from oil. When N,O is drawn from the cylinder the cooling effect can freeze the gas and a regulator with a built-in heater should be used. The operator should familiarize himself with these and many other safety precautions specified in the instrument’s manual and follow them.

5.11 Instruments The major necessary components of an atomic absorption spectrometer were mentioned in Section 5.7 and described in detail in the previous sections. Different manufacturers use different forms of these components but they all should be included in the instrument. Different manufacturers have different software to control the instrument’s operation and data handling. All this (including the rank or upgrading of the instrument which (depends whether the instrument is a single or double beam type, flame only or including graphite furnace, with or without hydride generator, a data station or an advanced computer, etc.) may run in the range of $20,000-$70,000. Contrary to emission spectrographs or ICP spectrometerswhich are simultaneous, i.e. can measure the signals of many elements at the same time, atomic absorption spectrometers are sequential, measuring one element at a time. One has to measure, for instance, copper in all the samples, then nickel in all the samples, otherwise one has to change the monochromator settings and recalibrate separately for each element in each sample. This sequential system makes the analyses much longer (operator time), a factor that has to be taken into account in efficiency or price/element calculations. Generally atomic absorption spectrometers are divided into two types, i.e. single beam and double beam instruments (each containing all the major components mentioned above). In a single beam instrument there is only one beam of light which passes from the source into the flame and optical system and then to the photomultiplier. Any change in the light source intensity or in flame characteristics will influence the final signal.


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In a double beam instrument, the light from the source passes through a beam splitter and is split into two beams of about equal intensity. One passes through the flame (I,)as in a single beam instrument and the other passes along the same optical system, but not through the flame, i.e. it is not influenced by the analyte (Io). This second beam serves as a reference beam to which the intensity of the first beam (the sample beam) is compared (Beer-Lambert). The intensity of each beam is read in turn (the beams become intermittent) by the photomultiplier and their ratio is amplified and displayed. Thus, any change in the source intensity, the detector sensitivity or the electronic amplification will affect both beams and the effect will be cancelled. The measurements of a double beam instrument are much more stable - less noise - the result of which is that higher amplifications are possible so that lower concentrations can be measured. On the other hand, this is not a real double beam compensation, as the reference beam does not pass through the flame and changes in the flame (for instance absorption by molecules formed from the solvent at high temperatures) or momentary changes in the nebulizer uptake will not be compensated for. In addition, the double beam system means more parts (the splitter, mirrors, half silvered mirrors) and a bigger, complicated instrument, hence its high price compared to that of a single beam instrument (but if one can afford it it's more than worth the difference). One system of a beam splitter is a rotating disc with alternate quadrants removed. The remaining quadrants which have mirrored surface reflect the light into the reference path, where through an array of mirrors, the beam avoids the flame and then a half silvered mirror recombines it with the sample beam along the optical path. The spaces (missing quadrants) pass the radiation into the sample path. This action of the beam splitter also modulates the beams into intermittent beams, so that only they will be read and not the continuous emission from the flame (see Section 5.10). In another system the splitter consists of a partially aluminized quartz plate. A part of the light is reflected to the flame and the other part passes through the splitter, through an array of mirrors, directly to the monochromator. A rotating reflecting chopper is located in front of the monochromator entrance slit, where it alternately interrupts the sample beam to direct the reference beam into the monochromator. This way a two-beam system with modulation is achieved.

5.12 Electrothermal atomization This technique is known by many names such as electrothermal atomization, graphite furnace, carbon rod, graphite tube atomization, etc. The function of both flame and electrothermal (electrically heated graphite furnace) atomization is the generation of free analyte atoms ready to be excited by the right wavelength radiation. Flame atomization is simple, reproducible and good results are obtained, but

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the method has a sensitivity limitation. For a large number of elements, furnace techniques are more sensitive than flame methods and can determine concentrations typically a hundred times smaller than those possible by flame, but the analysis is much longer (may reach 2-3 min per elemendsample). This is a technique which is complementary to flame absorption, rather than a technique which replaces it. Sample volumes in the microliter range can be analyzed. As combustible gases are not used, a furnace system can safely be left overnight unattended (provided no other trouble develops). Sampling efficiency of the flame is low as almost 90% of the sample are drained, residence time in the flame is very short (a very small fraction of a second) and dissociation depends both on temperature and the chemical nature (reducing, oxidizing or stoichiometric) of the flame. In electrothermal atomization the sample is very small but all of it is utilized and residence time of the atoms in the optical path is about a second. The concentration of analyte atoms is high, resulting in a high absorption, or, the proper absorption working range (about 0.1-0.8 absorbance) can be reached with lower concentrations. The dissociation depends on the final temperature, the rate of approaching it and the reducing nature of the graphite. The small sample volume (usually 10-40 p1) requires a very precise measurement and introduction into the furnace and this may become the limiting factor for reproducibility. At first micropipets with interchangeable plastic tips (to prevent contamination from sample to sample) were used for manual injection, but this proved not good enough. For reproducible results an automatic injector is a must. In the “cookbook,” relevant data for the instrument’s performance may be given in different forms such as: the typical characteristic concentration - the weight in grams of an element that would typically yield an absorbance equal to 0.0044 absorbance- 1% absorption - (in the peak height mode, under specified conditions), which is in the few pg range, based on a 20 p1 aliquot (which is the range of a few ng/ml-ppb) or/and as the typical response, for instance, a 10 pl aliquot of 5.0 ndml is expected to yield about 0.3 absorbance, and/or as the analytical working range, for instance (for a 20 p1 aliquot) 0.5-12 ng/ml (expected to yield 0.3-0.8 absorbance). In older instruments it was quite a complicated task to install the graphite furnace into the spectrometer. In modern instruments the furnace is interchangeable with the nebulizer-burner unit and only small adjustments (for correlation with the optical path) are needed. The sample holder (Fig. 5-5) is a replaceable graphite tube which is contained in a water cooled cell fitted at each end with quartz windows. A stream of inert gas (argon or nitrogen) flows through the cell protecting the graphite from oxidation. The graphite tube is a cylindrical tube, open on both sides (the optical path) with a hole in the center of the upper part of the tube for sample injection and through which the inert gas emerges with the products of drying and ashing. The tube fits into a pair of graphite electrodes (the form of which differs according to the manufacturer). A toggle mechanism locks the graphite tube in place, so that there

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GRAPHITE TUBE

\

INJECTION HOLE

'CARBON ELECTRODES APHITE TUBE

"'.EXTERNAL

CARBON ELECTRODES

QAS

FLOW

FLOW

Figure 5-5. A schematic diagram of two types of graphite furnaces. Toggle mechanism, cooling water, springs, connections, etc., are not shown.

is no gap between the tube and the electrodes (due to graphite corrosion) so that the same voltage will result in the same temperature. The tube is electrically heated in programmed steps by a high current (low voltage) passing along its length. The temperature is measured by an external sensor (a pyrometer) or by voltage-current measurements. Coating of the graphite tube with pyrolitic carbon (obtained by passing through the cell an inert gas and a hydrocarbon, at a high temperature) seals pores in the tube. This prevents soaking of the sample into the graphite, which would affect reproducibility and will lower sensitivity. This hard surface also prolongs tube life and minimizes carbide formation which can prevent analysis of carbide forming elements such as Mo, Ti, U, V. The autosampler (automatic injector) can inject into the furnace a programmed volume of a blank solution, a chemical modifier, a sample solution and can prepare (and inject) the programmed amounts of standard solutions by injecting the right amounts of diluent and of the analyzed element stock solution (up to a constant volume). It also can rinse the injector plastic tip between samples. Standard editions analysis is often used for the furnace analysis of samples. This involves the

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preparation of several solutions for each sample, Automatic preparation of these solutions saves time and nerves (and errors). A small volume of sample is place on the (inner) bottom of a graphite tube located in the place of the conventional flame of AA instrument. The process consists of 3 stages. In the first stage (drying) the solvent is evaporated. The sample is gently diied by programmed heating of the graphite tube. If the temperature is raised too fast or to temperatures above the boiling point, the sample will sputter, loosing material (which also may stain the quartz window). Because of this, if by mistake a wrong sample was introduced, one should be careful not to use the “clean” option as this raises the temperature immediately to very high temperatures (close to those of the atomization) and sputtering will occur. This option should be used for cleaning only before analysis or after atomization. The best results may be obtained, for example, by raising the temperature slowly to a temperature below the boiling point and hold it there for a time, then raise it to the boiling point and hold it there and then raise it to a temperature a few degrees above the boiling point and hold it there (in this example, a drying stage consisting of 6 programmed steps). There is a trade-off between temperature and time but one must remember that the longer the steps, the longer the analysis. The second stage is ashing, which is a further heating to a much higher temperature, to vaporize the major constituents (pyrolysis, combustion and distillation) leaving the trace elements. There is a danger that at high enough temperatures a part of the analyzed element will vaporize also and this is a serious danger when analyzing volatile or relatively volatile elements. Sometimes ashing at a relatively low temperature may solve the problem, causing volatilization of the interfering component and not of the analyte. On the other hand, when using low temperature ashing, sometimes not all the matrix is destroyed and at the atomization state this may result in non specific background absorption which needs correction (see Section 5.14 and Section 5.15). For refractory elements, ashing at high temperatures may be carried out without analyte. loss. With volatile samples there may be a problem and sometimes (for instance if the matrix is an organic matter) the use of a modifier can help, as it may stabilize the analyte at higher temperatures and this facilitates ashing at higher temperatures without analyte loss (see Section 5.16). In the case of an organic matrix, sometimes the use of oxygen as an auxiliary gas in the ash stage will help in getting rid of the matrix more easily. In planning the analysis, a temperature which is high enough to get rid of as much undesired elements as possible, without loosing any of the analyte, is desired. This may cost in a lot of trial and enor, as the suggestions in the “cookbook” are guidelines only, and can’t fit all matrices. Here again, several heating steps may be used. Finally, in the third (atomization) state, the tube is very quickly (at rates of up to 2000” C/sec) heated to between 2000-3000” C to atomize the remaining material and produce a transient cloud of atoms (in the optical path) to be analyzed by atomic


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absorption and quantified using peak height or peak area measurements. This stage is very short and may last less than a second, up to a second or two. During atomization, the inert gas flow is stopped so that all the atom population will remain in the tube and not be swept out. A small amount of inert gas still remains in the tube and it takes time for oxygen to enter and oxidize the graphite. By repeatedly injecting and drying (or drying and ashing) aliquots of sample, an effective concentration of the sample is obtained prior to atomization, thus increasing the analytical signal. This is equivalent to a better detection limit. With many injections the sample may be smeared and not concentrated in one place in the tube, causing a wider and lower signal. In such cases (can be checked on the graphic screen) one must measure the peak area. In Computerized instruments, programming of the different steps becomes a lot easier by using the graphic screen. The background and atomic signals are graphically displayed together with the furnace temperature program (T vs. t). Peaks are obtained also in the drying and ashing stages as the volatiles and solid particles block a part of the radiation. The operator can see the changes during the various steps. The need for higher temperatures or longer heating times is immediately obvious. Complex samples or samples close to the detection limit may require an investment of a day or two to prepare the right analysis method, by trial or error, finding the right time durations, the right temperatures, the right rates, to decide whether a modifier is needed and what modifier, etc. During the atomization stage, absorbance rises very quickly and then falls, so that the peak exists very shortly (even less than a second). A high speed recorder and later an electronic peak measuring device (built into the spectrometer) were used. Nowadays, with computerized instruments, this is done easily. Peak area or peak height are measured and compared to those of standards. For some elements, better results are obtained by using a L'vov platform. This is a pyrolitic graphite platform (with a central depression to contain the sample) which is placed on the tube's inner bottom, under the injection opening, touching the tube by lobes at the ends of the platform. The sample is injected and deposited on the platform instead of the furnace wall. During atomization the platform temperature lags behind the furnace wall temperature and reaches the designated temperature only after the tube has reached a stable higher temperature. The sample is vaporized into a stable higher temperature environment. Recombination of the vapor components is avoided, thus reducing interferences. For instance, in the determination of lead in a salt matrix, the lead vapors may reach a lower temperature zone, recombine with the salt anion (even redeposited) and not take part in the atomization and excitation. With the platform, the lead atoms will find a stable high temperature and recombination will not take place. The use of the platform may also cause a great reduction of background absorption due to the higher temperatures

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experienced by the sample after vaporization. A better reproducibility is obtained. The L‘vov platform is useful for the determination of volatile elements. Refractive elements which require high atomization temperature are better determined without the platform. Many times samples that are near the detection limit and can hardly be analyzed by flame atomization and have a matrix which causes interferences, can still be analyzed by furnace atomization. One can dilute the sample, for example, one hundred times, get completely (or partially) rid of the matrix effect and still, a high signal may be obtained. Even if the signal is small, it is still worth diluting, as the matrix effect is lowered much more. There may be a light scattering problem, as small solid particles (smoke) may be in the tube, scatter the light beam, thus increasing absorbance. In short wavelengths this is a problem and background correction has to be applied. If one has no idea of the concentration and the related signal size, the way to find it is to inject a small sample and check, increase the sample and check and so on, till a peak is seen. If one starts with a big sample, the tube may become totally contaminated and many cleaning cycles or a change of tube may be needed. The sample has a matrix and this may cause a problem as to what to use as a blank solution. If, as usual, a solution containing only the solvents is used, this may not fit the matrix and it may be critical. One can prepare a standard additions calibration line of the sample, dilute it by two and prepare another standard additions line. If the lines are parallel, then the blank can be used, if not, it means that this blank cannot be used for this analysis. One can try adding to the blank solution the proper concentrations of the major constituents of the matrix and determine what causes the problem. There are situations of chemical interferences where the standard additions method has to be used. This has to be done in the linear part of the calibration curve otherwise one does not know how to do the extrapolation, as, a small change in the curved part will yield a big change (or infinity) in the extrapolation. One can check in the “cookbook” what is the absorbance for the pure analyte, let’s say 0.0044 absorbance for 5 ppm analyte, which means 0.44 absorbance for 500 ppm (provided it is still in the linear range of the calibration curve). Let’s say that the sample yielded 0.200 absorbance. If one uses additions of 100 and 200 ppm the readings will be in the rnage of 0.3 and 0.4 absorbance and there will be 3 points for the standard additions line.

5.13 Hydride generation Hydride generation is used for the analysis (especially traces) of arsenic, antimony, tin, selenium, bismuth (and lead). The method is used to separate and preconcentrate analytes from sample matrices by a reaction that turns them into


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their hydride vapors (ASH,, SbH,, SnH,, SeH,, BiH,, TeH, and PbH,). The analyte is separated from the complex sample and also is concentrated. Such a procedure improves the detection limits by a factor of 10 to 100 (for lead there seems to be a small change only) and reduces chemical and spectral interferences. Determinations of traces of these elements in air particulates, sewage, alloys, foods, biological and geological materials, etc. were reported. Automated hydride generator systems (continuous flow with a peristaltic pump or a pressurized reagent pumping system) which are attached as an accessory to the spectrometer, are manufactured by a few spectrometer manufacturers. The elements mentioned above were determined by atomic absorption spectrometry, the sample being introduced by direct solution nebulization. There were problems with elements such as arsenic and selenium, whose optimal absorption lines were far into the ultraviolet region. One of the major problems was a very high background absorption in air-acetylene flame. The cooler argon-hydrogen flame reduced the background absorption (which still remained high) but caused other problems (such as chemical reactions in the fame). Turning the elements into hydrides and then passing them into the atomizer resulted in a recommended sensitive method. At first the hydrides were generated by a metal-acid reaction where the metal (usually Zn) liberated hydrogen atoms which combined with the analyte to form the hydride. This was a very slow reaction which resulted in broad peaks, not all of the elements formed the hydrides and some elements had to be prereduced. The common method today is NaBH,-acid reduction. An acidified aqueous solution of the analyte is combined with an aqueous solution of sodium borohydride. Sodium borohydride in acid liberates hydrogen atoms which combine with the analyte to form the hydride. After a short period the volatile hydride is swept (by an inert gas) into the atomizer where the hydride is dissociated. Pentavalent arsenic has to be reduced (by Kl in the borohydride solution) to trivalent arsenic prior to the borohydride reduction (this can be done, preferably separately, before the reaction, as it takes time). Copper, nickel, cobalt, oxidizing agents, etc. interfere with hydride formation and have to be dealt with. A precondensation (liquid nitrogen) is used to concentrate the analyte before the atomization, thus sharpening the response. The borohydride reduction is quite fast (several seconds) and all the above-mentioned elements form the hydrides. The atomizer can be the flame (usually argon-hydrogen) a graphite furnace or a heated quartz device which is a quartz tube heated to several hundred degrees. The radiation passes from the source through the tube to the monochromator. The vapor generator can be used for mercury analysis, without forming the hydride. Mercury compounds in acid are reduced to the free element with stannous chloride and the mercury vapor is swept by an inert gas into the quartz tube (no need of heating for dissociation). There are instruments in which the mercury is collected

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on a gold gauze and then all the collected amount is heated and swept into the quartz cell, thus improving the detection limit to the ppt level (good for ultratrace analysis of mercury in drinking water). 5.14 Interferences

A few of the interferences were mentioned in previous sections and will briefly be mentioned here again, to see the whole picture. One type of interference is instability of the hollow cathode lamp output (see Section 5.11). This interference can be corrected by the use of double beam instruments. The light from the radiation source is split into two beams, one which passes through the fame and sample, the other serving as a reference to which the first beam is compared. Changes in lamp output will affect both beams. A part of the factors that may cause the analytical signal of the samples to be different from those of standard solutions with the same concentrations are physical (or transport) interferences such as viscosity, caused for instance, by temperature or composition difference between sample and standards (sample taken out of a refrigerator and standards at room temperature, acids or organic materials in the sample and not in the standards) thus changing the rate of solution uptake or the aerosol droplets size. Cu in aqueous solution will have a calibration curve with a slope very different from that of Cu in the organic solvent MIBK. In furnace analysis (micro quantities) small variations in physical properties can have a large influence on the result. For example, differences in viscosity can cause a difference in spreading on the inside surface of the graphite tube and this may change the absorbance, because residence time of the atoms in the optical path will be dependent on how far from the center atomization takes place (because of this spreading, organic solvents should be injected deep into the tube, very close to the bottom, and for some, preheating is preferred). These types of interferences should be corrected by physical means (T), or careful matching of sample and standards (see Section 5.10), or by using the standard additions method. In many cases, dilution of the sample solves the problem. The analyte concentration is lowered but the interference may be lowered much more, or even cancelled. In atomic absorption spectrometry it is assumed that the absorbance is caused by the analyzed element only and is proportional to its concentration. An interference occurs when proportionality between the absorbance and the concentration varies. In practice, the analytical signal is changed hy factors other than concentration, due to spectral interferences or chemical interferences. The measured absorbance may be higher than expected by the atomic absorbance and will consist of the atomic absorbance and absorbance by other sources - background absorption (mostly in the ultraviolet region and very little in the visible). A background correction will be needed to obtain the true absorbance due to the analyte only. Sometimes the total


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absorbance will he lower than expected due to chemical reactions of the analyte and other types of corrections will he needed. Spectral interferences which cause hackground absorption occur when, during atomization, there is another species whose absorption lines are very close to the analyte absorption so that resolution by the monochromator is impossible and the result is as if there is a higher analyte concentration. This type of interference depends very much on the interfering element/analyte concentrations ratio. Emission lines of hollow cathode lamps are very narrow so that overlapping because of absorption of close lines of other elements is rare (emission lines of the elements in the sample, caused by flame temperature, are not modulated and are not read). In such a case usually absorption at another wavelength of the analyte should be used. Sometimes working with a narrower slit may solve the problem, or sometimes standard solutions with a matching matrix may be used (if the amount of the interfering element is known), or, sometimes the standard editions method may be used, or, sometimes instrumental background correction may solve the problem. In furnace analysis there is a real prohlem in the analysis of Se in blood where iron is present and has about 15 peaks (structured background). The analysis of 0.5% Cu,for example, in steel causes a problem. The iron has to be taken out (extraction), or if the amount of iron is known, standard solutions with a matching matrix are prepared, or the standard additions method may be used, or sometimes an instrumental background correction may be applied (depending also on the method of correction). The spectral interference in flames is usually caused not by another element, but by a broad absorption band (non-specific absorbance) caused by other molecules, combustion products, which originate in the flame or in the matrix. This may occur in flames when the flame is not hot enough to decompose all molecular compounds in the sample or formed in the flame. These molecules will absorb light and if the spectrum overlaps the analytical wavelength, this will be measured, increasing the signal. The interfering hroad absorption bands may be caused by molecules which originate in the matrix (e.g. by metals combining with OH radicals from the flame) which may overlap absorption lines of the analyte. Sometimes their dissociation by a nitrous oxide-acetylene flame will eliminate the interfering band. Non-specific absorbance can also result by radiation scattering particles in the flame (by the formation of metal oxides of refractory elements), thus lowering the beam intensity. Working with a fuel-rich flame may eliminate the formation of the scattering oxide. In flame atomization, background absorption is usually important when working near the detection limit or at low wavelengths. Sometimes the problem can be avoided by a change of operational parameters like flame temperature or fuel-oxidant ratio (to decompose oxides), measuring at a higher wavelength where molecular absorption is not high, or by the use of an instrumental background corrector.

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The most common commercial instrumental methods are the continuous source correction (deuterium background correction) and correction based on the Zeeman effect (see Section 5.15). The flame itself (without a sample) produces absorption, especially at low wavelengths. To correct for absorbance of absorbing products originating in the flame, i.e. fuel-oxidant-solvent system (e.g. the formation of OH by water dissociation), measurements with a blank solution should be made and subtracted (in modern instruments by the computer) from each analyte reading. Spectral interferences by matrix products are a much greater problem in graphite furnace than in flame atomization. In the ashing stage, one sometimes has to use a low temperature, not high enough to decompose all the matrix, to avoid loss of a relatively volatile analyte. Leftover molecular species will be present during atomization, and an incomplete breakdown during the brief atomization in the graphite furnace will result in broad molecular absorption bands. In samples containing halogens, stable halides with wide absorption bands can be formed, incompletely decomposed organic material can form absorbing molecules or particles which can scatter the light (the same as carbon particles from a much used graphite tube). In these (and other) cases, the use of a background corrector may be necessary. For some cases of furnace analysis the use of a chemical modifier can solve the problem (see Section 5.16). Chemical interferences are caused by various chemical processes, occurring during atomization, that alter the absorption of the analyte. Chemical interferences are more common than spectral interferences. The effect usually can be minimized by use of proper operating conditions. One type of chemical interference is ionization of analyte atoms due to high flame temperatures (mostly alkali and alkaline earth elements). While the effect is small in air-acetylene flame, it is very high in acetylene-nitrous oxide flame (may reach ranges of 8096, depending on the element) thus removing them from the ground state atom population (the ions will ahsorh at other wavelengths). One (not much used) solution is to work with a colder flame, but this may cause other problems. The usual remedy is the addition (to samples and standards) of high concentrations of an ionization suppressor, an easily ionized element which will saturate the flame with electrons and inhibit ionization (see Section 5.10). Another type of chemical interference is caused by the fact that in the hot environment during atomization, many dissociations and associations take place, among them, reactions leading to the formation of molecular species, for instance, metal oxides and hydroxides. Some of these are stable and their molecular bands are wide, bright and prominent in the spectra of the sample’s components (e.g. alkaline earth oxides). Changing the analytical conditions, for instance, working with a fuel-rich flame, may cause the dissociation of the oxide (see Section 5.10). Na in the presence of HCl can form NaC1, decreasing sodium atom concentration. The absorption of V


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may be enhanced by the presence of A1 or Ti in a fuel-rich flame. The concentration of oxygen and OH is relatively small, the other metals use a part of it and a part of the vanadium oxide is decomposed, increasing its atomic absorbance. In fuel-lean flames, there is enough oxygen so that the addition of A1 and Ti will not make much of a difference. In another type of chemical interference, low results will be obtained because of reactions (in the flame) of the analyte with anions forming low volatility compounds. For instance, calcium in the presence of phosphate will form a compound which is difficult to break down. Calcium chloride is volatilized and dissociated easily. Two solutions with equal concentrations of calcium will absorb different amounts of radiation, depending on the anion. Calcium nitrate in solution together with aluminium nitrate may form calcium aluminium oxides in the flame, which may dissociate only at very high temperatures. A good knowledge of chemistry is sometimes needed to understand the nature of the problem. In some cases the use of higher temperatures may cause dissociation of the compound, thus solving the problem. In most cases the addition of a releasing agent is needed. Lanthanum will bind the phosphate, when added in high concentrations, thus releasing the calcium (see Section 5.10). Sometimes the addition of the interfering ion (in about the same amounts as in the sample) or the addition of the main matrix components (in about the same amounts as in the sample, e.g. silicon and/or aluminium in the case of some geological samples) to the calibration standards, or the use of the standard additions method may solve the problem. Sometimes extraction of the analyte element or the interfering element may be needed. Usually the ion is extracted into an organic solvent. If the interfering ion is extracted, a quantitative extraction is not always necessary, just to remove most of the interfering species. In the determination of trace elements in geological samples, it is best to avoid interferences by getting rid of the silicon (evaporating with HF) but unless a specific compound of an analyzed element with silicon can be formed, the separation does not have to be quantitative,just removing the bulk of the silicon. The same may be applied to the extraction of iron into isobutyl acetate as the chloride complex, in the determination of trace metals in iron ores. If the analyte has to be extracted, care must be taken to ensure complete removal (for instance, an exact pH, a specific organic extractor) and the method should be checked with solutions of known analyte concentrations.

5.15 Instrumental background corrections For a good background correction, one must measure the background in the same wavelength as that of the analyte and at the same time. All commercial instruments measure the analtye and the background one after the other (a very short time difference) and not simultaneously. The time difference between the two

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measurements is very important, especially in furnace analysis and should be as small as possible (ideally none) as the backbround can change very fast (a fraction of a second). All background corrections are carried out by making one measurement of total absorbance (analyte and background) and one measurement of background only and then subtracting it from the total (one can read the total, the background and the analyte absorptions). As hackground can change with time, one could measure the background before the absorbance peak and after the peak and use the average. This may work only if the change is linear and no one can guarantee that. Four background correction methods were used in commercial instruments, of which one is not used anymore, one is rare, and only two are widespread, the continuum (deuterium) correction and a correction based on the Zeeman effect. The oldest, cheapest (and not used anymore) is the two line correction method. At one wavelength the total absorption (analyte and background) is measured and at another wavelength, as close as possible to the analyte line (but not absorbed by the analyte) the background absorption is measured, assuming that the background is the same for both wavelengths. The second line may be another line from the same hollow cathode lamp (A1 HCL, first line 3093 A, second line 3070 A), from the filler gas of the same HCL (Ba - 5536 A, second line Ne - 5409 or from another HCL (NA - 5890 A, second line Mo HCL - 5883 A). There is a big time difference between the two measurements, depending on how fast one can change wavelengths (may reach a few seconds) and the hackground may change). This means that the method can he used for flame atomization and not for furnace analysis, as two atomization heatings will he needed and the backgrounds may differ. Two monochromators can be used simultaneously but this will complicate the instrument structure and will be expensive. Another method, the newest, (Smith-Hieftje), rare with commercial instruments, is based on self reversal [15]. An HCL needs a certain small amount of current to emit radiation at precise, narrow wavelengths. At higher currents the emission lines are broadened and at still higher ones there is a high concentration of the lamp’s element unexited atoms, which absorb a great part of the desired wavelengths (see Section 5.8). The emission peak will look like a saddle and the result is that the source emits light at wavelengths other than expected (i-e. those absorbed by the analyte) close to and on both sides of the desired wavelengths. This liability is used for hackground correction. The hollow cathode lamp is cycled through periods (each precisely timed) of low and high currents. At low currents the spectrum (background and analyte) is determined, while at high currents the background only is determined and then subtracted from the total. The method requires special HCLs (to prevent arcing at high currents) and an electronic system. The most widespread method is the deuterium background correction. This needs a deuterium lamp to emit continuous radiation (see Section 5.8). The output of the

A)


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deuterium lamp is quite low at wavelengths above about 350 nm and this type of correction is not used above the ultraviolet region. The principle of this method is that the slit width is kept sufficiently wide. Absorption line of the analyte is typically very narrow in relation to the total spectral bandpass of the monochromator so that the background absorption of the sample may be measured using a suitable continuum source, but the fraction of the continuous source that is absorbed by the atoms of the sample (the analyte absorption line) is negligible. The radiation from the continuous source and the HCL are passed alternately through the analyte vapor (in the flame or the graphite tube). Measurements of the total absorption are made with the analyte HCL while measurements with the deuterium lamp give the background absorption only (which is subtracted by the instrument’s computer). The HCL and the deuterium lamp must be aimed at exactly the same line along the flame, as the sample vapor may be very inhomogeneous. This alignment used to be a hard task but in modem instruments the lamp is installed at the factory and only small adjustments are needed before use. When the background is not a continuum but consists of a fine structure molecular spectra (structural background), errors may occur with a continuum source, as a narrow line may overlap the analyte resonance line which may then contribute a big part of the measurement. There are only a few such cases. If this happens, one should make sequential measurements (and subtraction) of the absorption of the sample and of a standard solution whose composition with respect to the interfering species is very carefully matched to that of the sample (but does not contain the malyte). Another type of background correction [16-1 81 is based on the Zeeman effect (the splitting of spectral lines in a strong magnetic field). The advantages are: correction for spectral interferences, correction over the complete wavelength range, correction for structural background, correction for high background absorbances (2 compared to 0.7 absorbance with deuterium correction) and the use of a single line source with no possibility of misalignment. Less sample preparation (to reduce the concentration of materials which can cause interferences) is needed. This type of correction is also used particularly with graphite furnaces. This correction requires a strong magnet, a polarization device, etc. and these cannot be bought as accessories and attached on an existing commercial instrument (unless one wants to build his own instrument). One has to buy an electrothermal atomic absorption spectrometer manufactured for this type of correction. This is a very expensive instrument which is justified only in special cases, like the determination of ultra trace metals in drinking water (water standards), or the determination of trace metals in a difficult, problem causing matrix like seawater (high background because of reflecting salt particles) or blood, urine, milk, etc. (where the decomposition of the organic matrix may lead to the formation of absorbing molecules and

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very high background). For most analyses, deuterium background correction (alone or combined with the use of a chemical modifier or with extraction) is enough. There is a normal Zeeman effect (where both terms involved in the transition are singlets, or the values for both Lande g values are identical - some transitions of Ca, Cd, Ba, Be, Mg, Sr and Zn) and an anomalous Zeeman effect (where the terms are not singlets and the g values are not equal, for instance, other Zn transitions) where more splitting occur, either a relatively simple (Ge, Pb, Sn) or a very complicated (Cr, Mo) splitting. We shall deal with the simplest case of a singlet transition, a case of a normal effect. In this case the spectral line is split into three, one central (n)at the original wavelength, with half the intensity of the original line and polarized parallel to the direction of the magnetic field, and on each side a line (+a and -u,not at the analyte wavelength) each with a quarter of the intensity of the original line and polarized perpendicular to the magnetic field. The background is insensitive to the magnetic field. There are several methods to use this splitting for background correction. In all commercial instruments the magnetic field is perpendicular to the optical path (transverse). In most instruments the magnet surrounds the graphite tube. In this method, correction is performed at the resonance line, not at a slightly different wavelength, an important advantage. In another type, the magnet surrounds the hollow cathode lamp and the correction is at a wavelength slightly different from that of the analyte. In the case of a corrector based on the Zeeman effect where the magnet surrounds the HCL, the light emitted from the HCL (and not from the analyte atoms) is split into n and a components. A rotating disc transmits alternately the n and u components of the radiation. With the n component, the total absorbance and with the a component, the hackground absorbances are measured. In this method, the u component does not read the background at the same place that the n component reads the analyte (hut at the edges of the analyte peak, depending on the amount of separation caused by the magnetic field). In one method the graphite atomizer is surrounded by a permanent magnet that splits the analyte line (again, the simple case of splitting into a 7r and two u components). Unpolarized light from the HCL is passed through a rotating polarizer which separates the beam into two plane polarized waves, perpendicular to one another. These waves pass into the graphite tube. During that part of the cycle in which the radiation is polarized parallel to the field (n),the total absorbance is measured. When the radiation is polarized at 90"to the field (a),absorbance of the background (caused by molecular absorption and by scattering) is measured and subtracted from the total. In another method where the magnet surrounds the graphite atomizer, a polarizer is inserted in the optical path to remove the 7r component. The measurements are

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very fast (100 measurements per second). The total absorbance is measured with the magnet off (no splitting). The background absorbance is measured with the magnet on (acomponents only and the K component removed). A number of background measurements before and after total absorbance measurement and a polynomial interpolation are used to find the background at the time the correction should take place. In Zeeman atomic absorption there may be a curve rollover, where the curve of absorption vs. concentration rises, reaches a peak and falls again. One absorbance reading will fit two concentrations. To avoid the problem, the concentration working range is limited, depending on field strength (the maximum absorbance is predetermined by the manufacturer and can be changed by the user after investigation of high concentration standards) and the computer will indicate when the maximum permissible absorbance is acceded. 5.16 Modifiers When an analtye presents a problem due to its high volatility, or because the analyte and matrix volatilize at similar temperatures, the use of chemical modification should be considered. In some cases chemical modification will allow ashing at higher (or atomization at lower) furnace temperatures, getting rid of the matrix without loosing a relatively volatile analyte (or atomizing the analyte without the matrix). A good knowledge of chemistry or a study of a chemical handbook will help to determine if the analyte forms a compound that has a high melting point, its decomposition temperature and intermediate compounds and to decide if the addition of the proper atom or anion may help solve the problem (i.e. shift the atomization temperature to prevent atomic and background absorption at the same time, reduce the volatility of the analyte to allow a higher ashing temperature to remove all materials that produce background absorption, form a single compound of the analyte to ensure a single large atomic peak and not several small peaks due to the decomposition of several compounds). The modifier can be premixed with the sample or added to the sample in the furnace (a must if it will form a precipitate). A few examples will illustrate the modifiers effect. In the analysis of metal traces in seawater, NaCl has a high background absorption. One can add ammonium nitrate which will react with sodium chloride and get rid of the chloride as the volatile ammonium chloride. Addition of a large excess of phosphoric acid to Pb, Cd and Zn will raise the temperature corresponding to the maximum of the atomic absorption peak (peak atomization temperature). The addition of EDTA to lead will result in a complex that will atomize at lower temperatures than a nitrate or chloride matrix. The peak atomization temperature for lead in 1% NaCl is 9 10 C, with 0.5% EDTA 560" C and with 5% phosphoric acid 1180"C. This may become very useful O

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when one must shift the atomic peak from a coincident non atomic absorption. Nickel is typically used to stabilize arsenic and selenium to higher temperatures (by forming metal selenides). A relatively new matrix modifier is palladium (introduced to the furnace as Pd solutions, 50-1000 mg/l, preferably as nitrate). If the solution contains oxidizers such as nitric acid, sodium sulfate and sulfuric acid, Pd becomes a poor matrix modifier. With the addition of a reducing agent such as hydroxylamine hydrochloride or, better, ascorbic acid, Pd can raise ash temperatures by 400-800" C for several semimetallic elements and by 200-500" C for transition metals [ 19-21]. The reducing agent guarantees that the Pd will be reduced to the metal form early in the temperature program, before the analyte is volatilized (before this method was used, the Pd was preinjected and heated to 1000°Cprior to the injection of the sample). The metal retains the analyte element (believed to result from the formation of an intermetallic species) until a higher gas phase temperature is achieved. Scanning electron microscopy showed that the size and distribution of the Pd on the graphite surface (hence the effectiveness as a modifier) depend on the reduction method used. The development of a successful analytical method requires the analyst to carefully optimize the palladium concentration and temperature program. Increasing the palladium concentration shifts the atomization signal to higher temperatures, up to a limit (depends on the matrix) where broad irregular signals, with reduced sensitivity, are obtained. For clean water samples, 50-200 mg/l may be sufficient while for more complex samples, 200-1000 mg/l may be necessary. Ascorbic acid precipitates Pd so rapidly that palladium and ascorbic acid must be separately introduced into the furnace, so that the use of 5% hydrogen in 95% argon (a standard mixture that can be bought from gas suppliers) was recommended to be a better reducing agent [21].

5.17 Automation The different atomic absorption spectrometers on the market differ from each other in the amount and sophistication of automation, though several properties are common nowadays (in one form or another) in all modern instruments. Automation can relate to both analytical procedure (and instrumental parameters) and data processing (and reporting). Several examples of automation were described in former sections. The following is a partial list of examples of automation that can be found in AA spectrometers (by no means in all of them). Some operations need operator's choice and input to the computer and some are controlled by default values which can be overridden by the operator. Since the spectrometer performs sequential elemental analysis, unique analytical conditions can be established for each element. This can be done in a develop program mode (for completely new procedures) or modify program mode (for programs


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already stored in the computer). Automation may include provision for permanent storage and retrieval of complete analytical procedures. The computer commands the instrument to retrieve an analytical program, set up the proper physical (optical and atomizer) parameters, perform calibrations (ordinary or standard additions method, with automatic blank subtraction) and draw calibration graphs, analyze unknowns, draw absorption peaks, calculate means and standard deviations and print the results. Statistical data processing programs can be added or carried out through an external computer. The computer may inform the operator of various operational errors. Automation of spectrometer physical parameters may include monochromator wavelength, spectral bandwidth, controlling lamp turret position to select the desired lamp, lamp current, selection of the atomic line (and peaking), selection optimization and control of source intensity, selection of the continuum background source and its intensity, correction of background absorption throughout the ultraviolet and visible spectrum, using a double beam technique or Zeeman effect. Automation of atomizer systems may include selection of oxidant, determining and controlling gas flow rates, ignition and signal optimization (for flame atomizers), heating cycle program (for the graphite furnace) and operation of an autosampler (together with options for automatic analysis with hand held samples or non-automatic analysis with hand held samples). In electrothermal analysis, in addition to the possibility of programming the dry, ash and atomization stages, the computer can draw the appropriate absorbances together with the temperature curve and the atomization peak, making it easy to check the effectiveness of the cycle. There may be many options the operator may choose from. One can choose between measuring absorbance, flame emission or sometimes lamp emission (to check the lamp spectrum or work with a specific wavelength not for absorption purposes). One can choose an ordinary calibration curve, a standard additions calibration (including measurement and calculation of sample concentrations). One can choose the ordinary scale or scale expansion, remembering that scale expansion does not make the measurement more precise, as the noise is also amplified, just easier to read a very small absorbance. This is important if one wants to dilute the sample to overcome interferences. In these cases, where the noise is relatively high, the use of a higher HCL current may be justified, as the noise is inversely proportional to the square root of the current. The measurement can be peak height, peak area, integration (reading during a specified duration, e.g. 3 s, at 0.1 s increments and giving the average), or precision optimized measurement time, where the operator asks for a specified precision (for instance 1% relative standard deviation) and the computer reads only long enough to reach the specified precision. One can use sequence control to determine the order in which the elements will be analyzed automatically (each with calibration curve) and determine the last sample after which one can program an automatic turning off of the flame. Printing of results

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can be done after each reading (during run) or after all measurements (sequential or multielement). Automatic labeling - naming of samples can be used. Automation may include the possibility of editing analytical results, e.g. to disregard specific data points from the data processing procedures, or the possibility of calculating final concentrations in the sample, considering sample weight or dilutions. Automation makes life easier, the analysis simpler to carry out (also reducing operator’s errors), helps in sustaining precision and (maybe the most important) allows repeating analysis of samples at different times under almost identical conditions. A very important feature is automatic control of many safety precautions.

References 1 Skoog, D.A., Principles of Instrumental Analysis, 3d ed, Saunders, Philadelphia, 1985. 2 Mann, C.K., Vickers, T.J. and Gulick, W.M., Instrumental Analysis, Harper & Row, New York, 1974. 3 Strobel, H.A., Chemical Instrumentation, Addison-Wesley, Reading, MA, 1973. 4 Ewing, G.W., Instrument Methods of Chemical Analysis, McGraw-Hill Kogakusha, Ltd, Tokyo, 1969. 5 Slavin, W., Atoic Absorption Spectroscopy, Interscience, New York, 1968. 6 Kirkbright,G.E and Sargent, M., Atomic Absorptionand Fluorescence Spectroscopy, Academic Press, London, 1974. 7 Alkemade, C.ThJ. and Herrmann, R., Fundamentals of Analytical Flame Spectroscopy (translated from German), Adam Hilger, Bristol, 1979. 8 Analytical Methods for Flame Spectroscopy, Varian Techtron Pty, Ltd. 9 Analytical Methods for Graphite Tube Atomizers, Varian Techtron Pty. Ltd. 10 Analytical Methods for Atomic Absorption Spectrophotomelry, Perkin Elmer Corp. 11 Flanagan, F.J., Geo. Cosmo. Acta, 37 (1973) 1189-1200. 12 Abbey, S., 1977 Edition of “Usable” Values, Geological Survey of Canada, Paper 77-34. 13 Govindaraju, K., Geostandards Newsletter, 13 (July, special issue) (1989). 14 Pelly, I., Clin. Chim. Acta, 213 (1992) 51. 15 Maugh 11, T.H., Science, 220 (1983) 183. 16 Maugh 11, T.H., Science, 198 (1977) 3940. 17 Brown, S.D., Anal. Chem., 49 (1977) 1269A-1281A. 18 Fernandez, F.J., Myers, S.A. and Slavin, W.,Anal. Chem., 52 (1980) 741-746. 19 Voth-Beach, L.M. and Sharder, DE., Spectroscopy, 1 (1986) 49. 20 Voth-Beach. L.M. and Shrader, D.E., J. Anal. Atomic. Spectrometry, 2 (1987) 45.

21 Beach, L.M., Spectroscopy, 2 (1987) 21.

CHAPTER 6

Plasma optical emission and mass spectrometry

J.A.C. BROEKAERT University of Dorimund. Departmen1 of Chemistry. 0-44221 Dortmund. Germany

Contents 6.1

Atomic spectrometry with plasma sources . . . . . . . . . . . . . . . . . . . . . . . . 192 Historical development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Optical emission spectrometry . . . . . . . . . . . . . . . . . . . . . . . . 194 Plasma mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Plasma sources and sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 6.2.1 Arc and spark sources [7] . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 Flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 6.2.2 DCplasmajets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 6.2.3 6.2.4 Inductivelycoupled plasmas . . . . . . . . . . . . . . . . . . . . . . . . . 205 6.2.5 Microwave discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 6.2.6 Sample introduction for plasma spectrometry . . . . . . . . . . . . . . . . .209 6.2.7 Discharges under reduced pressure . . . . . . . . . . . . . . . . . . . . . . 215 Plasma optical emission spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . 217 6.3.1 Atomic emission spectrometry . . . . . . . . . . . . . . . . . . . . . . . . 217 6.3.2 ICP-Atomicemission spectrometry . . . . . . . . . . . . . . . . . . . . . . 220 MIP-Atomic emission spectrometry . . . . . . . . . . . . . . . . . . . . . . 223 6.3.3 Glow discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 6.3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 6.3.5 Plasma mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 6.4.1 ICP mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 Glow discharge mass spectrometry . . . . . . . . . . . . . . . . . . . . . . 239 6.4.2 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Power of detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 6.5.1 6.5.2 Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 6.5.3 Economic aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 6.1.1 6.1.2 6.1.3

6.2

6.3

6.4

6.5

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6.1 Atomic spectrometry with plasma sources 6.1.1 Historical devdopnrertt

Atomic spectrometry is the oldest instrumental method of inorganic analysis and in its origins dates back to the mid 19th century. In that time Bunsen and Kirchhoff established the spectroscopic characterization of elements by optical observation of the occurrence of typical atomic lines of the elements when spraying solutions in a flame. Apart from the observation of the emission, also absorption of elementspecific lines by an atomic vapour of the respective element was mentioned. These experiments are the basis of modern atomic emission and absorption as standard methods in optical atomic spectrometry. However, the experience gathered with the introduction of the solutions also gave rise to the overwhelming number of techniques we now have for sample introduction. Optical atomic spectrometry led to the discovery of several elements and became really important for elemental analysis when new sources were brought into use. Indeed the geometry and the stability of flames were excellent for analytical work but the low temperature (below 2500"K) was a serious drawback, as it hampered the sensitivity for a high number of elements and also led to the occurrence of a number of serious interferences. The availability of electrically generated so-called plasma sources changed this situation. With these sources a high temperature is achieved (above 5000" K) in a chemically inert environment, such as with one of the noble gases, and also the ionization is considerable. The development of arcs and sparks enlarged the applicability of optical atomic spectrometry to the analysis of solids. Both the survey analysis of non-conducting as well as electrically-conducting solids by DC arc methods and the analysis of metallic samples by spark analysis started to become important and remained of interest up to now. New impulses nowadays are coming from the electrically-generated plasmas treated in this chapter. They comprise the inductively coupled and the microwave plasmas as well as the low-pressure discharges and the laser sources. The first are mainly of use for chemical analysis, whereas the latter are applicable to electrically-conducting and non-conducting samples and became of interest both for bulk and for microdistributional analysis of solids. The development in atomic spectrometry not only was initiated by the research done on radiation sources but also profited enormously from the development in spectral isolation. Indeed, in the early decades of atomic spectrometry, prisms were the only way of producting optical spectra. The availability of high-quality diffraction gratings opened the way to highest linear reciprocal dispersion and a linear resolution, as they were asked for by new problems in the analytical chemistry of lanthanides and actinides, for instance. As a result of this development, we now have powerful spectrometric systems allowing sequential as well as simultaneous determinations. Also the developments on the detector side were of great importance. Indeed, at the beginning of optical atomic spectrometry only spectroscopic

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

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techniques and later on, photographic detection were available for the detection of radiation. The photographic plate, which is difficult and laborious to process and, moreover, of low precision, still has the broadest coverage of information. The spectrometers with photomultipliers as radiation detectors, in this respect, are of low flexibility and as single-channel detectors, are certainly very limited, however, they allow very precise radiation density measurements and deliver signals which can be directly processed in interfacing with computers. Therefore, they now are standard devices in commercially-available sequential and simultaneous spectrometers. With the development of the low-intensity radiation detection systems stemming from astronomy and space research, the use of multichannel detection came up and is still going on. It resulted in charge injection and charged coupled devices, which offer sensitive techniques for multiwavelength detection and are already available in commercial plasma spectrometers. Apart from optical emission spectrometry, atomic absorption appeared on the horizon in the beginning of the fifties. Flame atomic absorption spectrometry is still a standard technique in the routine analytical laboratory. Especially, however, with the use of electrothermal atomization, as it has been introduced by the work of L'vov and of MaBmann at the end of the sixties, atomic absorption developed to the most sensitive method in optical atomic spectrometry. The same level of power of detection also was obtained with atomic fluorescence spectrometry. This was especially the case when the furnace was used as atom reservoir and when saturation of the excited level was obtained, as it became possible through the use of tuneable dye lasers. This method might gain interest for the analytical chemistry with the rapid expanding availability of diode lasers. Both atomic absorption and atomic fluorescence spectrometry, though very sensitive, remained from their principle monoelement methods and from this side there was continuous motivation to develop other spectrometric methods with multielementcapacity but with a power of detection being similar to that of electrothermal atomization atomic absorption or laser atomic fluorescence. In the early eighties, plasma mass spectrometry was introduced and in one decade it developed to a mature method. This was possible as a result of the availability of relatively inexpensive but high-quality quadrupole mass spectrometers, on one side, and thesnormous experience available with classical mass spectrometric techniques such as spark source mass spectrometry and thermionic mass spectrometry, on the other side. Plasma optical emission spectrometry and plasma mass spectrometry are two fully developed and powerful atomic spectrometric methods for multielement determinations. Through the varieties of sources which can be used as radiation or ion source, and by the wide range of devices available for the introduction of the sample today, suitable analytical procedures for very diverse problems can be elaborated on the basis of these methods.

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6.1.2 Optical emission spectrontetiy 6.1.2.1 Optical spectra Optical emission spectrometry is based on the excitation of atomic spectra having their wavelengths in the range of 150 to 800 nm. Its principal is based on the measurement of the electromagnetic radiation emitted during transitions between the outer electronic energy levels of the atom. The electronic energy levels are characterized by the different quantum numbers, and thus the energy difference between these levels can be expressed in terms of them. In the case of the main quantum numbers, the relation is

A E = (27rmZ2e4/hz)(l/n; - 1/12;)

(6.1)

with m the mass of the electron (9 x 10-28g), 2 the nucleus charge, e the elementary charge ( 1 . 6 ~ erg s) and n the main Clb), h Planck’s constant (6.67 x quantum numbers ( q = 1,2,3,. . . and n2 = 2,3,4,5,. . .). When excitation from the ground level to an excited level and a subsequent decay of the excited level takes place, electromagnetic radiation is emitted with a wavelength given by Planck’s Law:

E, = E2 = hc/X or hu

(6.2)

cis the velocity of light (3 x 101Ocms-l), X is the wavelength and v is the frequency (in s-I). A complication of the atomic transitions arises from the fact that in Bohr’s atomic model further quantum numbers were required to account for different interactions. The 1 quantum number reflects the quantization of the angular momentum component along the electron orbit, the magnetic quantum number rnl accounts for the quantization of the momentum component along the direction of an external magnetic field and the spin quantum number s for the electron spin. In the case of atoms with more than one electron, the L-S coupling further increases the number of atomic terms and finally, both for the neutral atoms as well as for the ions of different charge, a complex term scheme exists. The complexity of such a scheme is shown for the sodium atom (Fig. 6-1). For most atomic and ionic species the term schemes are available in atlases (see tables published by Grotrian [I] and by the NIST or the former NBS [2]). The atomic and ionic spectra of the elements thus consist of a high number of lines with wavelengths from the VUV to the infrared region. These line spectra are the finger prints of the respective chemical elements. The spectral lines not only have well-defined wavelengths but also have defined physical widths. They are the result of several broadening processes:

1. Nutirrul broudening: resulting from the Heisenberg uncertainty principle. This contribution relates to the lifetime of the excited level and is normally at the 0.01 pm level;


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2. Doppler broadening: stemming from the velocity component of the radiating particles in the observation direction. This contribution is important especially in sources with high kinetic temperatures for atoms and ions; halfwidths of several pm may result of it;

3. Teniperaturebroadening: being due to the interaction of the radiating particles with other particles. It increases with pressure and accordingly in sources at atmospheric pressure, broadening up to several pm may occur, whereas in low-pressure discharges this effect is much smaller; 4. Other sources of line broadening such as isotope broadening, giving rise to more or less resolved hyperfine structures, resonance broadening resulting

Figure 6-1. Energy level diagram for sodium (reprinted with permission from Ref. 111

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196

from interaction of radiating and non-radiating species of the same element and Stark broadening resulting from the interaction of the electrons with the magnetic field in the plasma. The importance of these different components for the so-called physical line widths is well-reflected by high-resolution records of some rare earth lines obtained for the inductively coupled plasma (ICP) (Fig. 6-2) [3]. It should be emphasized

I11

I21

(3)

I&)

I51

16)

17)

Figure 6-2. Line profile of some rare earth atomic emission lines [3]. Photographic measurements. Theoretical resolving power Ro: 460 OOO, (1): Ho I1 345.6 nm, (2): Er I1 369.2 nm, ( 3 ) Yb I1 369.4 nm, (4): Y I1 371.0 nm, (5): Tm I1 376.1 nm, (6): Eu I1 381.9 nm, (7): La I1 398.8 nm

that these line widths differ from one radiation source to another, as a result of differences in temperature and pressure, as well as in the predominant excitation processes. The intensities of the spectral lines are related both to the properties of the discharges as well as to the number densities for the different atomic species, as written in the notations used in Ref. [4]:

Iqp= nqhvAq, Asp is the Einstein transition probabilityfor spontaneousemission, being the inverse of the lifetime of the excited state. In the case of an allowed transition, the latter is of the order of lO-'s, whereas when no allowed transitions start from the energy level, values may be up to some lo-' s. Provided so-called thermal equilibrium exists, the number density of excited atoms in a certain state is given by Boltzmann's law, namely: ng = ng,/Z(T)exp(-E,/kT) (6.4) g4 is the degeneration of the excited level ((2j + l), with j being the total quantum number defined as 1 f s), Z ( T )is the partition function for the species of a certain

level of ionization, Eq is the excitation energy of the excited level q (in eV) and


197

k is the Boltzmann constant (1.38 x 10-l6erg.s). When taking the possibility of ionization into account the ion and atom densities should be expressed as: ?a+ = Q

. 12 or 11 = (1 - a). 11

with Q the degree of ionization. The lifetimes of the excited levels are considerably dependent on the oscillator strengths of the lines themselves but also on the energy exchange processes possible. They may comprise:

1. Excitation by electron impact;

2. Excitation by collisions of the 1st kind with neutrals; 3. Excitation by collisions of the 2nd kind with excited species; 4. Excitation by absorption of radiation; 5. De-excitation by collisions with electrons;

6. De-excitation by collisions with neutrals;

7. De-excitation by spontaneous and stimulated emission of radiation. Further, the temperature of the plasma is included in Eqn. (6.4). Finger-print spectra of each of the elements will include the sequence of the characteristic line wavelengths. The line widths and relative intensities are also related to the atomic terms, but in addition, include influences of the radiation source. Therefore, the availability of atlases displaying the elemental spectra for particular atomic emission radiation sources is of prime importance for analytical atomic emission spectrometry. Such atlases have been made available for arc and spark sources and, however, still largely incomplete, are being made for modern sources such as glow discharges and plasma discharges. These complete finger-prints of the elements are the basis for an unambiguous qualitative analysis. Indeed, the presence of the most sensitive lines in the emission spectrum allows it to confirm the presence of an element in a given sample. 6.1.2.2 Optical emission spectral analysis As the emitted intensities are related to the element number densities Eqn. (6.3), the measurement of the intensities of the atomic spectral lines also allows quantitative determinations. Provided all dissociation steps required to generate the atomic vapour in the radiation source, the excitation processes and the ionization could be quantitatively described an absolute measurement of the atomic line intensities, would enable an estimate of the elemental number densities. However, the knowledge on these processes in the analytical radiation sources as well as of

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atomic constants and spectrometers are too poor for allowing a such absolute atomic emission spectral analysis. Accordingly, atomic emission spectrometry is a relative method of analysis, where a quantization is only possible after a calibration procedure with samples of known elemental composition and structure. The calibration procedure is an essential part of each optical atomic spectrometric procedure of analysis. It includes measurements of the intensities of the element-specific spectral lines for a number of well-characterizedcalibration samples with analyte concentrations covering the analytical range within which the concentrations of the unknown samples are likely to be, and with a matrix as similar as possible to the one of the unknown samples. The latter requirement depends on the interferences, which the matrix might induce in the case of a particular atomic spectrometric method and which are connected with the analytical accuracy of the method. In the case of wellcharacterized calibration samples with a matrix composition similar to the one of the samples to be analyzed are not available, combined analytical procedures including a separation of the analytes from the matrix and their matrix-free determination using synthetic standards, has to be used. Also, under well-defined conditions, mathematical procedures can be helpful for correcting some well-known differences in matrix composition between the calibration samples and the unknown samples. This may include the determination of the net analytical signals by stripping brutto signals from contributions of matrix line intensities determined by a separate procedure. Atomic emission spectrometry involves consequently a radiation source (for the development of this field, see Ref. [ 5 ] ) , into which a representative part of the sample is entered and in a first step atomized. In a second step, excitation of the atomic and ionic lines takes place. The first step mentioned requires the use of high energies, so as to atomize the sample material as complete as possible, the second step should be as selective as possible so as to obtain high power of detection and low matrix influences. Therefore, it can often be very useful to use so-called tandem sources, where both steps are performed in a separate and independently optimizable source. This concept which has been used throughout the development of atomic spectrometry has e.g. been described for modern sources in [6]. The radiation emitted by the atomic spectrometric source thus contains the element-characteristic lines for many of the elements introduced into the source but also the radiation emitted by the gas atmosphere. The radiation is conducted into the spectrometer with the aid of a suitable optical arrangement. One therefore can make use of lenses and minors and in addition, optical fibres. In spectrally resolving spectrometers, the radiation is brought as a parallel beam on a dispersive element, namely, a prism or a grating, and the spectrally resolved radiation including this at the analytical wavelength is measured with a multichannel multiwavelength covering detector (photoplates, diode arrays, etc.), or the radiation at the analytical wavelength only is isolated with the aid of an exit slit and measured with a photomultiplier. Accordingly, both flexible sequential determinations of several elements with a single-channel instrument


199

(monochromator) but also simultaneous multielement determinations with the aid of a multichannel spectrometer with exit slits prepositioned at a number of analytical wavelengths are possible. After suitable amplification of the photocuirents they are digitized and led to a data processing unit. Here calibration functions are calculated and for analysis the analyte concentrations are calculated from the measured intensities with the aid of the calibration functions eventually after matrix correction. Atomic emission spectrometry with spark excitation is now a standard method for production and product control in the metal industry [7].Glow discharge emission spectrometry for routine applications also is of great interest as it enables depthprofiling in solids, due to its layer-by-layer ablation. On the other hand, inductively coupled plasma emission spectrometry is now a routine tool besides atomic absorption spectrometry which is now present in almost any larger analytical laboratory. Apart from high-frequency discharges, microwave discharges and especially the microwave induced plasmas have also come under investigation as they proved to be low-cost sources which in some cases enable it to really achieve low absolute detection limits for the non-metals also. As from this assembly of methods (for a discussion see Ref. [S]) especially ICP-OES is of use for trace analysis in liquid samples or in solids, after dissolution it will be treated in detail in this chapter. Brief reference will also be made to the developments in MIP atomic spectrometry, when it comes to its coupling with special sampling techniques.

6.1.3 Plasma mass spectrometry The sources used for atomic emission spectrometry also enable the generation of ions for most elements. This is known from optical emission where, apart from the atom lines, sensitive ion lines may also be used in many cases. The ionization of an element in a high-temperature plasma, which is in local thermal equilibrium, is given by the so-called Saha-Eggert equation. With the symbols used in [4],the latter can be written as: logni . n,/?~,= 3/210gT - 5040/T Vi - log Zj/Z, - 15.56

(6.6)

where ni is the ion number density, n, is the electron number density, n, is the atom number density, T is the temperature in the plasma, V i is the ionization energy of the analyte (in eV), and Zi and 2, are the partition functions for the ions and the atoms, respectively. In elemental mass spectrometry, which in all its forms has been extensively treated in [9], the ions generated in a plasma are extracted from the source and lead into the high vacuum of the mass spectrometer via a suitable ion optics. After a separation or isolation of ions with different mass-to-charge ratio, signals for all ionic species can be obtained and detected.

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Ion sources used for elemental mass spectrometry were originally preferred to be operated at high vacuum so as to cope with the requirement of having a vacuum in the mass spectrometer below mbar. For such aims, high-vacuum sparks and arcs were very useful [lo]. This led to the development of spark source mass spectrometry as a technique for direct solids analysis or also for the analysis of dry solution residues. These techniques became commercially available with large double-focussing mass spectrometers. Originally one used photographic plate detection. Later on, however, photoelectric detection with electron multipliers and vidicon detection, including phosphors also became available. The method was very valuable for solids analysis especially in the case of high-purity metals and of semi-conductor materials, as the detection limits were in the sub-ng/g range. Apart from sparks, laser ablation has also been used and could be applied to the analysis of non-conductors even with a fair lateral resolution. With a Nd-YAG laser for evaporation and ionization, extremely sensitive microanalyses became possible in the LAMMA technique, where the absolute detection limits are down to the g level [ll]. The other very successful line in mass spectrometry for inorganic analysis made use of thermionic sources, in which the sample, which was generally a dry solution residue, was volatilized by heating from a refractory metal filament. This approach could be used for most non-refractory elements, as it has been shown in many applications, e.g. as described Heumann et al. [ 121, but also for refractory elements after volatile compound formation. The approach thus is extremely valuable for combination with extensive sample pretreatment involving matrix removal by chemical procedures,especially for micro- and ultratrace analysis. Whereas in spark mass spectrometry the analytical precision was rather limited (of 10 to 30%), thermionic mass spectrometry enabled high-precision work. This was especially true when applying isotope dilution techniques, which in the case of mass spectrometry, can be applied with stable isotopes, and has been impressively shown by high-accuracy work for u235/U238 ratio measurements in nuclear fuel, e.g. [ 131. Inorganic mass spectrometry went through a revival, as during the last ten years, high-quality quadrupole mass spectrometer instruments became available. Further reliable vacuum equipment now enables ion sampling from sources operating at higher working pressures. Thus it became feasible to use more sources known from atomic emission spectrometry as ion sources for mass spectrometry. The inductively coupled plasma was successfully introduced by Houk et al. [14] in 1981 and studies on the use of different types of glow discharges as ion sources started [15,16]. In these investigations much information known from diagnostic as well as from optimization studies performed for atomic emission spectrometry could be used and led to a rapid development of these techniques. To date, both inductively coupled mass spectrometry (ICP-MS) [ 171 and glow discharge mass spectrometry (GD-MS) [181 are important tools for extreme trace analysis, mostly for the analysis of liquid samples or for direct solid analysis, respectively. They will be treated in


20 1

detail in this work. Plasma mass spectrometry, due to its rather recent introduction, still will go through substantial further development. Here it can be expected that other types of mass spectrometers, in addition to other types of discharges, will gain interest in overcoming limitations of the method in its present form or to create possibilities for special application.

6.2 Plasma sources and sampling The term plasma in its physical meaning is a partially ionized gas. In analytical plasma spectrometry one mostly refers to those cases where the plasma is generated by the dissipation of electrical energy. The relevant sources (Fig. 6-3) can be divided into sources operated under reduced pressure and sources operated at atmospheric pressure [19]. In both cases, DC and also AC as well as high-frequency and microwave sources are used. As for their analytical use, the techniques applied for

-ARC

-+-

Q

-FLAME

,PLASMA

,LOW

SOURCES

PRESSURE DISCHARGES

-

ga,

GDL -GRAPHITE FURNACE

c ; L D ,LASER

PLUME

Figure 6-3. Sources for atomic spectrometry [8]

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J.A.C. BROEKAERT

introducing the samples are very important, a treatment of these techniques will be included in this chapter as well.

6.2.1 Arc arid spark sources [7] These sources have been used extensively at atmosphereic pressure and applied to the direct analysis of solids. Indeed, DC arcs have been used since the 1930s in combination with spectrography and since the 1950s with photoelectric registration. DC arc spectrometry thus became a very sensitive method for direct solids analysis. The method can be applied to metals, which then have to be brought in the form of chips and must be transferred into a cup. However, a rod can also be used directly as electrode or a powder can be analyzed after briqueting pellets. In the arc, currents of up to 20 A are applied and the burning voltage is below 60 V. The sample is volatilized by thermal evaporation and the analyte vapour is excited in the positive column of the arc located between the electrodes. The temperatures in this source are of the order of 5000"K, by which most of the elements vaporize. Reactions of the analyte with the electrode material (often graphite) or with the arc atmosphere may hamper the volatilization as then refractory carbides or oxides are formed. This can be avoided, however, by special precautions, such as halogenation or the use of noble gas atmospheres. Arc emission spectromery enables it to reach for many element detection limits down to the p/g level. Because of the volatilization of the elements by distillation, anions may cause matrix interferences and the elaboration of a reliable calibration may be very time-consuming. Therefore, DC arc spectrometry nowadays still has use for only semi-quantitative survey analysis. Spark sources are especially important for metal analysis. Here, the sample often is positioned in point-to-plane geometry and a tungsten or carbon rod is used as counter electrode. Sparks are generated in an R-L-C circuit, where the voltage over the capacitor is led to the electrodes. The R-L-C circuit is powered with AC or interrupted DC voltage. To date, medium voltage sparks (with voltage of 0.5-1 kV) often at high frequencies (1 kHz and more) are used under argon atmosphere. In order to keep the sparking times low, as it is required in on-line production control for steel, e.g. the preburn times are kept low by applying high-energy pre-sparking. Accordingly, spark analyses can be performed in as less as 30 s. For the simultaneous determination of a high number of elements including C, P, and S spark emission spectrometers are now available as large instruments, but also in portable form. They enable the direct determination also of trace constituents in electrically-conductive samples. For elements such as Mg, B, Cu a.o., the detection limits are at the pg/g level. By using intensity ratios of the analyte lines and a reference line (often a matrix line) as a function of concentration ratios, a high precision (better than 1%) can be obtained. For accurate analyses extensive sets of calibration samples must be used and mathematical procedures may be helpful so as to perform corrections for matrix interferences. Both in arc as well as in spark emission spectrometry

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the spectral lines used are situated in the ultraviolet (200-400 nm), in the visible (400-550 nm) and even in the vacuum ultraviolet (below 200 nm) region.

6.2.2 Flames Since the work of Bunsen and Kirchhoff in the mid 19th century, flames were used as emission spectroscopic sources [20] and they are currently being used as atom reservoirs for atomic absorption work [21]. Their possibilities for atomic emission spectrometry, however, are limited by their low temperatures. Indeed, the temperatures for the adpropane (2000' K), the airhydrogen (2300' K) and the aidacetylene (2400 K) flame, for the nitrous oxide/propane (2900' K), the nitrous oxide/hydrogen (2900 K) and the nitrous oxide/acetylene (3200 K) flame are well known [22]. Except for the latter case, only for elements with low excitation potentials such as the alkali elements, a sufficient excitation can be expected. In flame emission spectrometry the sample, which as a rule is a liquid, is brought into the flame by pneumatic nebulization. Here, both concentric and cross-flow nebulizers are used, which are now standard devices also for plasma spectrometry. Their origin goes back to work in the 19th century, where Gouy [23] first described a concentric nebulizer. Advanced methods for sample introduction are e.g. the use of the Delves cup [24], as a technique for dry solution residue analysis in atomic absorption spectrometry. A further possibility is explored by Berndt et al. [25], who used a combustion of organic material in an oxidant gas stream by heating with focussed radiation from a tungsten lamp and lead the vapors are led into a flame atomic absorption system. Both techniques show how innovative sample introduction still leads to improvements of mature methods. Despite its age, flame emission spectrometry is still one of the most sensitive methods for the determination of the alkali elements and is still widely used, e.g. for the determination of sodium and potassium in body fluids. Due to the flow temperature, the detection limits for other elements are insufficient for trace analysis work (much above the &ml level). Moreover, the low temperature is also responsiblefor matrix effects due to the formation of thermally stable compounds. In this respect the Ca-phosphate interference is well known. The latter hampers the determination of calcium and is based on the following reactions: O

O

O

Ca3(P04), e +P205 e CaO e + C e Ca+CO Here the formation of the stable Ca,(PO,), hampers the formation of atomic Ca vapors and thus causes signal depressions. The spatial and temporal stability of flames as well as the diversity of techniques which could be used for sample introduction, in fact, were an impetus for the rapid development of the plasma spectrometric analysis of liquid samples later on. The only point to overcome in flame spectrometry was the establishment of sources operating in a chemically inert environment and at higher temperatures.

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6.2.3 DC yiusniu jets Electrical DC discharges can be operated in argon or helium at atmospheric pressures and with currents of up to some A at a burning voltage of about 50 V. In such a discharge we have a so-called normal characteristic, as at increasing current the material evaporation increases and the discharge thus can be sustained at decreasing voltage. Most of the voltage drop occurs in the immediate vicinity of the cathode (cathode region) and the voltage drop in the other zones (positive column and anode region) is low. Such argon plasma discharges have temperatures of the order of 5000"K and are in local thermal equilibrium, as the kinetic energy distribution of all plasma constituents (electrons, ions, neutrals) are equal and thus the electron temperatures, the gas temperatures, the ionic temperatures and the excitation temperatures, as well. Two types of such plasma discharges exist, which have substantially different properties. In the current-carrying type the analyte material is introduced in the current-carrying region of the discharge. Here the excitation and the ionization of analyte constituents may greatly change as a result of small changes in the concentration of easily ionized elements. This is due to the fact that as the electron and ion number densities change, the burning voltage and the energy delivered to the plasma greatly change. When the aerosol loading of the plasma changes, its temperature and thus its conductivity would also change. However, the burning voltage would then immediately increase by which the temperature is adjusted automatically back to its original value. Thus, current-carrying plasmas are strongly influenced by changes of the alkali element concentrations in the samples, but are rather insensitive to changes in sample supply to the plasma. Such plasma discharges were widely used for the analysis of liquids and described by Korolev and Vain'shtein [27] and by Owen [28]. A second type of DC plasma jet is the transferred plasmas type. Here the sample is introduced into the plasma plume which lies outside the current-carrying region. In this type of plasma, easily ionized elements will hardly influence the electrical conductivity of the plasma and thus only cause minor matrix effects. Their only influence may lie in an ambipolar diffusion which causes changes in the plasma volume. Indeed, due to the analyte dissociation there is an overpressure in the central part of the plasma. As now the electrons diffuse with a higher velocity to the outer zones than the heavier positive ions, there is an additional field directed from the inner to the outer part of the plasma, by which the latter tends to increase its volume. A cooling of the plasma resulting from an increased sample supply will have a drastic influence as changes in plasma temperature do not couple back to the burning voltage and thus cannot be compensated for. Such a plasma has been described by Kranz [29] and has been used successfully for the spectrochemical analysis of solutions. Both types of plasma sources have been studied extensively. They have their own approach to cope with the problem of entering a cold aiialyte vapour into a hot plasma, which is always hampered by the high viscosity of the latter. These


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studies led to a commercially available three-electrode plasma [30], which is now a standard device for atomic emission spectrometry(Fig. 6-4). This source operates at

Figure 6-4. Electrode arrangement for three-electrodedirect current plasma jet

2 x 10 A current and a total argon consumption of below 8 I/min. Two gas-shielded carbon anodes with separated electrical supply and a common well-shielded cathode are used. The sample aerosol is introduced just below the point where both positive columns come together and is prevented from entering the cathodefall. Accordingly, the excitation in this source is very efficient. 6.2.4 Inductively coupled plasmas Due to the temperature restrictions imposed by flames and the possible contamination from electrode material encountered with arc sources and plasma jets, efforts were made to realize an electrodeless source with high temperature and high sampling efficiency for atomic spectrometric analysis. Such a source was realized with the indictively coupled high-frequency plasma (ICP), which was described for crystal growth studies by Reed 1311. This source was first used for atomic emission spectrometric analysis by Greenfield et al. [32] and by Wendt and Fassel[33], who both applied it for solution analysis and obtained detection limits in the ng/ml range. The ICP is operated in an assembly of three concentric quartz tubes (torch), in which argon flows (Fig. 6-5). The high-frequency energy is coupled into the argon with the aid of a working coil having 2-3 turns around the outer quartz tube. The argon is seeded with electrons with the aid of a Tesla discharge and at a frequency of 2-40 MHz and at suitable gas flows, a toroidal plasma is formed. In a tube with ca. 22 mm diameter, an outer gas flow of 6-10 ymin and a power of about 1-2 kW are required. A wet aerosol can be introduced with an argon carrier gas

J.A.C. BROEKAERT

206

data acquisition and processing

-

r.f. generator

ICP-torch

L

nebulization chamber

Figure 6-5. Instrumentation for ICP-AESusing convenlional pneumatic nebulization

flow into the ICP so that the analyte flow pierces the plasma along its central axis. This is the result of the skin effect, by which electrical currents only flow at the outer surface of the plasma while the internal part of the plasma is current-free. ICPs are operated normally with argon, however, at higher power, also nitrogen, oxygen or air can be used as outer gas flow, as shown by Greenfield et al. [32] and studied by Ohls et al. [34] and Broekaert et al. [35]. The high-frequency generator normally consists of an R-L-C circuit using triode tubes for power amplification, however, solid-state generators have also come into use. They became especially of interest since so-called low consumption torches, as studied by Rezaaijaan et al. [36], became available and the working frequency was increased from 27 to 40 MHz. The ICP is not completely in local thermal equilibrium. Indeed, the excitation temperatures determined from intensity ratios of lines originating from the same ionization level of an element are about 6000°K [37], whereas the rotational temperatures as determined, for example, from OH bands or N; bands are about 4000°K [38]. It was also found that the electron number density determined from the broadening of the H, line is 10'6cm-3 and the one from intensity ratios of an ion and an atom line only lOI4 ~ m - ~Moreover, . calculated ion to atom line intensity ratios were a factor of 100 smaller than the measured ones, which would demonstrate an over-ionization in the ICP. This discrepancy vanishes when ionization temperatures are taken for the calculation, as shown in a state-of-the-art paper by De Galan et al. [39]. Some of these facts could be explained by considering the relevant excitation and ionization mechanisms:


207

-

(a) Electron impact Ar + e

Ar*, Ar'and Aim (metastable levels of argon at 11 eV) M + e +M*,M*+

(b) Radiation trapping

Ar + hu

-

Ar'

(c) Radiative recombination M++e-M+hu (d) Charge transfer Ar'+M (e) Penning effect Af'+M

-

Ar+M+*

-

Ar+Mt*

The latter mechanism could explain the over-ionization of the analyte whereas the fact that metastable argon is easily ionized (ionization potential of argon is 15.9 eV, i.e. only 4 eV above the metastable levels) could explain the high electron number densities [40]. This would explain why ionization interferences caused by alkali elements are moderate [41] as the plasma is buffered with electrons. However, in more recent work where eIectron number densities and electron temperatures are determined with high spatial resolution and with the aid of Thomson scattering, it was shown that many of the effects described might also be due to the predominance of different mechanisms at different locations in the ICP [42,43]. Atomic emission spectrometry with the ICP now is a mature method in trace element analytical chemistry. Due to its analytical figures of merit and also because of the wide range of possibilities for sample introduction, which are treated in Section 6.2.6, it can be used for a wide variety of analytical problems and will be treated in detail. 6.23 Microwuve discharges

Microwave discharges are operated in the GHz region and have since long been applied in sealed tubes at low pressure. Here they served as line sources. They really became of interest when they also could be operated at atmospheric pressure as then sample introduction became more feasible. Microwaves are generated with the aid of a klystron (at a power below 100 W)or in a magnetron. Here a ultra high frequency field is applied perpendicular to a circular current which flows in a series of cavities so that at suitable frequencies, amplification by interference is obtained

208

J.A.C. BROEKAERT

and a high-power microwave can be transported with the aid of a coaxial cable and fed to a resonant cavity or to a wave guide. The part of microwave energy which is reflected to the magnetron can be minimized by realizing resonance conditions in the wave guide or in the cavity. Mavrodineanu and Boiteux [44] first realized a plasma at the top of an electrode through microwaves. An outer metal cylinder hereby served as grounded electrode. This capacitively coupled microwave plasma (CMP) (Fig. 6-3) was obtained with argon, helium, nitrogen or even air flows of as low as 2 l/min at a power between 200 and 600 W, it is not in local thermal equilibrium. This can be understood as the frequency is higher than in the case of the ICP. Therefore, the electrons follow fairly well such frequencies, however, not the much heavier ions. Major drawbacks of the CMP lie in its less favorable geometry with respect to analyte introduction and in the high matrix effects caused by alkali elements [45]. Electrodeless microwave induced plasmas can also be obtained. They became attractive for spectrochemical analysis, since Beenakker [46] described a TMol0 resonator in which, at a power below 100 W and with less than 1 Vmin of argon or helium, an MIP could be stabily operated at atmospheric pressure (Fig. 6-6). In

swfatron

Figure 6-6. T h f o l 0 resonator according to Beenakker [46] and surfatron [51] sources for MIPatomic spectrometry

this cavity the microwave power is introduced with the aid of a loop or an antenna. MIPS operated in a resonator according to Beenakker are very weak plasmas with respect to their atomization capacity. This hangs together with the low power


209

dissipated in these sources and the low gas temperatures. Indeed, when taking the rotational temperatures as an approximation, one should consider that values of only 2000'K are obtained [47]. When a capillary with a diameter of 1 mm is used as discharge tube, a filament-type plasma is obtained. However, with wider tubes also multi-filament and even toroidal plasmas can be obtained, as described by Kollotzek et al. [48]. The latter could only be obtained, however, with a wet aerosol. Also by the use of multitube torches with threaded inserts a toroidal plasma could be obtained [49]. With higher power, also MIPs operated in nitrogen have been described [50]. Apart from the cavity described by Beenakker, other devices were also useful for sustaining MIPS with good analytical properties. Hubert et al. [5l] described a so-called sui-fatron (Fig. 6-6). Here the microwave energy is coupled into the discharge with an antenna, which causes a surface wave to propagate through a slit between the quartz discharge tube and the front plate of the device along the plasma surface. This type of MIP easily forms multifilament and quasi-toroidally shaped discharges, which are less influenced in their stability by the entrance of an analyte in the plasma. They can be operated in argon and also with helium, the latter, however, with more difficulties than the cavity according to Beenakker. The spectroscopic properties of the surfatron have been investigated by Moussanda et al. [52], who performed rotational temperature measurements and found values on the order of 2000' K. Further types of microwave discharges were produced in the so-called strip-line cavity [53] and in the microwave plasma torch (MPT) described by Jin et al. [54]. In the latter, both argon and helium discharges can be obtained. The device can be made wholly of brass and the microwave power is coupled into the plasma by an antenna. The latter has a ring at its end, which glides over the central tube through which the analyte is introduced. Microwave discharges can also be obtained at high power and with a torch similar to the one used in ICP-spectrometry. For power coupling, Leis and Broekaert [55] described the use of a rectangular wave-guide with a hole, through which a threetube quartz torch was positioned. With such a device, MIPs could be sustained with argon and nitrogen at powers of up to 500 W. The field of microwave discharges itself is an actual field of research; many different plasma arrangements may be obtained through widely varying shape and volume, within a wide range of power and with different gases, Therefore, MIPs together with a suitable sample introduction can be tailored to solve special analytical needs rather than be a general purpose work horse for routine analysis. 6.2.6 Saniple introductionfor ylusniu syectsonietry Sample introduction techniques are a very important link in an analytical plasma spectrometric method, as they enable it to optimally use the potential of the method,

J.A.C. BROEKAERT

210

in view of the nature of the sample to be analyzed, i.e. its state of aggregate, its volume, its homogeneity, etc. Accordingly, the choice and optimization of the sample introduction techniques to be used must be made very carefully. For sample introduction in atomic spectrometry, a wide variety of techniques exists (Fig. 6-7) and they have been the subject of many studies (for its treatment see Ref. [56,57]). Here a brief review and discussion of the techniques used in combination with plasma discharges operating at atmospheric pressure will be elaborated. For the analysis of solutions, pneumatic nebulization and introduction of the aerosol into the plasma is well-known from flame emission and flame atomic absoiption spectrometry. For plasma spectrometry with an ICP, DCP or MIP, the pneumatic nebulizers that can be used include the concentric nebulizer, the cross-flow nebulizer, pneurnotic nebulization

7 t

-Thermal spmy

- Ultmsonic

nebulizotion

- Electrothermol

U

evaporation

Furnoce

L

Rloment

-Hydride

w-

- Electroerodon

Loser ablation . .

- Direct sample insertion Y

m-tt Figure 6-7. Sample introduction techniques for ICP atomic spectrometry

21 1


1.

SAMPLE

A

F

t

G

ARGON I

Y

h

II

SAMPLE

3.

I

MPLE

Figure 6-8. Pneumatic nebulizers for plasma atomic spectrometry [56]. (1) concentric glass nebulizer; (2) cross flownebulizer;(3) Babington-typenebulizer: (4) fritted-disc nebulizer

the Babington nebulizer and special types such as the fritted-disc nebulizer (Fig. 68) [56]. The concentric nebulizer, which in the case of ICP-spectrometry, often is made of glass or of quartz and is known as Meinhard nebulizer, is self-aspirating. All other types make use of forced sample feeding, as it is possible with a peristaltic pump. The liquid is nebulized by a high-velocity gas flow through a nozzle which splits off small volumes of liquid from a liquid flow as a result of viscosity drag forces. Pneumatic nebulization results in the formation of droplets of which the particle size can be described with the Nukuyama-Tanasawaequation [58]. where vG is the gas velocity, QG is the gas flow rate, QLis the liquid flow rate, 7 is the viscosity, Q is the surface tension of the liquid and c', c" and c"' are constants. With an increase of the gas pressure and the gas flow the Sauter diameter, 4,i.e. the diameter of the droplets of which the surface to volume ratio equals that of the entire aerosol, decreases. The aerosol then becomes finer and its loading also increases. Both improve the sampling capacity and the power of detection obtained but also make the sampling less dependent on the physical properties of the solutions, by which the nebulization effects decrease. Most of the nebulizers consume about 1 ml/min of analyte solution and are operated with a nebulizer gas flow of 1-2 l/min, by which residence times in the plasmas in the ms range can be obtained. At these gas flows the nebulizers already obtain their maximum efficiency, which is in the 1-3% range. The large droplets are removed by placing the nebulizer in a nebulization chamber, of which the form and

212

J.A.C. BROEKAERT

eventual impact surfaces must be optimized for the type of nebulizer used. Then the mean droplet size is in the pm-range. The cross-flow nebulizer is less sensitive to clogging, in the case of solutions with high salt contents. With the Babington nebulizer this also applies and even suspensions and slurries can be nebulized, as first described by Ebdon and Cave for atomic absorption spectrometry 1591. Nebulizers made of PTFE are now available for work with solutions containing hydrofluoric acid. For the case of organic solutions, special attention must be paid to the tubings used, but also to the clean-up times. For aqueous solutions and glass spray chambers they are in the 30 s range, for organics and especially in the case of PTFE, which is difficult to wetten, they may be considerably longer. From the early beginning of plasma spectrometry ultrasonic nebulization has also been used for aerosol generation in the case of solutions (for a report on the state-of-the-art, see Ref. 1601). Here the sample liquid is brought on the transmitter of an ultrasonic vibrator operated at up to 1 MHz, by which for the case of aqueous solutions, an aerosol with a mean particle size around 5 pm is formed. The latter occurs as a result of geisers built as droplets split off from the vibrating soIution surface. Due to the small droplet size, aerosol generation efficiencies of up to 30% are possible and thus increased sensitivities as compared to pneumatic nebulization can be expected. However, in the case of solutions containing high salt contents, deposits may be formed which shorten the lifetime of the transducer. Further, memory effects occur and accordingly, the clean-up times a e long. The overall aerosol generation constancy is inferior to the one of pneumatic nebulization. Very promising is the use of high-pressure nebulization, as described by Berndt [6 11. Here the liquid is supplied by a high-pressure pump as used in high-performance liquid chromatography and nebulized directly by the pressure drop at a nozzle and without use of a nebulizer gas. The aerosol particle size is low and the aerosol production efficiency high [62], also for viscous solutions. The system, because of its operation at high pressure, is ideally suited for use in speciation work, when ICP-spectrometry is used as an element-specificdetector in HPLC. Partially similar features can also be expected from thermal spray systems, as described for use in ICP spectrometry [63]. In all three systems and for the analysis of solutions, a desolvation of the serosol may be required, so as to bring the solvent loading of the aerosol to a reasonable level and this in turn may introduce interferences and memory effects. For the elements which form volatile hydrides such as arsenic selenium, etc., hydride generation, as known from atomic absorption spectrometry, may be used to bring the analyte in an isolated form and to deliver it with a high efficiency to the plasma. In the case of the ICP, flow-cell hydride generation (Fig. 69) [64] may be used to generate the hydrides, as the excess of hydrogen can be tolerated by the plasma. Here detection limits for As, Se, Sn, Ge, etc. are in the 0.5-2 ng/ml range [65]. In the case of weak plasmas such as the MIP, the excess of hydrogen must mostly be removed and an isolation of the hydrides by freezing

213

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY to

solution

If I waste Figure 6-9. Flow-cell hydride generator for ICP spectrometry [641

them out and a subsequent sweeping into the plasma may solve the problem, as it was already described by Fry et al. [66] for the case of the ICP. In the case of microsamples, electrothermal evaporation, as known from AAS, may be of use. The samples may be evaporated from a graphite rod [67], from a graphite furnace [68], or from a tungsten filament [69]. Here, the consumption of the whole sample volume and the high sample introduction efficiency generally allow it to obtain lower detection limits than in pneumatic and in ultrasonic nebulization (Table 6- 1). Moreover, as a dry aerosol is generated, the approach is very valuable also for the low power MIPS [70]. In the case of direct solids sampling, a number of approaches have been shown to be useful for plasma spectrometry. One aimed at the same power of detection and ease of calibration as in the case of solution analysis is described. For the direct analysis of powders, slurry nebulization can be used, however, limitations arise from the particle size which may cause difficulties in the nebulization process, but also in the subsequent thermochemicaldecomposition processes in a plasma, as discussed in the case of ceramic powders in Ref. [71]. Powders as well as metal chips can also be directly analysed by insertingthem directly into a high-temperature plasma (Fig. 6-10) [73]. As then, the sampling efficiency is very high, the power of detection can be expected to be the largest of all sampling techniques. This can be expected both for dry solution residues [72] and for impurities in powders [73]. In the case of electrically conductive samples, spark ablation may be very useful, and was proposed already in early work on ICP spectrometry [74]. Here, an aerosol having a low particle size (in the pm range) may be produced and for a wide variety of samples, selective effects are low [75]. In the case of electrically non-conductive samples especially, laser ablation is very useful. Both when using the laser plume

2 14

J.A.C. BROEKAERT

Table 6-1. Detection limits in ICP-AES ~

Element

~~

Pneumatic nebulization [88]

Electrothermal evaporation [68]

(Pglnll)

(Cldml)

(Pg/g>

Hydride generation [65] (ng)

at lOg/l Ag A1 As Au Ba B Cd Ce

co Cr cu cs Ge Fe Hg In La Li Mg Mn Mo Ni Pb Rb Se Sn Te

Th Ti TI U

V W

zn Zr

0.007 0.02 0.05 0.02 0.00 1 0.005 0.003 0.05 0.006 0.006 0.005 42 0.05 0.005 0.02 0.06 0.01 0.8 0.0002 0.001

0.008 0.01 0.04 37 0.07 0.025 0.04 0.06

0.004 0.004

0.25 0.005 0.03 0.002 0.001

0.7 2

5 2 0.1 0.5 0.3 5 0.6 0.6 0.5 4200 5 0.5 2 6 1 80 0.02 0.1 0.8 1 4 3700 7 2.5 4 6 0.4 0.4 25 0.5 3 0.2 0.1

0.3

0.4 0.2 0.5

0.1

0.005 0.02 4 6

4

0.3

(Pg/llN


215

dc motor adjustable

Figure 6-10. Direct sample insertion device for high-power ICP [731j. (1) switches, (2) lead screw with 2 guidances, (3) carriage gliding on 2 guidance stubs

itself as emission source [76] or as atom reservoir for fluorescence [77], a high power of detection was obtained (below the pg/g level) and the matrix interferences were low as well [78]. The technique, moreover, enables microdistributional analysis and has become a strong tool for multielement determinations, especially when the sample vapor produced is led into ICP mass spectrometry [79].

62.7 Discharges under reduced pressure In a discharge under reduced pressure, one can position the sample as one of the electrodes. The ions and energetic neutrals produced in the discharge and directed onto the sample can volatilize the analytes and bring them into a suitable region of the discharge where they can be excited and ionized. When a DC voltage is across two electrodes, ionization takes place and positive ions impact with high energy on

216

J.A.C. BROEKAERT

the cathode from which material is ablated and at the same time the electrons flow to the anode. In this way, a high positive space charge is built up in the neighbourhood of the cathode. The electrons are found back at high number density in the luminous negative glow further away from the cathode. The cathode thus will be volatilized partly by ion impact, during which material is ablated by momentum transfer from the impacting particles to atoms or ions on grating locations. When the impulse transferred exceeds the grating energy, the particle is released. The fraction of the energy of the impacting particles which is uansferred is a function of the masses of the impacting (772) and of the grating (Ad) particles and is given by

Em = E[4mM/(m+ M y ]

(6.8)

As a result of the bombardment of the cathode, however, its temperature increases and thermal volatilization also can occur. The ablated material then enters a lowpressure plasma, which is not in thermal equilibrium. Excitation occurs through electron impact where the high-energy electrons excite high-energy terms and low energy electrons are available for radiative recombination, by the Penning effect, and also by radiation trapping. As a result of the absence of local thermal equilibrium, the background continuum is also abnormally low. The kinetic temperatures in a low pressure plasma are also low by which line broadening in the optical spectra will be small. As analytically important low pressure discharges (Fig. 6-1 l), hollow cathode

Hollow cathode

Glow discharge lamp

FANES

Figure6-11. Glow discharge sources for atomic spectrometry

discharges are known since the work of Paschen in the 1930s. They are interesting as atomic spectrometric sources as the analyte species have a long residence time in the negative glow of the plasma, which is totally contained within the electrode. Therefore, they are used as atomic emission sources for microanalysis (see Ref. [80]). The most important glow discharge sources for atomic spectrometry are the discharges with flat and pin cathodes. The first one was introduced by Grimm in 1968 [81]. The Griinm lainp is an obstructed glow discharge where the cathode is cooled and thus volatilization is uniquely the result of cathodic sputtering. The


217

discharge is confined to the free surface of the cathode as the distance between the anode or the floating restrictor and the cathode is below the free path length of the electrons at the working pressure of 1-3 mbar. Due to the high burning voltage in argon (up to 1-2 kV), an intensive layer-by-layer ablation of the cathode occurs (some mdmin in the case of an 8 mm burning spot diameter) and the material is excited and eventually ionized mainly in the negative glow. Accordingly, the source is very useful both for atomic emission and for mass spectrometry, as will be discussed.

6.3 Plasma optical emission spectrometry 6.3.1 Atomic eniissiori spectrometry

In optical emission spectrometry, the radiation emitted by the source is led with the aid of an illumination system into the spectrometer, where the radiation is spectrally resolved. The monochromatic radiation at the analytical wavelength is then measured with the aid of a suitable detector and the signal obtained fed into a computer, which has often also the role of controlling the operation of the plasma and the spectrometer. As spectrometers both sequential and simultaneous dispersive spectrometers are used. As optical emission spectra are very linerich and the line widths are of the order of 1-3 pm, the spectrometers used should have both a high resolving power (of the order of A/ A A > 40000), determined by their dispersive element, as well as a large reciprocal linear dispersion (expressed in nm/mm in the focal plane), determined by the focal length of the spectrometer as well. For sequential spectrometers, computer-controlled monochromators are used. They mostly have an Ebert or a Czerny-Turner set-up (Fig. 6-12) [ 5 ] , of which the latter has less chromatic aberrations. As dispersive element, a grating is used. In the case of a holographic grating, the response over the whole range is rather uniform, whereas in the case of a blazed grating, a maximum response at the blaze angle and the respective wavelengths in the different orders is obtained. The latter type of grating can be used for working in higher orders, which enables it to obtain a high resolving power even with gratings having a low grating constant, i.e. a low number of lines per mm. The slit widths used in the spectrometers may be down to the 10 pm level, which improves the practical resolution but reduces the optical conductance and finally may lead to detector noise limitations. For wavelength selection, a computer-controlled stepping of the grating across the profile and the immediate environment of the analytical lines is very useful. In this way both accurate peaking of the line as well as a measurement of the spectral background intensities are feasible.

218

J.A.C. BROEKAERT

s.3

Figure 6-12. Mountings for optical emission spectrometers. (3) Ebert monochromator; (b) CzemyTurner monochromater; (c) Paschen-Runge spectrometer. sc: entrance slit, sa: exit slit, g,: plane grating, &: concave grating, m: mirror.

In simultaneous spectrometers, a Paschen-Runge set-up (Fig. 6-12) with a concave grating is often used. Here, often more than 30 exit slits with detectors can be provided in the focal plane. The practical resolution because of thermal stability reasons mostly is low as wider slits are used. A measurement of the spectral background can be achieved by a computer-controlled displacement of the entrance slit by which a scan over all analyticallines simultaneouslyis possible. Apart from these conventionaltypes of spectrometers, also Eschelle systems with crossed and parallel dispersion are used. Here an Eschelle grating is used as first dispersive element. It normally has a low grating constant but is used at a high order (30-80) where its reflection qualities still are good. In addition, a second dispersive element (prism or rating) is used as order sorter. In a crossed arrangement, the spectrum then appears in different rows above each other, each containing the wavelength-dispersed spectra for a given order with a height equalling the entrance slit height. Accordingly, a very high resolution can be obtained with a compact spectral apparatus [82]. Radiation detector photomultiplier tubes are used in most cases. They have a photocathode at which a photoelectron flow is produced with a given quantum efficiency and it is amplified with the aid of up to 15 dynodes, across each of which up to 100 V is provided. Accordingly, an amplification of up to lo5 is possible. The lower working limit is determined by the dark current, which is in


219

the nA range. Different types of photocathodes are available, enabling it to work in different spectral ranges, among which is the S 20 type for the UV-VIS region and the bialkali type for the VIS-IR. One can also use filters so as to remove radiation from unwanted orders, or so as to isolate low wavelength radiation (with solar blind tubes). Apart from the single wavelength detector consisting of an exit slit and a photomultiplier, a multichannel detection for different wavelengths simultaneously, now is possible with the aid of advanced photodiode arrays and more recently also with charge coupled and charge injection devices [83]. They provide for the electronic registration of small spectral areas, however, with limited signal to noise ratios. In the case of image intensification with the aid of microchannel plates, this disadvantage no longer occurs. In optical emission spectrometry, the net spectral line intensities are used as analytical signals and are proportional to the analyte concentrations. The detection limits of the methods are determined by the spectral background and its fluctuations and can be calculated as:

Here the line-to-background intensity ratio determines the background equivalent concentration and a(Iu) is the absolute standard deviation of the spectral background. When measurable blank contributions are present, the noise level is determined by their fluctuations and the detection limit then will be given as:

CI-= F[IB + 3&a(lB)]

(6.10)

when F is the functional relation for the calibration. Thus, a reliable calibration is only possible with the true net signals. In order to measure these signals an accurate evaluation of contributions from the spectral background, which includes stray radiation in the spectrometer, continuum radiation, band spectra and wings of matrix lines, must be performed. This must be done very carefully, especially in trace analysis. Indeed, when accepting for (~(1,) the value of 1%at a concentration of only 10 times the detection limit, we have an error in line intensity and in concentration which is twice as high. In plasma emission spectrometry spectral interferences therefore will be the main source of systematic errors. They cannot be avoided by standard addition. In plasma emission spectrometry further systematic errors may arise from ionization interferences or from influences of concomitants on the sampling rate, which then can be corrected for by standard addition or by the use of an internal standard. The latter approach is very well known from classical arc and spark work and is very worthwhile for the new plasma sources as well [84,85]. The selection of the reference element as well as the selection of the suitable reference line are an important point of the working procedure. In direct solids analysis one often can select a matrix constituent but in solution analysis, any element, which is not present in the sample to be analyzed, can be used.

J.A.C. BROEKAERT

220

6.3.2 ICP-Atomic eniissiori syectronzetiy 186,871 6.3.2.1 Figures of merit ICP-atomic emission spectrometry using pneumatic nebulization is a very powerful method both for the sequential and for the simultaneous determination of trace elements in solutions. Especially for the elements which form refractory oxides (such as Al, Nb, Zr, rare earths) the detection limits are in the ng/ml range (Table 6-1) [88,89]. This especially applies when sensitive ion lines are available and when the ionization potentials are not too high, as is the case for Be, Mg, the rare earth elements, etc. However, also for elements such as B, P and S, the detection limits are in the 0.1 pdml range. The optimization of ICP-AES with respect to the power of detection necessitates a careful optimization of the nebulizer gas flow, of the power and of the observation height, for each element. The parameters are interOBSERVATION HEIGHT

p:E:i-'

TEMPERATURE atoms, ions are excilet

molecules dissociote

material evapomtes c,droplets

I

dry

0.

.*

*:

X

-

.'.' .. .. 7 7 .i

X

droplet size

-

biscosity,.I

GAS FLOW

Figure 6-13. Importance of optimizationparameters in ICP spectrometry

related (Fig. 6-13). Indeed, the nebulizer gas flow determines both the nebulization efficiency and the droplet size but also the temperature and the residence time for the analyte in the plasma. All three are important for the free atom concentration in the ICP, which determines the line intensities. Further, the observation height is important as the kinetics of the evaporation and dissociation steps depend on the time travelled through a plasma region of a given temperature, which again depends


221

on the observation height. Finally, the power of the ICP again influences the plasma volume and therefore the temperature distribution. Especially for the optimization in the case of multielement determinations, simplex procedures are very useful [90]. The analytical precision obtainable with ICP-AES is high. Already at integration times in the s-range and concentration levels of 10-20 times the detection limits, relative standard deviations in the 1% range are achievable. The most important noise sources are nebulizer fluctuations, resulting mainly in white noise, but also gas supply fluctuations causing plasma pulsations [9 1J and frequency dependent noise [92]. Also at high concentrations standard deviations of 0.1% can be obtained when using a reference line. This could be shown for the analysis of binary alloys with ICP-AES at a concentration range of 500 to 900 mg/g [93]. The linear dynamic range in ICP-AES extends from the detection limits to over 5 orders of magnitude from the side of the radiation source. This is a result of the absence of analyte material in the cooler plasma zones and therefore of rather low self-absorption. Matrix effects in ICP-AES may arise from: 1. Nebulization effects, which can be minimized by acid matching in samples and standards;

2. Effects in the plasma resulting from easily ionized elements, such as ionization shifts, geometry changes by ambipolar diffusion, etc. These effects are low as, for example, in concentrations of up to 500 pg/ml Ca, Na or Mg were found to cause no matrix effects [3]. At higher concentrations these matrix effects occur but can be corrected for by matrix matching or by standard addition;

3. Spectral background interferences, which are the most important source of systematic errors. Here line selection with respect to freedom from interferences by matrix lines should be aimed at, and for this task the spectral atlas published by Winge et al. [94] and the tables of Boumans [95] including critical concentration ratios are very helpful. Further, instrumental background correction is of prime importance. 6.3.2.2 Applications and special techniques ICP-AES is of use for a wide range of applications. A number of applications are in the field of geological samples. Here the sample dissolution used is of prime importance both with respect to possible nebulization effects and the longterm stability, as it is known already since the early applications of ICP-emission spectrometry [96], as well as with respect to ionization interferences. It has been shown in the case of determination of the rare earths in bastnaesite and monazite that minor components of the samples almost have no influence from the side of

222

J.A.C. BROEKAERT

the excitation [3]. The concentrations of the fluxes used, such as Na,B,O,, cause matrix effects because of different reasons [97] and have to be matched in the standard solutions and i n the samples. In the case of particular minerals, further work has to be done on the optimization of wet chemical dissolution procedures, as shown, in the case of phosphate minerals [98]. This particularly is true as microwave heating may have some impetus especially for routine work. Extensive work on the analysis of minerals subsequent to sample dissolution has been published regularly (see for example, Ref. [99]. The analysis of enviroonmentully-reele~~u~it samples is a main field of application for ICP-AES. Based on the early work of Garbarino and Taylor for water analysis [ 1001, a method was proposed for waste water analysis by EPA [loll and later on by DIN [102]. In the latter, the sample decomposition, the analytical range for 22 elements and possible spectral interferences are treated. For the analysis of natural waters, the use of various preconcentration techniques is of prime importance. They may be based on liquid-liquid extraction of complexes (see Ref. [ 103,1041 for the case of dithiocarbamates), but also on coprecipitation, e.g. with In(OH), [ 1051 or sorption on acetylated cellulose [ 1061. Special impulses came from the use of flow-injection, by which concoinitants can be removed and analytes preconcentrated, as discussed by Ruzicka and Hansen [107]. The use of microcolumns allows it to shorten the preenrichment-release cycle considerably, as shown by Hartenstein et al. [ 1081. For environmental analysis, speciation now becomes very important [ 1091. Here, a coupling of ICP-AES with column chromatography is especially powerful. This has already been shown by early works on the speciation of Cr(II1) and Cr(V1) compounds with the aid of A1,0, columns which were eluted with acid or basic solutions alternatively [ 1101. For water analysis, attempts were made to make ultrasonic nebulization [ 1113 and thermal spray [1121procedures more reliable, e.g. by concomitant removal during sample preparation. However, this did not lead to a general use of these techniques. They are in principle capable of bringing an improvement in power of detection as they increase the sampling efficiency. From this point of view, hydride generation in the case of the elements which form volatile hydrides is also interesting. It has been introduced as flow-cell technique by Thompson et al. [ 1131 already and proved to be useful for the determination of As in waste water down to the &l-level. However, it should be remembered that the As compounds, which often are organic substances, must be mineralized. This can be done by treatment with H,SO, and H,O, but must be performed carefully so as to avoid analyte losses. Moreover, hydride techniques are especially prone to interferences of transition metals, which must be masked, e.g. by complexing with tartaric acid, must be eliminated by coprecipitation with La(OH), or of which the interferences can be avoided in some cases by calibration by standard addition [64]. In soil analysis, the development of suitable complete dissolution procedures, which can be used in routine, is particularly difficult.


223

A further series of applications of ICP-OES is in the field of biologicul suniyles. It has already been shown very early that Ca, Fe, Cu, Mg, Na and K can be simultaneously determined in human serum. This even applies for the case of microamounts of serum, for which after a 1 5 dilution, a direct analysis of 50 pl samples is possible [ 1141, similar to earlier flame atomic absorption work [115]. In the case of microamounts of biological samples, also evaporation from a carbon rod [67], from a graphite furnace [68] and even direct insertion is very useful. The latter from its principle certainly is the most sensitive ICP-AES technique and allows it to obtain detection limits in the sub-ng level [72]. All approaches, however, suffer from anion and matrix interferences as a result of the thermal evaporation. In the case of plant tissues, ICP-AES has also been used extensively (for an early review see Ref. [ 1161. However, it suffers from the lack of power of detection for toxic elements such' as Cd, Pb, TI, etc. For the analysis of industrial materials ICPAES nowadays finds wide application in all fields, ranging from ore analysis to the analysis of steel and slags as well as to the analysis of high-purity chemicals. In the actual field of high-purity powders for advanced ceramics, both analysis subsequent to a suitable sample dissolution as well as direct analyses of powders by slurry nebulization are possible. The limiting factors are the sample nature and the powder granulometry which influence the nebulization and vaporization in the ICP [71]. It has been shown that particles larger than 15 pm cannot be kept in the aerosol at the gas flows used. However, for very refractory substances such as ZrO,, volatilization already becomes incomplete above the 8 pm level. The approach could, however, be shown to be very valuable within these limits for the analysis of A1,0, [117], S i c [118], and Zro, powders [119,120]. Apart from slurry nebulization also direct insertion of powders [73] has been shown to be very useful for the determination of volatile elements in A1,0, powders. This technique can also be applied to the analysis of used oils, which even may contain particulate material [ 1211. In the case of metal analysis, ICP-OES finds wide application, as it has been demonstrated by a collaborative study [ 1221. Especially for this field, direct methods such as spark ablation El231 are very useful. As shown for the case of A1 alloys [75], as well as in the case of steels [123], a wide variety of alloys can be analysed with a high precision with the same calibration curve. The same advantage also applies in the case of laser ablation, as shown by Uwamino et al. [ 1241 in the case of ICP-OES and with more advanced Nd-YAG lasers in general, recently [125]. The laser ablation technique allows it to perform also laterally resolved measurements and in addition, also is suitable for non-conducting samples, such as advanced ceramics.

6.3.3 MIP-Atomic emissiotl spectrometiy MIP-AES in the case of a TM 010 cavity according to Beenakker is ideally suited for combination with graphite furnace evaporation and allows the multielement analysis of microsamples. The detection limits in the case of this approach

224

J.A.C. BROEKAERT

are at the sub-ng level [126]. Analyses of real samples require, however, a very careful calibration, as shown for serum samples. It also has been found that with improved plasma forms such as the toroidal MIP, not only wet aerosols can be taken up with limited power of detection [127], but also the sampling from a graphite furnace can be much improved [128]. The possibility of entering wet aerosols in the case of MIP-OES made them in principle also useful for element-specific detection in liquid chromatography [129]. The helium MIP is very suitable for the excitation of non-metals and thus is of use for element-specific detection in gas chromatography. For this aim, instrumentation including a gas chromatograph, a MIP and a diode away emission spectrometer is commercially available. This is not only suitable for pesticide residue analysis, but also for the speciation of metals such as Pb and Sn [131]. Considerable improvements of the stability of MIPS are obtained with the surfatron. This has been shown by comparative studies of a surfatron and a TM 010 cavity both in combination with evaporation from a tungsten filament [132]. Indeed, for Cu and Cd,both the power of detection, the linear dynamic range and the interferences of Na, in the case of a surfatron, were more favorable. Also the microwave plasma torch described by Jin et al. [54] constitutes a progress in terms of discharge robustness. This approach enables it to operate a stable argon and helium plasma. In the case of an argon discharge, samples can be introduced as desolvated aerosols, enabling detection limits in the sub-pg/ml range, but also as volatile hydrides, vapours generated by electrothermal atomization, etc. Also the CMP (capacitively coupled plasma) remains an interesting source for atomic emission spectrometry. Despite it is highly prone to interferences from concomitants such as Na, which is well-known [133], it remains of interest [134], also because high-power nitrogen discharges could be operated. This is also possible with electrodeless discharges, as published by Leis and Broekaert [55]. In all cases, however, the detection limits were found to be considerably higher than in the case of ICP-AES. The use of helium, which is possible in ICP-AES, as shown by Montaser et al. [ 1351,again might be easily possible with the aid of the high-power microwave discharges. Therefore they also remain an interesting area of research. 63.4 Glow discharges Glow discharges have been used as spectroscopic plasma sources since the early work of Paschen [136]. They especially have the advantage that through the use of noble gases with high ionization energy and by the absence of local thermal equilibrium, very highly energetic terms of also 0, N, S, and the halogens can be excited. In the case of the hollow cathode, the analyte is held for a long time in the excitation region. As a result of these facts, Mandelstam and Nedler 11371 and Falk [138] showed that this plasma source is the most sensitive atomic emission source. This has been proven by microanalyses performed by Russian groups [ 1391.


225

In hot hollow cathodes the analyte may even selectively vaporize from the matrix, which in a number of cases, is a supplementary advantage. A hollow cathode lamp for routine use has been constructed by Thornton et al. [ 1401 and shown to be useful for the direct determination of volatile elements with high excitation energies such as As, Se, Bi, etc. in high-temperature alloys [ 1411. However, the source is difficult to handle and moreover, a general technique for the sampling of solids is not available. The main advantage of the hollow cathode sources is that the sample composition largely influences the material volatilization, especially in the case of the hot hollow cathode. Therefore, a separation of the evaporation by external heating of a graphite furnace under vacuum was combined with excitation in the negative glow of a DC discharge. This approach, known as furnace atomic non-thermal emission spectrometry (FANES) was described by Falk et al. 11421 and could be shown to enable detection limits in the pg-range. As in every low-pressure discharge, easily ionized elements disturb the favorable excitation conditions. Therefore, the approach is very valuable as a detection principle in combined analytical procedures, where the elements to be determined are separated from the matrix and are present as a trace element concentrate in a microvolume. The method, however, is more difficult to handle than the MIP,as it is operated at low pressure and its multielement capacity may suffer from selective volatilization. A further interesting type of discharge under reduced pressure is the Grimm type glow discharge lamp, which was introduced as source for atomic emission spectrometry in 1968 [81].Due to its time-stable operation, this glow discharge lamp can be used both for simultaneous as well as for sequential emission spectrometric determinations in electrically-conductive samples. The latter should be available in the form of flat discs and are used as cathode [143,144]. At a power dissipation of up to 100 W (currents of up to 100 mA at 1 kV), ablation rates of some mg/min can be obtained. Detection limits in the case of metals can lay in the pg/g range or even lower [145]. Due to the optically-thin character of the plasma, calibration curves were found to be linear over a wide range of concentration, excepted in the case of resonance lines where self-reversal seems to be considerable. Glow discharges are very stable sources and display no flickering noise [146]. Therefore Fourier Transform spectrometry can be used in glow discharge atomic emission spectrometry. This easily enables it to perform line profile measurements. The line widths determined accordingly were found to be in the pm range. In the case of resonant lines of major constituents, severe self-reversalmay take place, as shown for the case of Cu I 324.7 nm line. The material ablation in the case of Grimm type glow discharges, uniquely takes place as a result of cathodic sputtering. Therefore matrix effects related with selective thermal evaporation do not occur. The Grimm type glow discharge source is mainly used for the determination of minor components in metals rather than for trace analysis. However, as the samples are ablated layer-bylayer, glow dischages are now also important sources for depth-profiling of metal

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samples [ 1471. Glow discharges may also now become important for bulk analysis and for depth-profiling in electrically non-conducting samples, as r.f. discharges with sufficient ablating capacity now become available [ 1481. Also gas sampling glow discharges have been described. They consist of a cathode plate having an opening of below 0.1 mm for sampling and being attached to a discharge chamber which is grounded. The assembly is connected to a highdisplacement rotary pump. These devices were introduced as soft ion sources in organic mass spectrometry, and go back to the work of McLuckey et al. [149]. These sources have been shown to be extremely sensitive also for molecular ions of large organic substances. They can be operated at currents of up to 200 mA and voltages of up to 700 V in argon, but also in helium and in neon at pressures of 1 to up to several mbar. It recently has been shown that such discharges in helium are capable of exciting the optical spectra of halogens and S [150]. In the case of each of the three gases, the device could be coupled also with hydiide generation even when allowing the excess of hydrogen produced to enter the discharge [ 1.511 and the detection limits for As in the case of atomic emission spectrometry are at the 10-30 ng/ml level. The capabilities of the gas sampling glow discharge for trace analysis, however, certainly will only be fully explored when applying mass spectrometry.

6.3.5 Coriclusion Especially as a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multielement trace determinations. As summarized in Table 6-2, absolute detection limits with several techniques are below the ng level. This applies especially when the total sample really can be brought into the radiation source, as it is possible with direct sample insertion, for example. In this case, however, volatilization interferences are very high, so that the techniques are only useful when the elements to be determined are present in matrix-isolated form. Atomic emission spectrometry only in a restricted number of cases can be used for the trace determination of elements such as the halogens, P and S. This requires the use of radiation sources where also helium can be used.

6.4 Plasma mass spectrometry Plasma mass spectrometry, with ion sources at moderate vacuum (mbar range) and atmospheric pressure, was started in the sixties when ion sampling at various sources such as flames was realized mainly for diagnostic purposes. Its breakthrough occurred with the work of Houk et al. [ 141, who made use of the inductively coupled plasma as ion source (ICP-MS). Already in their first work, they succeeded in showing that the power of detection of ICP-MS was superior to and that the spectral interferences, especially at higher masses, were low as compared to atomic emission


227

Table 6-2. Power of detection of plasma optical atomic emission spectrometric methods Method

Detection limits Re1ative

Absolute

Flame AES

1 ng/nil (alkali)

-

DC arc AES

0.1-2 pLg/g

1-20 ng

Spark AES

1-5 Pg/g

10-100ng

Glow discharge AES

1-5 Pg/g

1-5 ng

Hollow cathode AES

0.05-1 /lg/g

lo-loopg

FANES

0.05-1 pg/g

10-100 pg

Laser AES

1-10 pg

ICP-AES

1-10 PLg/lg 1-20 ng/ml

0.1-1 ng (ETV)

DCP-AES

1-20 ng/nil

-

MIP-AES

2-40 ng/nil

0.05-0.5 ng

spectrometry. This development was only possible because in the seventies mass spectrometers went through a thorough development both with respect to the price performance ratio as well as their resolution. In particular, the development of quadrupole mass spectrometers of high quality was of prime importance. ICPmass spectrometry in the meanwhile developed to a strong tool for trace analysis; developments in the case of other plasmas and especially in the case of the microwave plasmas and of the glow discharges are also to be discussed. 6.4.1 ICP mass spectsontetiy

6.4.1.1 Instrumentation (Fig. 6- 14)

In ICP-MS, ions are extracted from the analytical zone with the aid of a conical metal aperture (sampler). The diameter of the aperture is between 0.3 and 1 mm. Towards smaller values the diameter is restricted by the critical thickness of the cold boundary layer. Under this limit value, a sampling of analyte ions from the ICP is not possible. Towards higher values, the diameter is limited by the pressure in the intermediate stage, which should not be higher than some mbar. At these dimensions of the aperture, a powerful oil rotation pump or a turbomolecular pump must be used, so as to provide for the vacuum mentioned. The sampler can be made of different metals. Both copper and nickel can be used. When aggressive acids such as HF or HNO, are present in the analyte solutions, as it is often the case in

J.A.C. BROEKAERT

228

,

IOIFFERENTIALLY PUMPED REGION AORUPOLE MASS

R

Figure 6- 14. Instrumentationfor ICP-MS

the analysis of geological samples, samplers made of Pt may be useful. In that case, it was also found useful to nebulize a solution containing Ti for a longer time, so as to cover the sampler with a protective layer [152]. When using a sampler with a cone angle of 120°, the stability of the plasma and the ion extraction were found to be optimum. In the intermediate vacuum, the beam of the sampled ions expands. From this beam the central part is sampled with a second aperture (skimmer) and then led into the high vacuum of the mass spectrometer. The cone angle of the skimmer is ca. 55". The vacuum in the mass spectrometer is maintained with the aid of a diffusion or a cryopump. A high ion transmission is obtained at a distance of 5-10 mm between sampler and skimmer. In the intermediate vacuum high-energy ions, radicals, molecules and electrons give rise to a number of reactions, by which a series of compounds are formed, namely:

(a) Metal compounds MO-MO++e M + C1, N, . . .

__t

MCl', MN', . .

(b) Argon compounds

-

Ar + H,O --+ ArO', ArH', ArOH; Ar + C1, N, . . . ArCl', ArNH', . . . 2Ar Ar; + e

(6.1 1)

The presence of these cluster ions gives rise to spectral interferences with the analyte ion signals. This is especially the case with quadrupole mass spectrometers, which have only unit mass resolution. In order to keep these inteiferences to a


229

tolerable level, acids such as HCl, H,PO,, and H,SO, should not be present in the measurement solution and thus must be replaced by HNO,. However, also the working conditions such as the sampling depth, the aerosol gas flow and the parameters of the ion optics influence the degree of interferences. In commercial equipment, mostly quadrupole mass spectrometers are used. They have the advantage that no electrical field is required at the sampler for realizing a sufficiently high ion transmission. In the ion beam, one often provides a photon stop so as to prevent UV radiation from entering the detector, and uses several electrostatic lenses to focus the ion beam. The mass spectrometer itself consists of 4 equidistant and parallel rods (diameters 10-12 mm), between which a highfrequency field is applied and superimposed on a DC field. The transmission for anions with a given mle thus becomes maximum at a certain value of the field and the system then acts as a filter for this species. This can, however, be done in a rapidscanning mode. The spectral information can further be stored in a multichannel analyzer. Apart from quadrupole instruments, also high-resolution double-focussing mass spectrometers can be used. With such very expensive spectrometers, spectral interferences in a number of cases can be successfully eliminated, however, at the expense of ion transmission. The latter can only be compensated for by applying a high voltage at the sampler. Recently, time of flight mass spectrometers have also bee proposed for ICP-MS [1531 and potential for the use of ion traps exists. For ion detection in mass spectrometry,electron multipliers and pulse counters normally are applied. However, other detectors can also be used, such as microchannel plates. The operation of the plasma and of the mass spectrometeras well as of the data acquisition are computer-controlled. An external computer is mostly used for calibration, for interference as well as for drift corrections, graphics and spectrum treatment. Equipment including an ICP combined to a quadrupole mass spectrometer has been commercially available since 1982 and an apparatus using a double-focussing mass spectrometer and an ICP since 1987.

6.4.1.2 Analytical figures of merit ICP-MS has the advantages of all ways of sample introduction known from ICPAES, the possibilities for an easy calibration with synthetic solutions or by standard addition as well as the possibilities of rapid and flexible multielement determinations. Its power of detection is at the same level for most of the elements and is much higher than in ICP-AES. Also isotope dilution can be applied for calibration. (a) ThelCP niuss spectru (Fig. 6- 15) in the case of quadrupole mass spectrometers have a resolution of 1 dalton. Accordingly, cluster ions may cause considerable spectral interferences with analyte ions, especially at low masses. Cluster ions may be formed as a result of different processes and may stem from:

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J.A.C. BROEKAERT

5 ." 3

----L 0

E

I.

N 0.. 0 0 -

v,

7

1.

8

1. o

00-

0

NO

0

r.

4

N

0

c

dd

0

,

+2{


23 1

0

Solvents and acids: H', OH', H,O+, . . . , NO+, NO2', .. ., C1' (in the case of HCl), SO', SO;, S0,H' (when residual H,SO, is present).

0

Gases from the stmounding atmosphere: 0;. CO', CO;, N;, NH', NO', . ..

0

Reactions of the above mentioned species with argon: ArO', &OH+, . . ., ArCl', . . ., Ar;.

These cluster ions cause interferences especially in the mass range below 80 dalton and may seriously hamper the determination of the light elements. Moreover, also a number of compounds of the analyte ions with different species are found. Compounds such as MO', MCl', MOH', MOH;,. ..are formed as a result of the dissociation of nitrates, sulfates or phosphates in the ICP or they are formed by reactions of analyte ions with solvent or oxygen in the plasma and eventually also in the intermediate vacuum. The formation of these species very much depends on the working conditions, as it has been extensively studied by Horlick et al. [154] and others [ M I . For the recognition of interferences in ICP-MS, the use of the isotope ratios of the elements can be very useful. The ICP-MS spectra accordingly are much less sophisticated than the ICP emission spectra, however, the resolving power of the mass spectrometers used in most cases also is much lower. Due to the different kinds of cluster ions which may be formed, spectral interferences in many cases will occur and correction methods will have to be worked out. In the ICP mass spectra the spectral background is low and is mostly determined by the dark current of the detector used or by scatteringof ions in the mass spectrometer. Other than ICP-AES, the ICP itself only slightly contributes to the spectral background, by which in ICP-MS it will be much easier to apply standard addition; (b) For the optimization of ICP-MS with respect to power of detection, freedom of spectral interferencesand of signal enhancements or suppression as well as of analytical precision, the operation parameters of the ICP, namely, the gas flow rates, the power, the position of the sampler and the parameters of the ion optics have to be optimized. The nebulizer gas flow is much more important than the outer and the intermediate gas flow. Its influence on the plasma temperature, as well as on the droplet size and accordingly, on the power of detection and the nebulization effects is similar as in ICP-AES. However, the effect is much larger, which can be understood from the fact that the sampling location is much more defined than in ICP-AES. Moreover, also the formation and decay of cluster ions and the ion energies are severely influenced. The

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J.A.C. BROEKAERT

Figure 6-16. Energy maxima for 68Cu+and A 0 ions as a function of the aerosol gas flows [1551; power: 1.5 kW

latter is also the case for cluster ions, as shown for ArO+ and Cu+ in Fig. 616. The aerosol gas flow also influences the geometry of the aerosol channel and accordingly, also the ionic number densities at the sampling aperture and the analyte signals. The power influences the plasma volume more than the plasma temperature and has to be optimized together with the aerosol gas flow and the sampling position (Fig. 6-17). The voltages at the different ion lenses must be optimized so as to get maximum transmission, which is eventually a compromise for elements with widely different masses; (c) The detection h i t s of ICP-MS are at the sub-ngJml level (Table 6-3). The detection limits for most elements are of the same order of magnitude. The fact that for some elements the detection limits are higher is due to isobaric interferences. This applies for As (interference of 7 5 A ~with + 40Ar35C1'),for Se (interference of "Se' with 40Ar40Ar+)and for Fe (interference of 56Fe+ with &Arl60'). For these elements the acid present in the analyte solution also is important. Also for the elements of which the sampler is made (e.g. Ni, Cu, ...), the limits of detection may be higher. In the case of elements with high ionization energies such as the halogens, low detection limits may also be obtained with negative ions (e.g. for C1+: 5 and for C1-: 1 nglml) [156]. The analytical precision mainly depends on the stability of the aerosol production, of the ionization in the plasma, of the ion sampling and of the detection. Relative short-term fluctuations are of the order of 1%. Further improvement just as in ICP-AES can also be achieved with the aid of a reference line [157,158]. This may occur especially when the salt content in the analyte solutions is high, as it is in the case with sample decomposition by fusioii with fluxes. Then salt deposits at the nebulizer, the torch or the sampler may lead to drifts. Whereas deposits at the nozzle of the nebulizer can

1,O

1,l

1,2

1,3l/min 1,L

0,9

1,0

1,l

1,2

lJI/minl,L

0,9

1,O

1,t

1,3I/minl,4

Aerosol gas flow

1)

Figure 6-17. The influence of the aerosol gas flow and the power on the signals of Ga+, Ge+, As+, In+, Sn+, Sb+, TI+, Pb+ and Bi+ (sampler position of 15 mm above the coil) (reprinted with pemiission from Ref. [2101)

0,9

w

w w

J.A.C. BROEKAERT

234

Table 6-3. Detection limits in ICP-AES and ICP-MS Element

Ag

AI As AU

B Cd

Ce co Cr Ge Fe Hg In La Li Mg Mn

Detection limit ICP-AES [SS] 7 20

50 20 5

3 50 6 6 50 5 20 60

10 80

Pb

0.1 1 10 40

Se

70

Te Th Ti

40

Ni

U V W

zn

60

0.03 0.2 0.04

0.06 0.04 0.06 0.05 0.01

0.3 0.02

-

0.02 0.07

0.05 0.1 0.7 0.1

0.05 0.8 0.09 0.02

4 250 5

0.03 -

30

0.05 0.2

2

efficiently by avoided by wetting the aerosol gas, deposits at the sampler are more problematic. Therefore, the total salt contents of the solutions should not be above 0.1-0.5 dl00 ml depending on the salts used. These effects lead not only to signal depressions and signal fluctuations but also to memory effects. In the case of solutions with low salt contents they are only 1-2 min (before the signals go back to < 1% of the sample signal).

Znteifereizces as a result of changes in the matrix composition may be due to changes in nebulization, in the ionization in the plasma, in the geometry of the aerosol channel or in the ion energy. The first are known as nebulization


235

effects and have been discussed earlier (see Section 6.3.2.1). By increasing the aerosol gas flow, nebulization effects decrease and at the same time the influences of the analyte on the geometry of the aerosol channel go back.

As in ICP-MS mass signals are measured, a differentiation between different isotopes of an element becomes possible. At the low mass resolution of quadrupole mass spectrometers (about 1 dalton), this leads to a series of isobaric interferences. Therefore, corrections with the aid of suitable software have to be made in the evaluation of the spectra. These kinds of interferences do not much change with the working conditions. This is not the case with the spectral interferences arising from doubly charged ions, background species or cluster ions, which depend very much on the working conditions. The background species at low masses lead to considerable interferences [ 1591: for example, 28Si+interferes with I4NL4N+, 31P+with 14NOH+,and 80Se+with 40Ar40Ar+.Species like 40Arl60+ not ony interfere with 56Fe+but as a result of a series of isotopic combinations and hydrides they lead to spectral interferences for a whole series of transition metals (53Cr,54Cr,54Fe,55Mn,56Fe, 58Ni,58Fe,and 59C0). When HCl is present, Cl', ClO+, ClN+,Cl', and ArCl+ species will give rise to inteiferences for further transition metals. At fixed working conditions, these interferences again are rather constant. The occurrence of doubly charged ions and cluster ions with the analyte ions greatly depends on the power and the aerosol gas flows. They are especially important for elements such as Ba, Sr and Mg which have rather low ionization energies and have thermally rather stable oxides [ 1591. Many inteiferences are mentioned in the literature. For instance, the isotopes of T i with the masses 46,47, 48,49 and 50 have oxide ions with I6O+,which interfere with 62Ni+, 63Cu+,65Cu+and 66Zn+and these interferences widely differ with the plasma locations sampled [MI; (d) Isotopic dilution can be used well in ICP-MS. This enables it to eliminate a series of systematic errors as well as to perform tracer studies. This technique [ 1601 can be applied for each element which has at least two stable or longliving isotopes. A known amount of the element with a known but different isotopic composition as compared to the one where the sample is added and intensively mixed. The isotope ratio (R: isotope l/isotope 2)) is then given bv: (6.12)

Np is the number of atoms of the element to be determined in the sample and NAthe number added. up and aA are the abundancees of the isotope (1) and (2) in the sample and in the added amount, respectively. Accordingly, from R we can calculate Np

236

.

J .A .C BROEKAERT

Isotopic dilution ICP-MS was very useful for studies on Pb (see Ref. [ 1611). Also tracer studies with Fe have been performed in biological materials [ 1621. The precision of the determination of isotopic compositions is of the order of 1% when the respective isotopic concentrations do not differ by more than one order of magnitude; (e) Apart from pneumatic nebulization of aqueous solutions some alrernative methodsfor sanzple introduction are particularly useful for ICP-MS. Similar to ICP-AES the analysis of organic solutionsis somewhat more difficult [ 1631. The use of ultasonic nebulization is also known from ICP-AES, but due to risks for high water loading of the aerosol and the related influences on the pressure in the intermediate stage, it is still more difficult to optimize than in the case of ICP-AES [164]. A new approach lies in the use of high-pressure nebulization [165]. It promises to be ideally suited for a combination of ICP-MS and HPLC, which is one of the strongest tools in speciation now available. With the formation of volatile hydrides, as it is known from AAS [166] and from ICP-AES (see Ref. [56]), one can considerably improve the detection limits for elements such as As, Se, and Sb. As shown in 11671, improvements were obtained also for Pb. They are due to the increased sampling efficiency but also the production of a water-free analyte flow resulting in a decrease of cluster formation. However, also in ICP-MS the many chemical interferences in the hydride formation reaction may hamper the accuracy in the case of real samples. From ICP-AES electrothermal evaporation (ETV) is known to be very useful for the analysis of micro-samples (see Sections 6.2 and and 6.3.2.2). In the case of ICP-MS it brings the additional advantage of introducing a dry analyte vapor into the plasma. It has been found to be extremely useful for the elements of which the detection limits are high, as a result of spectral interferences with cluster ions. In the case of 56Fe,which is interfered by 4 0 A ~ O Park + , et al. [ 1681showed that the detection limit could be considerably improved by ETV. Also the direct insertion of samples into the plasma has been described for the case of ICP-MS [169,170].

As techniques for direct solids sampling, spark ablation [171] and especially laser ablation have been investigated. The latter is commercially available and is very suitable for the analysis of electrically non-conducting samples also as it enables rapid survey analysis of sample inclusions. It has been shown for Sic, for example, that with a Nd-YAG laser operated at 110 Hz and an energy of some 0.1 J, a sample ablation of 1-10 ng and detection limits in the 0.1 pg/g range can be obtained [79,172].


237

6.4.1.3 Applications ICP-MS has found use in all areas where ICP-AES is applied but where further progress in power of detection was required. This is the case in trace analysis for geological samples and then especially for hydrogeological samples as well as for trace determinations in metals, in biology and medicine as well as in environmental analysis. The latter in particular is true when it comes to speciation work. (a) In the case of geologicalsamples, the determination of traces of rare earths in minerals has been described [ 173,1741,when ICP-AES is not suitable because of interferences or of limited power of detection. Calibration in most cases is done with standard addition and sample concentrations of < 0.1% were used. A further example is the determination of Pt subsequent to pre-enrichment by a "fire assay" with NiS [175]. Also the analysis of hydrogeological samples has been widely applied, as described by Garbarino and Taylor [ 1761. They applied isotopic dilution in the determination of Ni, Cu, Sr, Cd,Ba, TI and Pb in pure water samples. Also alternative sample introduction techniques such as ETV for the determination of Tl 11771 or suspension techniques for the analysis of powders of coal [ 1781 have been investigated; (b) In nietuls und ceiantic niutr-ices trace determinations down to the sub-pg/g level are possible as the analyte concentrations may be up to 5 g/l. In contrast to ICP-AES, considerable signal changes may occur already at this sample concentration and have to be considered in the calibration. ICP-MS is especially useful for determinations in metals with linerich emission spectra. They include high-temperature alloys which are of use for reactor technology or in aeronautics [179]. McLeod et al. [181], however, showed with the example of nickel alloys that also in ICP-MS, finally the spectral interferences stemming from matrix lines limit the power of detection. Therefore, procedures for matrix removal, as possible with extraction, in the case of uranium [182] will bring progress in power of detection and in accuracy. Also direct solids sampling has been applied. In uranium as nuclear fuel, direct determinations can be performed by laser ablation coupled to ICP-MS [180]. It was also of use for the case of ceramics [ 1721. With arc ablation coupled to ICP-MS detection limits at the 0.1-1 pg/g were obtained in steels [183]; (c) For biological atid ntedical saniples ICP-MS allows it to considerably enlarge the number of elements which can be directly determined in body fluids. Indeed, in serum, e.g. only Na, Ca, Mg, Fe, Cu and Zn can be directly determined with ICP-AES whereas with ICP-MS any other elements also can be determined. Thus ICP-MS will be a strong tool also for the study of bioavailability of trace elements. ICP-MS was used already fairly early for the determination of normal concentrations of trace elements in clinical

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samples [185]. In proteins Mg, Al, Cr, Mn, Fe, Ni, Cu, Zn, and Se have been determined. In urine, a good agreement with results of other techniques was found for elements with masses higher than 81 (Pb, Cd, Hg, and T1) but for As, Fe and Se, deviations were found. The latter could be eliminated by removing C1 from the analyte solutions by precipitation. It also has been shown that by internal standardisation the accuracy of ICP-MS as well as the precision could be considerably improved [158]. ICP-MS was also used for the determination of Pb in blood [186] and for bioavailability studies of Pb [81]. Flow injection allows it to work with small sample volumes or to avoid difficulties stemming from samples with a high viscosity or with a high salt content [ 1881. By isdtopic dilution, metabolic studies with stable isotopes can be performed [ 1891, which is a very promising technique for medical studies; (d) For environmental arralysis ICP-MS is a most promising technique, because of its high power of detection. Especially for drinking water a determination of almost any relevant element can be directly performed. For the case of waste waters, the possible interferences were studied in detail by Herzog and Dietz [ 1901, who described optimization procedures for minimizing the interferences, in addition to factorial analysis studies for the treatment of large data collectives. For direct determinations in seawater, its salt content is too high. Here liquid-liquid extraction procedures or sorption techniques should be applied to isolate the elements to be determined. Beauchemin et al. [I911 described the use of a SiO, column loaded with 8-hydroxyquinoline, which enabled it to perform a 50-fold pre-enrichment of Ni, Cu, Zn, Mo, Cd, Pb, and U. In river water Na, Mg, K, Ca, Al, V, Cr,Mn, Cu, Zn, Sr, Mo, Sb, Ba, and U could be determined directly and Co, Ni, Cd, and Pb could be measured subsequent to the above mentioned pre-enrichment [ 1921. Isotopic dilution also delivered the most accurate results [ 1931. Also for the analysis of marine sediments [1941 and the characterization of the according reference materials ICP-MS was very useful. With ICP-MS also Sn could be determined in environmentally relevant samples [ 1951 and even its speciation is possible. For this aim it is to be mentioned that ICP-MS coupled directly to HPLC offers unique possibilities.

6.4.1.4 Outlook Further trends in ICP-MS lie in the development of new instrumentation, both from the side of the plasma as well as from the spectrometer and in the improvement of sample introduction. With respect to the first point, the use of MIPS as sources for mass spectrometry has to be mentioned, as it has been described, for example, by Caruso et al. [196].


239

With this low-power plasma the possibilities for sample introduction are limited, which is less often the case when high-power microwave plasmas are used, as recently mentioned by Furuta et al. [197]. Also high-power MIPS operated with nitrogen, such as the MINDAP described by Wilson et al. [198], could be very useful. The use of alternative gases in general is interesting, as it opens up the possibility of reducing interferences in particular cases. In this respect the use of mixed-gas ICPs [199] and also the use of helium ICPs, as described by Montaser et al. [200] are promising. Apart from quadrupole mass spectrometers, the high-resolution double-focussing mass spectrometer certainly enables it to cope with a number of interferences; however it is very expensive. Here, the use of ion traps, time-of-flight systems, double quadrupoles, etc. is to be further investigated. Finally, in the field of sample introduction, much of the achievements in ICPAES may deliver the inspiration to cope with particular analytical problems. Due to its high power of detection, the coupling of ICP-MS to chromatography certainly is a challenge for speciation work. Here, innovative work such as the optimization of new sampling techniques is underway. In summary, it can be stated that with ICP-MS a powerful method is available which has the high power of detection of graphite furnace atomic absorption on one hand, with the multielement capability of plasma spectrometry, on the other hand.

6.42 Glow discharge muss spectrometry Glow discharges became especially attractive for trace analysis since they are used as ion sources for mass spectrometry. In the sources developed for emission, spectrometry ions can be sampled very easily with the aid of a skimmer, as it is done in ICP-MS. Both quadrupole-based as well as double-focussing mass spectrometers can be used and for both approaches instruments are commercially available. The scope and the analytical figures of merit of glow discharge mass spectrometry have been described in papers of Harrison et al. (see Ref. [ 151) and of Jakubowski and Stiiwer (Fig. 6-18) [ 161. In the case of DC discharges, both with a pin cathode as well as with a flat cathode, a sputtering equilibrium and constant ion signals are obtained after a certain time, by which both sequential as well as simultaneous determinations in compact electrically-conductive solids are possible. Detection limits in the case of quadrupole mass spectrometers are in the ng/g region and for the light elements they may be limited by spectral interferences from clusters [201]. However, it could be shown that in the case of steels, reliable analyses at sub-pg/g concentrations can be performed. In the case of a double focussing mass spectrometer, the detection limits are considerably lower, as shown by work on analysis of materials used in microelectronics [202]. In both cases relative sensitivity factors, which in mass spectrometry reflect the influence of the matrix mainly as a result of phenomena in

J.A.C. BROEKAERT

240

~7 ~~

'

SOURCf

OPTICS

FILTER ,L

d

m n l f f f BUS

0AKING

DIFRRENIIAL PUMPING

FLOW

Figure 6-18. Glow discharge mass spectromelry including a Grimm-type glow discharge and a quadruple mass spectrometer; (1) exit aperture, (2) skimmer, (3) ion optics, (4) quadrupole rod system, (5) ion detector (reprinted with permission from Ref. [16])

the ion source as well as in ion transmission, are much closer to 1 [203] as compared to the spark source values. Glow discharge mass spectrometry with flat cathodes has the unique feature for depth profiling, which is important for the analysis of protective layers on working materials and for the characterization of multilayer materials now used in microelectronics. For the latter, it has been recently shown in an impressive way how multilayers can be sputtered and analyzed by mass spectrometry using a Grimm-type glow discharge [204]. Accordingly, it can be understood that glow discharge mass spectrometry is also a very powerful tool for the analysis of dry solution residues which are formed by the evaporation of microvolumes of analyte solutions of low total salt contents on a suitable carrier. In the case of the noble metals, one can make use of cementation in the case of copper targets to fix the analyte, so that it can be enriched and sputtered reproducibly. There has been found a way to fix pg amounts of Ir on a copper plate prior to their determination with GD-MS (Fig. 6-19) 12051. As r.f. glow discharges become available, they can be expected to become also of use as sources for mass spectrometry. They will certainly enable both direct bulk and depth-profile analysis of solid ceramics, as has already been demonstrated in the case of emission spectrometry. Further impulses for the use of glow discharges as sources for plasma mass spectrometry may also come from the use of other types of mass spectrometers [206]. This approach will be particularly valuable at first in model studies on ion formation. The latter as well as the modelling of the sputtering itself, as described in a paper of Van Straeten et al. [207], are actual topics of methodological studies by now.


0.1

1.

0.01 0.1

I 1

10 1 0 0 loo0

Ir content. ng

-

24 1

time, s

Ir

-b&

ground

-_--- detection limit

Figure 6-19. Determination of pg amounts of Ir by dry solution residue glow discharge mass spectrometry; (a) calibrationcurve obtainedwith disk-shapedcathode, quadratic regression, (b) single ion monitoring profile of 1 pg Ir (reprinted with permission from Ref. [205]

6.5 Conclusion In comparison with other methods of elemental analysis, plasma atomic spectrometry has a number of interesting analytical features. Its features are to be compared with those of atomic absorption spectrometry, spectrophotometry and fluorimetry as well as with X-ray spectrometry and electrochemical methods, in terms of power of detection, analytical accuracy in terms of systematic errors, but also with respect to its instrumentation and operational costs when it comes to its application in trace analysis. A survey of the analytical figures of merit of the atomic spectrometric methods is given in Table 6-4. 6.5.1 Power of detection

The detection limits for plasma emission spectrometry are at the 1-20 ng/inl level and for ICP-mass spectrometry at the sub-ngml level. Therefore the latter is one of the most sensitive techniques in elemental analysis. Only for the light elements, such as Mg, B, et al. or for interfered elements such as Fe, ICP-AES and in the latter case, other methods such as total reflection X-ray fluorescence (for a feature article, see Ref. [208]) may be more suitable. For solids analysis, both ICP-AES and MS make use of solutions, apart from some techniques allowing direct solids sampling. Therefore, both the loss in power of detection as well as the time involved with sample dissolution are disadvantages. This point also applies for AAS and for electrochemical methods, not however, for glow discharges which especially in the case of direct solids mass spectrometry, now is the most sensitive method in many cases.

J.A.C. BROEKAERT

242

Table 6-4. Figures of merit of methods for atomic spectrometric elemental determinations [I91 ~

Method

Power of detection

Matrix effects

costs

Atomic absorption * flame * furnace

++ +++

+ +++

++

Atomic emission * flame * arc/spark * inductively coupled plasma * microwave discharges * DC plasmas * glow discharge * laser

+ ++ ++ ++ ++ ++ ++

+++ +++ + +++ ++ + +

Laser atomic fluorescence

+++

++

+++

Laser enhanced ionization

+++

+

+++

+++

+++ ++

+++ +++

+++ ++

+++ ++ +++

Mass spectrometry * themiionic * inductively coupled plasma * glow discharge

X-rayspectroinetry

* fluorescence * total reflection * electron micmpmbe

+++ +++

++ +++ +++

+

++

+

+

+++

++

+ ++ ++ ++

++

+ low, ++ medium, +++ very high 6.5.2 Ititegerences As compared to graphite furnace AAS, interferences in ICP-AES and ICP-MS are low. In ICP-AES, they are mostly the result of spectral interferences, by which a matrix separation especially in the case of matrices with linerich spectra (e.g. Zr)is the only solution and this despite the availability of highly sophisticated instrumental background correction. In ICP-MS the influence of matrix constituents on the signals is much larger. However, when spectral interferences do not occur, which can be intelligently tested at the hand of the isotope patterns, calibration by standard addition often can solve the interference problem. In the case of the


243

light elements especially, cluster ions may cause spectral interferences. This is also the case for a number of heavy elements, but here a control is more easily possible and as more isotopes are present, the problem can often be solved. In the case of solids, low detection limits often cannot be obtained with a determination of the trace constituents in the presence of the matrix, as a result of the fact that ICP-MS only can tolerate low salt contents. Here, combined analytical procedures including matrix removal must be applied. Then, however, apart from ICP-MS, also total reflection X-ray fluorescence becomes very useful as its absolute detection limits are in the pg-range and multielement determinations are very rapidly done (for further literature, see Chapter 1). However, the light elements here cannot be determined. Graphite furnace AAS is a cheaper but monoelement solution for the problem as well as are electroanalytical methods. In the case of direct solids analysis, glow discharges especially in mass spectrometry have a high power of detection and are highly accurate, especially when compared to earlier spark source mass spectrometry.

65.3 Economic aspects Whereas ICP-AES in the latest years became considerably cheaper, as a result of the rationalization of the instruments design and the development of lowconsumption ICPs with lower gas and power consumption, ICP-MS remains an expensive method. However, when considering the multielement capacity and the power of detection, it becomes affordable for routine analytical laboratories. For a number of analytically challenging problems such as speciation' in life sciences and biological problems as well as for the characterization of refractory metals for microelectronics, it often becomes the only analytical approach available. Also glow discharge mass spectrometry is an expensive method, however, at least in combination with quadrupole mass spectrometers, its instrumental costs become similar to other direct solids analysis methods. It is much more reliable than the earlier well-known spark source mass spectrometry.

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119 Lobinski, R., Broekaert. J.A.C., TschOpel. P., arid TUlg, G., Freseiiius J. Anal. Chem., 342 (1992) 560-580. 120 Lobinski, R., Van Borm, W., Broekaert, J.A.C., Tschopel, P., and TOlg, G., Freseiiius J. Anal. Chem., 342 (1992) 563-568. 121 Sommer, D., Ohls, K., Fresenius Z. Anal. Chem., 304 (1980) 97-103. 122 Bosch, H., Broekaert, J.A.C., Hirschfeld, D., Lohr, W., Ohls, K., Petesch, P., Thierig, D., and Uttech, R., ICP Inform. Newsl., 14 (1987) 87-97. 123 Lemarchand, A.. Labarraque. G.. Masson, P., andBroekaert, J.A.C., J. Anal. Atom. Spectrom., 2 (1987) 481434.

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129 Kollotzek, D., Oechsle, D., Kaiser, G., TschOpel, P., and TOlg, G., Fresenius Z. Anal. Chem., 3 18 (1984) 485489.

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186 Delves, H.T. and Campbell, MJ.,J. Anal. Atom. Spectrom.. 3 (1988) 343-348. 187 Serfass, R.E., Thompson, JJ., and Houk,R.S., Anal. Chim. Acta, 188 (1988) 73-84. 188 Dean, J.R., Ebdon, L., Crews, H.M., and Massey, R.C., J. Anal. Atom. Spectrom., 2 (1987) 607-6 10. 189 Ting, B.T.G. and Janghorbani. M., Spectrochim. Acta, 42B (1987) 21-27. 190 Herzog, R. and Dietz, F., GewBsserschutz, Wasser. Abwasser, 92 (1987) 109-141. 191 Beauchemin, D., McLaren, J.W., Mykytiuk,A.P., andBerman, S.S., J. Anal. Atom. Spectrom., 3 (1988) 305-308. 192 Beauchemin, D., McLaren, J.W., Mykytiuk, A.P., and Berman, S.S., Anal. Chem., 59 (1987) 778-783. 193 McLaren, J.W., Beauchemin, D., and Berman, S.S., Anal. Chem., 59 (1987) 610-613. 194 McLaren, J.W., Beauchemin, D., and Berman, S.S., J. Anal. Atom. Speclrom., 2 (1987) 277-281.


25 1

195 Brzezinska-Paudin. A. and Van Loon, J.C.. Fresenius Z. Anal. Chem., 331 (1988) 707-712. 196 Satzger. R.D.. Fricke. EL.. Brown. PG., and Caruso. J.A.. Speclrochim. Acta, 42B (1987) 705-712. 197 Furuta, N., Abstracts of the 1992 Winter Conference on Plasma Speclrochemistry, San Diego, R.M. Barnes (Ed.). The University of Massachusetts, p. 64. 198 Wilson, D.A., Vickers, G.H., and Hieftje, G.M.. Anal. Chem., 59 (1987) 1664-1670. 199 Lam,J.W.H. and Horlick, G., Spectrochim. Acta, 45B (1990) 1313-1325. 200 Montaser, A., Chan, S.K.,and Koppenaal, D.W., Anal. Chein., 59 (1987) 1240-1242. 201 Jalrubowski, N.. Sluwer. D., and Vieth. W., Anal. Chem., 59 (1987) 1825-1830. 202 Mykytiuk, A.P.. Semeniuk, P.. and Berman, S.. Spectrochim. Acta Rev., 13 (1990) 1-10. 203 Vieth, W. and Huneke, J.C., Speclrochim. Acta, 46B (1991) 137-153. 204 Jalrubowski, N. and Stuwer, D., J. Anal. Atom. Spectrom., 7 (1992) 951-958. 205 Jalrubowski. N., Slilwer. D., aid TO&, G., Spectrochim. Acta, 46B (1991) 155-163.

206 McLuckey, S.A., Glish. G.L.,Marcus. R.K.. Anal. Chem., 64 (1992) 1606-1609. 207 Van Straaten. M.. Vertes, A.. and Gijbels. R., Spectrochim. Acta, 46B (1991) 283-290. 208 Klockenkwper, R., Knoth, J., Range, A., and Schwenke, H., Anal. Chem., 64 (1992) 1115A1123A. 209 Tail, S.H. and Horlick, G., Applied Speclrosc., 40 (1986) 445-460. 210 Horlick, G..Tan., S.H., Vaughan. M.A. and Shao, Y., in: Iiidictively Coupled Plasmas in Analytical Atomic Spectrometry, A. Monlaser and D.W. Golightly (Eds.), 1st ed., Verlagsgesellschaft mbH, VCH Weinheim. 1987, p. 369.


CHAPTER 7

Instrumental neutron activation analysis (INAA)

ZEEV B.ALFASSI Department of Nuclear Engineering. Beti Gurion University of the Negev. Beer-Sheva 84I20. Israel

Contents 7.1 7.2

7.3

7.4

7.5

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 Basic nuclear physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 7.2.1 Nuclides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 Radioactivedecay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 7.2.2 7.2.3 Kinetics of decay of radioactive nuclides . . . . . . . . . . . . . . . . . . .260 Kinetics of formation of radioactive nuclides by irradiation . . . . . . . . . . 261 7.2.4 The chart of the nuclides . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 7.2.5 Gamma detection systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 NaI(TI) - scintillation detector . . . . . . . . . . . . . . . . . . . . . . . . 268 7.3.1 Solid-state ionization detector . . . . . . . . . . . . . . . . . . . . . . . . . 268 7.3.2 7.3.3 The shape of y spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 IiTadiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 7.4.1 Neutronsources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 7.4.2 Samples introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Instrumental neutron activation analysis (INAA) . . . . . . . . . . . . . . . . . . . . 280 7.5.1 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 7.5.2 Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 Nuclear interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 7.5.3 7.5.4 A test case - INAA of trace elements in silicon . . . . . . . . . . . . . . . .285 7.5.5 EpithermalINNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 Fast neutrons INAA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 7.5.6 7.5.7 Cyclic INAA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 7.5.8 Prompt Gamma Neutron Activation Analysis (PGNAA) . . . . . . . . . . . 301 7.5.9 Depth profiling by INAA . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

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7.1 Introduction Chemical analysis by nuclear activation is an elemental analysis, i.e. it determines the contents of the various elements, but cannot tell in what chemical forin (compounds, valence states) they are. The analysis is based on a reaction of the analysed element with nuclear projectiles (neutrons, accelerates small charged particles, for example protons, deuterons, 3He or a particles, or gamma photons). The reaction can be written in the same form as a chemical reaction: Target

+ projectile

-+ light product

+ heavy product

or in the more concise form of writings of nuclear physics: Target (projectile, light product) heavy product. Thus, for example, the first production of artificial radionuclide by Joliot and Curie was done by the reaction of alpha particles with aluminum metal.

27Al+ a +30P+ n

( a is the4He nucleus)

or in the physics notation 27Al (a,n)30P. The light product is similar to the projectile, a neutron, a small charged particle or a photon. The basis of the nuclear activation method for chemical analysis, is the measurement of the amount of either the light or the heavy product produced in a known flux of projectiles for a known length of time. The amount of the products is proportional to the number of the target atoms, and hence, the measurement of the amount of the product yields the amount of the target atoms. The amount formed of the products is too small to be measured chemically (except in very rare cases [ 11). and the only way to measure them is by the nuclear physics methods of pulse countings. The light product can be measured due to its energy, if it is a photon or high kinetic energy charged particle, or due to its reaction if it is a neutron. The common denominator of all these processes, is that they must be done at a very short time after the formation of the product, otherwise the light product will lose either its energy or its identity, by reaction with the surrounding media. Consequently, the measurement of the light particle must be done during the bombardment of the target with the projectiles. This kind of measurement is called Prompt Activation

Analysis. In the case where the heavy product is radioactive, its amount can be measured by its radioactivity. This is the more common case, Since the radioactivity can be measured after the end of the interaction between the target and the beam of the projectiles, this method of analysis is called Delayed Activation Analysis. Since the heavy product must be radioactive, not all elements can be measured with each projectile. However, by the choice of the bombarding projectile, every element can

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255

be determined. Since many elements are activated simultaneously, the radioactivity measurement must yield both the identity of the radioactive nuclides and their amounts. The sometimes used term radioisotopes is wrong, since when saying isotopes we should say of what element. Nuclide is the term in nuclear physics which is parallel to the atom in chemistry. It consists of a nucleus (composed of neutrons and protons) and electrons around it. As the identity of radionuclides (short term for radioactive nuclides) in a mixture cannot be determined from pemission (unless very few are in the mixture), almost only y ray emitters are used in activation analysis. The radionuclides used for identification are called Indicator Radionuclide and abbreviated IRN. Only a few - p--emitting IRNs as 32Pare used in activation analysis. In this case the phosphorus should be separated from the other radionuclides in the activated sample. The analysis by nuclear activation with activity measurements only after chemical separation, is named Radiochemical Activation Analysis (RAA), whereas analysis by nuclear activation with direct measurement of the nuclear activity is named Instrumental Activation Analysis (IAA). RAA is used not only for p- and p’ emitters, but also for y-emiting IRN’s when they are masked by the more active nuclides in the sample. Since the most common projectile is neutron, this chapter will be devoted to Instrumental Neutron Activation Analysis (INAA).

7.2 Basic nuclear physics This chapter gives only briefly the basic knowledge. Readers who are interested in further reading can use common textbooks [ 2 4 ] . 7.2.1 Nuclides

The nucleus of an atom is composed of protons and neutrons. A general term to describe a particle in the nucleus (either proton or neutron) is nucleon. The total number of nucleons in the nucleus is called the mass number - A (A = N + 2, when N is the number of neutrons in the nucleus, and Z is the number of protons in the nucleus). Z is called the atomic number. A nucleus is defined unequivocally by its A and 2 values. The usual way to write a nucleus is i X . Nuclides of constant Z but different values of A are called Isotopes. The writing of Z is parallel to the use of the chemical symbol of the element. For example, oxygen nucleus has 8 protons and consequently the use of the symbol for oxygen is the same as writing Z = 8. If we write for the nucleus the symbol 0 we do not have to write the subscript 8, although in some cases we will do it for clarity purposes. Oxygen has three stable isotopes with 8,9 and 10 neutrons. They are written as l60, 170and l80.A common misuse is the designation of any nuclear species, as e.g. 14C, 13N,1 7 0 , by the word isotope. The word isotope should be used only in connection with a special element, as e.g. saying that l60,1 7 0 and l80are the three stable isotopes of oxygen or saying that

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34Cl,35Cl,36Clare isotopes of the same element. However, "F and ''0 are not isotopes. The general term to designate any nuclear species is nu.clide. A general term for radioactive nuclide should be radionuclide and not radioisotope, although it is correct to say for example that "'1 is a radioisotope of iodine. 7.2.2 Radioactive decuy

The neutrons and protons are held together in the nucleus due to the nuclear forces, These forces together with the Coulombic force lead to the result that only special combinations of neutrons and protons are stable. This is clear from the fact that each element has only a limited number of stable isotopes. Saying that some nuclides are unstable does not mean that they cannot be formed, it means that they are transformed to the other nuclides which have lower energy (i.e. lower mass according to Einstein's equation of mass and energy). This transformation of nuclides is done by the transformation of a proton to a neutron or vice versa, thus changing the ratio neutrons/protons. The transformation of nuclides can be done also by emission of an alpha particle (4Henuclide), however, this emission occurs only for the very heavy nuclides and it is almost unimportant in activation analysis. Very few radionuclides are transformed by emission of a neutron or a proton or by a spontaneous fission. All the transformationsof neutron +proton and proton +neutron are called /?-decays. They are characterized by the mass number - A - remains unchanged, whereas the atomic number is changed by f l . There are three transformations of that type. A neutron is transformed to a proton by a /?- process.

The proton remains in the nucleus, while the electron (p-) and the antineutrino (Ti) are emitted outside from the nucleus, with high kinetic energies. The electron

is written as /?- and not e-, to show that it is not an atomic electron. A proton can be transformed into a neutron by two processes. One process is p' decay, in which a proton is transformed into a neutron and a positron p' = e+ emitted from the nucleus, an entity similar in mass to electron but which has an opposite charge. p+ +n + / ? + + u

In this process there is also an emission of a neutrino (v). Another process is the reaction of a proton with one of the surrounding electrons in the atom to yield a neutron. This process is called electron capture (EC). Neutrino and antineutrino are very small particles which have no charge, and consequently are very difficult to be detected. They are not important at all to activation analysis, except for their effect on the /? energies, which will be explained later.

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257

Most of the radionuclides are not pwe ,8 emitters, but they emit simultaneously also a y photon (more accurately the y emission is a very short time of less than lO-’Os after the ,8- emission). Gamma-rays are electromagnetic waves, as light and radiowaves, but they have much higher energies (much shorter wave lengths). They have usually lower energies than X-rays, although the main difference between X-rays and y-rays are their sources. X-rays are due to atomic transitions (transitions between different energy levels of electrons), where y rays are due to nuclear transitions (transitions between different energy levels of the nucleons). The ,8 decay does not yield in most cases the ground state of the product nuclide, but forms it in an excited state. The excited state nuclide decays very rapidly (in most cases) to the ground state by the emission of y rays. A very important difference between p processes and y decay (besides the difference in A Z, f1 in the /3 process and 0 in the y decay) is the fact that in /3 processes there is emission of two particles, p- and V or p’ and v, while in y decay there is emission of only one photon. There are cases of emission of a few photons froin the same nuclide, but these are successive emissions and not a simultaneous emission as in the /3 processes. The importance of this difference lies in the fact that each nuclear decay is a transformation between two discrete energy states, resulting in a definite energy released in the process. If only one particle is emitted from the nuclei, this particle has a defined energy. When two particles are emitted, the released energy is distributed between them, and each particle can have a spectrum of energies ranging from zero up to a maximal energy, equal to the released energy. Since the emitted y photons have definite energies, they can be used in order to identify their emitters, by the measurement of the energy of the photons. As /3 particles do not have definite energies, they cannot usually be used to identify their emitters. In nuclear activation analysis, we need to identify the different radionuclides (in order to know from which nuclide they were formed), in addition to the measurement of their activities. This is the reason why nuclear activation analysis is employing almost exclusively the measurement of the spectrum of emitted y photons. In a few cases, where the IRN (Indicator Radionuclide) is a pure-p- emitter it can be measured either by chemical separation of this element (Radiochemical Activation Analysis - RAA) or for long-lived IRNs, by long waiting periods between the end of the irradiations and the starting of the counting of the activity. This waiting time is usually called “cooling time.” The end of irradiation is called also “pile out” in case of nuclear reactor irradiations, or EOB (end of bombardment) in case of accelerators. Some of p’ emitters do not have deexciting y photons. However, they are not measured by their p’ emission due to its short range, but rather by the 5 11 keV y produced in the annihilation process. When a positron collides with an electron, the masses of both particles annihilate and is transformed to two photons of 5 11 keV each. Since each /?+-emitter,emits the 51 1 keV y line, 51 1 keV y line can be used only for quantitative measurement, but not for identification. Thus, in many cases, NAA

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with IRNs which are only p' or /?-emitters are done only after chemical separation of the element. If there are only a few (2-3) non-y-emitter IRNs in the sample with quite different half-lives, their separate activities can be measured even without chemical separation by measuring the activities at different times (measuring the decay curve), and extracting the various activities from the time dependence of the measured activity. The y photons emitted are due to the deexcitation of the nuclide produced in the p process. However, this deexcitation has not to be always by only one photon. Moreover, not all /3 decays lead to the same level of excitation. These two facts lead to the possibilities that one radionuclide can have more than one kind (energy) of photons, and that the number of photons should not be equal to the number of nuclides which as been decayed (disintegrated). The number of photons of specific energy emitted per 100 disintegrated nuclides is called the intensity of that y line, in percent. Let us examine different cases of y lines intensity in four examples of radionuclides.

1. 28A1decays completely to a 1.78 MeV excited state of %i. This level decays to the ground state by only one photon. Thus, it means that 28A1has only one y line of 1.78 MeV with intensity of 100%. This fact can be drawn in the following scheme (called decay scheme) of decreasing energies;

energies 4.64

..-I--

p -= 2.86 MeV 1-78..--.-I..I...... .....-

28

Si* (1.78 MeV ) y (1.78MeV)

28

Si

2. 24Nadecays almost completely (> 99.8%) to one level of 24Mg. However, this level decays to the ground state only by two successive photons. The first photon of 2.75 MeV is followed by a second photon of 1.39 MeV. This decay scheme explains why 24Nahas two y lines of 2.75 MeV and 1.39 MeV, each with 100% intensity;

3. 27Mg decays to two different excited levels, resulting in two p- with different energies 1.59 MeV (31%) and 1.75 MeV (69%). The higher excited level (1.01 MeV) decays only 98% directly to the ground state while 2%

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259

(0.31 . 0.02 = 0.006 = 0.6% from the total nuclides disintegrated) decays to the lower excited state. The lower excited level decays completely to the ground state by one photon. This decay scheme explains why 27Mghas three y lines of 1.01 MeV (31%), 0.84 MeV (69%), 0.18 MeV (0.6%);

4. 38Cldecays directly to three different states of 38Ar, both to two excited states of 38Ar and to the ground state of 38Ar. This is the reason for 38Cl having thee different energies p- particles (remember, it is only maximal energy, since each p- leads to spectrum of 0-energies, up to Emaximum), 4.81 MeV (53%), 2.77 MeV (9%) and 1.77 MeV (38%). The higher excited state (3.77 MeV) decays almost completely to the lower excited state and

0.03 % )

E (MeV)

260

ZEEV €3. ALFASSI

only very little (0.06% of the 38%) decays directly to the ground state. Consequently, 38Clhas three y lines 3.77 MeV (0.03%), 2.10 MeV (38%) and 1.60 MeV (38% + 53% = 91%). This data together with more physical data on the nuclides can be found in books collecting all the decay schemes of the nuclides, however, for application of radionuclides it is sufficient to use collection of all the y lines of the radionuclides without the detailed decay scheme [8,9]. These collections are for all known radionuclides, however, for the most cases of activation analysis it is better to use a smaller collection of radionuclides, only of those potentially formed by the specific kind of activation [7,10].

7.2.3 Kinetics of decuy of radioactive riuclides The decay of radioactive nuclides is a statistical process. Thus, the number of atoms decaying per unit time (rate of decay) is proportional to the number of atoms of that specific radionuclide present in our sample. If the number of nuclei (atoms) of a specific radionuclide at time t is N*, their rate of disappearance (decay or disintegration) is given by the equation

--dN' -AN* dt

The constant A, which is different for each radionuclide, is called the decay constant of the radionuclide. Integration of Eqn. (7.1) lead to the decay equation of radionuclides - the equation describing the time dependence of the number of atoms of the specific radionuclide: N * ( t )= N p ' (7.2) where N,' is the number of atoms at time chosen as t = 0. Eqn. (7.2), which is the same as the integrated equation of any first-order chemical process, indicates the existence of constant life for the half of the atoms, which is called the half-life. It means that independent of the value of N;, it takes the same time (for a specific radionuclide) for disintegration of half of the atoms, leaving only AZ/2 atoms of that radionuclide. Substituting N*(t) by N;/2 in Eqn. (7.2) leads to the correlation between the half-life (tllz) and the decay constant (A)

N*(t)= N,"/2 ==$

= ln2/A

or

t , / 2= 0.693/A

Eqn. (7.2), the decay equation, can be written also with tIl2 instead of A:

(7.3)

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

26 1

The advantage of Eqn. (7.4) on (7.2), is that it gives easier “feeling” for the extent of the decay. Thus for example, if the half-life is t1,2 = 2.5 days, we know that after 2.5 days, only half of the original atoms remain, and after 5 days, the number of original atoms reduces to one-fourth. Eqn. (7.2) was easier to use when people were using a table of exponents. Nowadays, with computers, there is no difference which equation is used. In tables of data, only tIl2 - the half-life is given. In order to use Eqn. (7.2), A should be calculated from Eqn. (7.3). Data on half lives and y line intensities of all nuclides can be found in several sources. The most common source to non-experts is the CRC Haildbook of Chentistiy and Physics.

7.2.4 Kinetics offorniatiori of radioactive nuclides by irradiation When a thin target is bombarded with a beam of projectiles, the rate of nuclear transformations (number of produced nuclides per unit time) is proportional to the beam intensity (I- number of incident particles per unit time), proportional to the target nuclei density (n - number of target nuclei per unit volume) and the thickness of the target (dz). The proportionality to the thickness of the target is limited to sufficiently small dx, such that both the intensity of the beam and the energy of the projecticles remain practically unchanged. The proportionality constant is assigned u and is called the reaction cross-section. This name comes from a simple model, when assuming that each geometrical collision leads to a nuclear reaction. In this case, u is the geometrical cross-section of the collision pair - ~ ( r+,r2)2. dN* -= u .I . n - dx dt

(7.5)

Since the nucleus radius is few times fm cm), the cross sections are in the order of 10-24cm2. This is the reason why it has been accustomed to express crosssections in units of barns: 1 barn = 10-24cm2. Eqn. (7.5) assumes that the beam cross-section is smaller than the target size facing the beam, and consequently each projectile particle is transversing through the target. In the case where the target size is smaller than the beam cross-section, the beam intensity, I, should be replaced by the product 4 . A, where 4 is the beam flux (number of projectile particles per unit area and per unit time), and A is the area of the target facing the beam. dN* -=u-q5-A.n dt

edx

A dz is the volume of the target and consequently 11 . A dz is the total number of target atoms - N:

ZEEV B. ALFASSI

262

Since we are speaking of areaction of a specific nuclide, N is the total number of the atoms of that nuclide. Eqn. (7.6) is used mainly for irradiation in a nuclear reactor, where the target is located inside a uniform flux of neutrons. Eqn. (7.5) is used in cases of a narrow beam from charged particle accelerators. In the case of prompt activation analysis, the only important number is the number of nuclei which were transformed by the nuclear reaction. Hence, the number of nuclei transformed is given by the integration of Eqns. (7.5) or (7.6) over the irradiation time. However, for delayed activation analysis, the important factor is the number of nuclei decaying during the measurement period, which is delayed and done sometimes a long time after the end of the irradiation. In calculating the number of nuclei of the newly formed radionuclide at the end of the irradiation, it will be wrong to integrate those equations. The radionuclides are not only formed during the irradiation, but they are also decaying. Combining Eqns. (7.6) and (7.1) leads to the real equation for the rate of the change of the number of radioactive nuclei during the irradiation:

dN' -dt

- U - 4 . N - A . N*

(7.7)

Integration of Eqn. (7.7) with the initial condition N,* 10, leads to the equation for the number of radioactive nuclei at the end of irradiation for time t,:

(7-8) ti is the irradiation time. EOI stands for end of irradiation. For cases where Ati 99%) was sufficient for determination of 18 elements with concentrationlevels of 0.1 ppb. For concentration levels of 1 ppb, an extra 20 elements can be measured. The 24Nais removed by passing HF/HCI solution of the digested irradiated aluminum on a 1.5 cm height HAP (hydrated antimony perntoxide) column, at a rate of about 1ml/min. More than 99% of the Na was retained on the column, whereas 48 elements were eluted with yields of above 99% and an additional five elements with yields of 93-98.5%. HAP columns or HAP powder in batch are utilized in many uses of RNAA to remove 24Na interference 139-421. However, most studies use 6-12 M HC1 for which retention of Ta, As and Se, besides Na, were also found. Egger and Krivan showed that the use of HC1-HF (10 ml concentrated HCl + 1 ml40% HF) followed by elution with concentrated HCI leads to quantitative recovery also of these elements. 8.3.1.2 Separation of a single element Although silicon is one of the most important impurities in high-purity metals, very few studies were able to determine its concentration at the level of ppm or below. The main limiting factor for all these studies is the high blank involved in decomposition of the sample and silicon separation. Due to the high concentration of Si in the earth crust (28%), it is almost impossible to obtain a detection limit below 1 ppm, even using clean rooms and specially purified reagents. Only activation analysis does not suffer from the blank problem. However, silicon is not an easy element for delayed neutron activation analysis. Silicon has three natural stable isotopes 28Si,29Siand 30Si. Only the last one, which is the least abundant (3.1%), gives a radionuclide on radiative capture of neutrons. The cross-section for this reaction, while not too small, is still smaller than for most of the elements. However, the main problem in determining silicon by NAA stems from the fact that its indicator radionuclide emits very few 7-rays. Its y intensity is 0.07% (meaning that for every 10000 decaying 31Si nuclides, only seven photons of y-rays are emitted). Thus, the activity of 31Si must be determined by its /3 activity. While different y-ray emitters can be distinguished by their different energy ?-rays using HPGe or Ge(Li) detectors, the activities of a mixture of p emitters can be separated into its components only for small numbers of components (- 3 4 ) with sufficiently different energies. Another way to distinguish between different p emitters is by the analysis of the decay curve (activity vs. time) and convolution to different half-lives. However, the analysis of decay curves is accurate only for at most, 3-4 components with sufficiently different half-lives. 32P,which has a relatively long-life, can be measured after long decay [49,501 for various matrices. 31Si has a relatively short halfliife (2.62 h) and in order to determine Si in metals, 3'Si should be completely

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

3 19

separated from any other radionuclides, and then its P activity should be measured. Rouchaud et al. [51] determined the concentration of silicon in molybdenum by RNAA, separating the 3 1Si from all other radionuclides. After irradiation, the molybdenum was digested by alkaline fusion with NaN0,-NaOH (2: l), which was found to attack both molybdenum and silicon. The fused cake is dissolved in hot water. The solution is neutralized and H202is added to transform the molybdenum quantitatively to peroxy ,complexes needed for the following separation. To the solution are added HC1 and HF, to form HCl(O.03 M), HF (0.1 M) solution, which is passed on an anion exchange column (Dowex 1 X8,C1- form), and the column is washed with an addition 25 ml of HC1 (0.03 M/HF 0.1 M). Most of the trace elements are eluted and on the column remain Si, Mo and its daughter Tc, and W. Silicon was later eluted with 30 ml of HCl(8 M)/HF (0.1 M). In order to concentrate the 31Sito smaller volume, to allow higher detection efficiency, the solution was brought to pH 9 and passed through a small column of hydrated alumina, on which ,lSi remained fixed and counted. The removal of all radioactive interferences was checked by looking for y activity. It was found that no 7 or X-ray emitter was present. In a later paper, Rouchaud et al. [52] developed a similar method for Fe and Al, using this time cation exchange Dowex 50 x8 for the retainment of the matrix. Si is eluted with HF + HCl + acetone, and Si is finally retained on A1203for counting. Schmid and Krivan [53] developed another process for separating solely Si. This process is based on distillation of 31Sias SiF, from HF + HBr solution, followed by measurement of the /3 activity, with the very efficient liquid scintillation counting system. The HBr was added to the sample before its decomposition, in order to prevent distillation of As together with the Si. They used this method for determination of Si in vanadium and niobium. 8.3.1.3 Group separation of several elements Since several IRNs can be determined simultaneously, as long as they do not interfere with each other, there is no need for complete separation. Park et al. [54] studied the concentration of 13 trace elements in high purity molybdenum by separating them into three groups, which are separated from the major interferences, which are Mo and W. A cation exchange column (Dowex 50) in dilute HC1 medium is used to remove Mo and W, which are only weakly adsorbed on this column, while many of the impurity elements are strongly absorbed. Na and K are eluted by using additional dilute HCI. Nine trace elements were eluted with 6 M HCI. Together with Mo and W, 239Npand 233Pawere also eluted, which are the IRNs for U and Th, respectively, due to p- decay following the (n,y) reaction.

Np and Pa were separated from Mo and W using anion.exchange (Dowex-1) in

ZEEV B. ALFASSI

320

Dipslcd Mo. hcaced lo dryness

1. Heat to dryness

2. + 16 mL 13% HzO,

3.

+ 15 mL conc. HNO,

0.5M HCI i0.3%

H202

Rb,Zr, 1%

Group

Zn.CO.La

Na

u

l-l

Dowcx

column

7M

HNO,

(0.3% H203

1M HCI

Figure 8-1. Park et al. [541 method for separation of 13 elements into lhree groups for determination of Lrxe elements in molybdenum

strong nitric acid media. Pa and Np are strongly absorbed on the anion exchange resin, while molybdenum and tungsten are not. Figure 8-1 gives a schematic representation of the process of separation of Park et al. [54]. Before counting, each eluate was concentrated by partial evaporation. Theimer and Krivan [55] developed a group separation for the determination of 20 elements in high-purity molybdenum (Fig. 8-2). They developed two modifications, the first one involves removal of Mo and its daughter Tc and the main contamination of W, by absorption on anion exchange resin (Dowex 1x8). Five other elements are also absorbed on the column, but 20 elements are eluted and can be determined simultaneously in the eluate. The second modification allows determination of only 11 elements, although separation into two groups is done. The advantage of this special modification is the separation of 233Pa(the IRN of Th) only with Sc, enabling determination of small amounts of Th. Both modifications are based on elution with various concentrations of HF,

321


(1 mLconc.HF+ 0.5 mL conc. HNO3)

evaporation Procedure B

Procedure A .r

dissolution (2 mL 20 M HF/ 3%H2W

(0.5 mL 20 M HFI 0.5 mL 30% H 2 W

elution with 27 mL 20 M HF/3% H202 v v

r

elution with 20 mL 2 MHF/3% H202

'-r

elution with 25 mL

OE

Nb,Re,Sb, Ta, Tc. W,Zr, (Sn)

1 M HF/lM NH4F

adsorbed: Hf, Mo,

W c (

I

'I

eluted: Ag, Cd, Co, Cr, Cs, Cu.Fe, Ga. XnJr, K, Mn. Na, Np, Pa, Rb, Ru, Sc. Se, Zn,0,

(Sn). (W

Eluate 1: Ag. Co, Cr.Cs, Fe. Rb. Ru,Se,Zn,(Sn)

Eluate 2: Pa;Sc

Figure8-2.Flow chart of the two procedures for radiochemical separation of trace element in molybdenum (reproduced Ref. [551, with permission)

322

ZEEV B.ALFASSI

similarly to all procedures developed in recent years by Krivan and coworkers for determination of trace elements in metals. The flow chart for the two radiochemical separation procedures are given in Fig. 8-2. The two methods are based on Mo, Tc, and W absorbing on anion exchange column in HF media, while the trace elements are eluted with HF. For selective separation of Pa (together with only Sc), lower concentrations of HF are used. The elements given in parentheses are eluted but not completely, so they are found both on the column and in the eluate. The elements determined via medium-lived IRNs and which had to be in eluate 1 according to their distribution ratios, as e.g. Cu, Ga, K, Mn and Na, do not appear in procedure B, since about 1%of the technetium is eluted with this fraction and masking the activity of these IRNs. However, if (NH,),S20, is added to the sample solution prior to its evaporation, in order to oxidize all technetium species to the heptavalence state, less than 0.01% of the technetium is found in fraction 1, and also the other elements can be determined in this fraction. A similar technique was developed by Krivan's group to determine the content of 19 elements in high purity tungsten [56]. However, a change was done in order to measure the concentration of Ta and Sb, which in the previous processes were adsorbed on the anion exchange resins together with Mo, Tc and W, and could not be determined. Ta and Sb are extracted with organic solvent (dichloreothane-DCE) after forming complexes with diantipyrlmethane (DAM), prior to the introduction of the solution of the dissolved irradiation tungsten on the anion exchange column. In order to measure the P content via 32P, only /3--emitter, 32P is substoichiometrically extracted as molybdophosphate with tetraphenylarsonium chloride in dichoromethane [57]. The flow chart of this separation is given in Fig. 8-3. Caletka et al. [58] developed a radiochemical NAA procedure for determination of 26 elements in niobium by irradiation and processing of two samples, one for measurement of medium-lived IRNs (Indicator Radionuclide - the radionuclide used for the determination of an element in NAA), by irradiation for 12 h. The second sample was irradiated for 5 days for the sake of determination of long-lived IRNs. In the determination of medium-lived IRNs, they wanted to measure also 56Mn ( t I l 2= 2.58 h) and 65Ni ( i l l , = 2.52 h). This is possible since the main activity produced from the matrix e4"'Nb) is short-lived ( 11,, = 6.24 m) and one hour of cooling leaves a sample that is not too radiactive to handle. For matrices where the main activity is longer-lived, IRNs with half-lives of a couple of hours can be determined only by pre-irradiation separation of the matrix, or treatment in special hot laboratories, where very high radioactivity can be handled. Different schemes of separations were devised for the medium and long-lived IRNs, however, they have many common features. Both are using solvent extractions followed by separation on columns. The same two solvent extractions were used in the two procedures, and the only difference is in the columns and acids used for separation of the last aqueous phase into three groups. The first extraction step is for cations which form


1

Surface etching

323

I

Carriers ~~~~

Evaporation near to dryness, dissolution in 12M HF I

in DCE,

I

1

Aqueousphase

r.

1 1

Ta,Sb,(Re)

1

16M or 31M HF

fraction B

1 determination Figure 8-3. Flow chart for (he radiochemical separalion of trace elements in tungsten (reproduced from Ref. [56],with permission)

chelates with dithizone and are extracted with chloroform. This fraction includes Cu, Au, Pd and Pt in the medium-lived IRNs and Ag, Se in the long-lived IRN samples. The second extraction of metals is for those which form complexes with diantipyrlmethane (DAM). The organic solvent is dichloreothane. This extraction is used to remove tantalum and niobium, and the organic phase including them is discarded. The elements remaining in the aqueous phase are separated into three groups using two columns in series. A group is absorbed on each column, and the third group contains the elements which were eluted from the two columns. For the long-lived, the first column is the reversed phase di (2-ethylhexyl)orthophosphoric acid on a solid support and then a column of anion exchange resin-Dowex 1. For the medium-lived IRNs the anion exchange-Dowex 1 is the first column, followed by an HAP (Hydrated Antimony Pentoxide) column. In high concentration HCl solution, HAP retains only the Na ions, however, under the conditions used here,

ZEEV B. ALFASSI

324

Oissolulion in the HF-HNO] mixture

r--l Carriers

drgness. assolution in K)M HF

I 1 E.lmcllon

1

Extrachon wlth O.COSM dlthizone. Washlng with CHC13

I Wllh

Cu ,AU ,Pd, Pl

1

1

Dorri-1-column

w. ~

0 ini.zri .

HAP-column Na. K

‘r’ I

I

Figure 8-4. The two procedures for RNAA of trace elements in niobium (a) for medium-lived IRNs; (b) for long-lived IRNs (based on figures in Ref. [%I, with permission)

325


K also remained on the HAP column. The flow chart of the separation is given in Fig. 8-4. A most thorough determination of the trace elements in ultra pure metal was done for aluminum [59]. Previously, we saw that 38 elements were determined in levels of ppbs, by selective removal of 24Na with a HAP column. Later, the same group extended this study, and in order to improve the limits of detection, an additional separation into 10 groups was done [59]. Forty-three elements have limits of detection below 10 ppb, and for U and Th the limits of detection are 50 ppt (5 x lo-"). The separation scheme is given in Fig. 8-5. First, Na' is retained on

I

Activalion ( - l W mp SamOlOl Surfaca doamtamination I

Sample dnompovlbn

' Eluoon rnlh l2molllHCLnOmolllHF S:l O0W.I 1 x 0

C.a d s o m : (AS). Au. BI. Cd. CO. (Cul. fe. Ga. np. I. (In). MO. W. A& sb. (S1I. sn. Tr. w. zn

'

"'I

(+

1+

"2 (Iraciion Co. (Cu). (In). C) (AS).

"2 Fe.

Nb. W

Ga. Mo (banion 0 )

-

EluaCs L h k i i 01 Ilwnde ions ri l h bong acid

Elution win tZmoll1 HCI

ads0rb.d: Np. Pa (Iractlon A)

Eluate

0.8moi/l

nci

CI. (Nal. NI (Iractbn HI Ca. Cs. (Cu). K. hln. Fib. (bacllon I)

-

I

Ce. W.Eri Eu. Gd. Ho. La. Lu. Nd. Pr. Sc. Sm. Tb. Trn. Y. Yb

(fraclion J)

Figure 8-5. Procedure of separation of 52 elements in irradiated ultra pure aluminum to 10 fractions

HAP (hydrated antimony pentoxide). With 12 M HC1 elution, this is the only cation to be retained. Twenty elements from the eluate are adsorbed on anion exchanger resin (Dowex lXS), and later divided into five groups - by elution of four different groups with different elutions. The fifth group are those that are retained on the column after the four eluants. To the original eluate from the Dowex 1x8column, boric acid is added and the solution is introduced into another Dowex 1 x 8 column.

326

ZEEV B. ALFASSI

The masking of the F- anions by the boric acid, leads to retaining of Pa and Np (IRNs of Th and U) on the column. In order to change the medium of the eluted solution for further separation, the solution is evaporated to dryness and dissolved in 0.1 M HC1. The solution is added to cation exchanger Dowex 50 X8 and washed with 0.1 M HCI. The eluate is one group of five elements, while the 27 elements remained on the column are separated into three groups by two elutions. The flow chart of the separation in Fig. 8-5 shows the separation of 52 elements as studied by radiotracers. Not all of them were found in the ultra-pure Al. Only those not in brackets were actually found in the irradiated Al.

8.3.2 RNAA of geologicti1 arid ciiviroiiinciitul suniples 8.3.2.1 Removal of tlie main interferences This method was not used for geological samples, but it was used for trace element determination in sea or river waters [60,61]. Ndiokwere and Guinn [60] irradiated preconcentrated river water (by evaporation either at room temperature or at low temperature under reduced pressure) and removed the main interferences after the irradiation. 24Nawas removed by a HAP column, 82Brwas removed by oxidation to Br, followed by the extraction of Br, into CHCI,. This allows the determination of elements with IRNs which have half-lives larger than a couple of hours, after a decay of 12-14 h (mainly for the decay of Grancini et al. 1611 removes besides 24Na and 82Br, also 32P. 24Na was removed an a HAP column, which was found to retain also Ta (in 12 M HCl), 32P was removed by an AAO (Acidic Aluminum Oxide) column. This column removes also As, W and part of Mo, Ta, Sn,Br and Sb. The remaining Br is removed by oxidation with H20, and distillation of the Br2 formed. C1, I and part of 0 s are removed also in that way.

8.3.2.2 Separation of one special element The separation of a single element is done usually in order to obtain lower limits of detection for elements which are important for some reason. Thus for example, Munita et al. [62] determined the low concentration of thorium in some geological samples. In the section on material science, we saw that Th was determined via the IRN 233Pa. It is done for many geological samples instrumentally, without any chemical separation (INAA). For microamounts of Th,long irradiation times and often long “cooling” times are required to allow the decay of the matrix activity. However, a fast determination can be done using 233Thas IRN ( t1,2= 22.3 min). Its low-energy -pray (main peak = 86 keV) cannot be detected in the presence of the matrix and other contaminants activity, and it can be measured only after complete isolation of Th. 233Th was adsorbed on a cation exchange column (Bio-Rad AG 50W) which was previously saturated with thorium cations. The interfering radionuclides were eluted with a dilute solution of Th in 0.5 M HCl. Singh and

RADIOCHEMICALNEUTRON ACTIVATION ANALYSIS

327

Sawant 1631 used RNAA for determination of Se in geological environmental and food samples. Sediments, phosphate rocks and dust deposits were dissolved in HNO, + H,SO, + HF mixture. The solution is evaporated to small volume and HC1 is added. Se is reduced to elemental red selenium with ethyl-a-isonitroso acetoacetate in 6.5 M HCl. This reagent was found to reduce Se compounds selectively and quantitatively within 10 min on heating the mixture at about 60' C. Cooling in ice lead to settling of the red colloidal precipitate of Se. 8.3.2.3 Separation of a few elements Many studies were done on the determination of precious metals in geological samples. Nadkarni and Morrison 1641 digested the irradiated sample (200-500 mg) by peroxide fusion and the noble metals are separated on Srafion NMRR selective chelating ion exchange resin. However, yields are not well reproducible and reirradiation of each sample is necessary to determine them [65]. Only Pd, Pt, Ir and Au can be determined by this process. The use of the ion exchanger Srafion NMRR was questioned by Stockman 1661 who cannot repeat the results of Nadkarni and Morrison, and attributed it to changes in manufacturing of the resin. There are also some previous results which contrast the specificity of this resin for d8 ions [67,68]. Cocherie et al. [69] developed a procedure for the determinationof all platinum group elements (=PGE - platinum, ruthenium, rhodium, palladium, osmium, iridium and palladium) except Rh. Ag is also determined. Ir is determined instrumentally by biparametric y - y coincidence up to levels of 0.5 ppb. The recovery yields are not reproducible by this method, but instead of determining the recovery yield by reirradiation together with the carriers, they used direct yield determination by the use of radiotracers. This method must use different radioisotopes than those produced by the neutron activation. Thus for example, Pd is determined by lWPd (loSPd(n,y) '@Pd), whereas its recovery is measured with 'O'Pd. These radioisotopes are added to the irradiated sample before the digestion and they are digested together. Different separation procedures were used for different rock materials, according to their matrix. We will use as an example the chromites, which are geological samples with large concentrations of QO,. An important step is the removal of the radioactivity of the matrix. The irradiated samples together with carriers and the radiotracers are digested by alkali fusion. The fused cake is dissolved by 12 M HC1 and is brought to pH 7. Some of the elements remain as a precipitate of hydroxides. The Cr is in the solution. The solution is filtered and the Cr is precipitated as barium bichromate by bringing it to pH 5 and the addition of BaCl,. The previously precipitated hydroxides are dissolved in HCI and added to the filtrate from the barium bichromate precipitate. Pd, Pt, Au and Ag are precipitated from this solution together with Se and Te by reduction to the elemental state with SnCl,. Se and Te solutions are added together with SnCl, solution. The solution is filtered and Pt, Pd, Au and Ag IRNs are determined on the filter paper together

328

ZEEV B. ALFASSI

with measurement of their radiotracers. Thus both their contents and their recovery yields are measured in the same measurement. In this method 0 s and Ru were not determined, since 0 s is very volatile in hot acid solutions, and since there is no good radiotracer for Ru. 0 s and Ru are determined by second, longer, irradiation of another fraction of the sample. In the shorter irradiation the radiotracer 9 7 R is~ the same as the IRN. In the longer irradiation the IRN used is Io3Ru ( t l , , = 39.4 d). The 9 7 R formed ~ in the activation is decayed at the cooling time ( f1,2 = 2.88 d) of two to three weeks. At the end of the cooling time, new 9 7 R is ~ added to the irradiated sample and can be used for determination of the recovery yields. The Ru concentration is determined from the activity of lo3Ru. The digestion process is similar with one exception. In the first fraction after dissolution of the fused cake it is heated to remove H,O2, while for these two elements this step is omitted. Most of the Ru is found in the hydroxide precipitate and it is used for its determination, whereas most of the 0 s (90%) is found in the filtrate. The hydroxide precipitate is dissolved in acid and Ru is precipitated with Se and Te by SnCI, reduction. From the filtrate, Cr is precipitated as BaCr,O, and the 0 s is retained on an ion-exchange column (Srafion NMRR). Chai et al. 1701 developed two different method for radiochemical separation of the noble metals. They found that Nadkarni and Morison were right and that Srafion NMRR resin can be used to retain 0 s and Ir. Ten ml of 0.05 M HCl eluted 100% of Sc, Cs, and Fe, whereas only 10% of 0 s were eluted. Nadkarni and Morrison reported that thiourea can be used to elute the noble metals, whereas Stockman argued that thiourea elutes only gold, while Ir remains on the resin. Chai et al. [70] found that Ir and Re can be eluted with NH,, whereas only one-fourth of 0 s was eluted and none of Au or Pt. Chai et al. [70]found that all PGE including Au is retained by various productions of this resin. Chai et al. suggested a scheme for radiochemical separation for noble metals which is given in Fig. 8-6. They found that their digestion process forms mainly Ir(1II) which is not absorbed onto the chelate resin. However, oxidation of the chelated Ir with hot H202transforms all Ir to the +4 oxidation state, which is absorbed on another column with the same resin. An alternative process for the separation of the noble metals suggested by Chai et al. [70]is based on extraction of noble metals with long chain primary amines (19-23 carbon atoms) in dichloroethane. All the noble metals are extracted in high yields into the organicphase. The base metals were only negligibly extracted. Oddoneet al. [713 measured very low levels of platinum group elements in standard rock material, as well as coal, ash and botanical samples. They determined the concentration of all platinum group elements (PGE - ruthenium, rhodium, palladium, osmium, iridium and palladium besides platinum itself) and also gold. The samples were fused with N%O, + NaOH. The 0 s and Ru were successfully oxidized to tetraoxides (OsO, by KMnO, and RuO, by Ago) and extracted with CCl,. The remaining cations were retained as the chlorocomplexes in HCl solution on anion exchange resin. Pd, Pt

329

RADIOCHEMICALNEUTRON ACTIVATION ANALYSIS

I

ca. 200 ng srnple irradiated in f,facto_r2 s - l for 60 hrs at 8x10 n.un

.

1-

add c a r r i e r of os I r Ru,Re,Pt, and anticarrier of Sc,Cr,etc.

]decayed for 1 day melted in ruffle furnace at at 65OoC for 20 min

7 :z

1 rinse the melt with hot m t e r ,

Be;,: ‘$3

- - .- .

acidify with H=l and m k e solution transparant

1

adjust pH to 1.5 with 7”Hj

1

transfer solutibn to chelate ion exchange colum 1,100 mesh,&x90 mn

I resin phase and base metals

1

count i ng

heat and adjust .1

transfer solution to chelate ion exchange colum I I ,10(mesh,g8x90 mn

m resin phase

+.

count 1%

Figure 8-6. Radiochemical separationprocedure of noble metals based on chelate ion exchange resin (reproducedfrom Ref. [701, wilh permission)

and Au were eluted with 0.1 M HCl aqueous solution of thiourea. Ir and Rh remains on the resin. Each fraction was counted with y-ray spectrometry to determine the concentration of each element. As the separation yields are not reproducible, the yield of the recoveries have to be determined for each element by removal of the extracting or eluting solvents or the resin, reirradiating and determining the amount of each element vs. its amount added as carrier. Two radiochemical separation methods are based on fire-assay procedures which are more frequently used to preconcentrate the low concentrations of noble metals in large samples of ores [72,73]. Hillard [74] irradiated with epithermal neutrons 500 mg samples and collected Ir, Au, and Ag in 1 g lead buttons using the mini fire-assay technique. The main contaminants found in the buttons were As and Sb.

330

ZEEV B. ALFASSI

They were removed by heating the button with a mixture of Na202 and NaOH. The resulting 0.2 g lead-bead is counted in a Compton-suppression spectrometer. The chemical (recovery) yields are very irreproducible (15-6095 for Ir in two-thirds of the samples, or 47-77% for Au) and the carrier yields are determined for each sample by reirradiation of the lead beads with neutrons. Parry et al. [75] developed a method for fire-assay of 50 g samples with NiS in order to preconcentrate PGE before analysis. Later, in order to reduce the blank value due to chemicals in the fusing mixture, they irradiated the sample before separation, followed by radiochemical separation via the NiS-fireassay technique. However, special hot labs should be available to handle irradiated 50 g geological samples. Pietra et al. [76] developed many radiochemical separation procedures, and one of them was developed for determination of noble metals and Hg in geological samples. The method involves the fusioii in a closed system of the sample in a Ni boat with recovery of the distilled Hg. The fused sample is dissolved in H,SO,. 0 s and Ir are distilled as tetraoxides from this solution. The other precious metals are separated on the ion exchanger IONAC SR3. Another group besides the noble metals, which are determined many times by RNAA, is the rare earth elements (REE) group. Zilliacus et al. [77] described two procedures for the separation of the REE group from irradiated geological samples. A simple and faster method was developed for samples with concentrations above 0.5 ppm, while a method with more separation steps was developed for samples with lower concentrations. The more time-consuming long process is required for obtaining a clear REE fraction. The two processes are described in Fig. 8-7. Both methods are based on fusion with Na202and cycles of precipitation as hydroxides and as fluorides. In the simple method there is one more step of removal of Sc and other metals by extraction with tributylphosphate. For the lower concentration samples, Si02 is removed by precipitation with gelatin in acidic solution, the main interferences are adsorbed on an anion exchange column in HCI-medium, while the REE are eluted. Sc is removed by extraction with ether from SCN- solution. Purification of the REE fraction is done by a cycle of precipitation of hydroxides and fluorides. A similar process was developed also by Laul et al. [78]. They don’t have the step of Sc removal by extraction; however, they are doing in the final step, three cycles of hydroxides-fluorides precipitation. Meloni et al. [79] dissolved the samples in HCI/HF mixture, converted the digested material to chlorides by drying and dissolving in HCl, and used cation exchange column for separation. Two washings (0.5 M HCI + 0.1 M oxalic acid, and 2 M HNO,) were done to remove all interfering elements. The REE are collected on the third elution with 6 M HC1. However, their method has problems when analyzing geological samples with high silicate concentrations. A similar method was developed by Wandless and Morgan [80]. They did not use the fluoride-hydroxide precipitation cycles in order to save time, and used more

33 1

RADIOCHEMICALNEUTRON ACTIVATION ANALYSIS Irradiated sample Fuse with Na202 + carriers Dissolvc the melt in H20

I

I

Method 1

Add HCI until clear green solution Liquid t o wastePrecipitate to waste

-

I I

I

100 x 10 rnm Dowex-2 column

I . I

3

Y1StC

Dissolve with HCI Add thiocyanate solution Extract uitb ether

Liquid to wnstc-

Precipitate with NaOH

Org. phase tci

-

Centrifuge -Liquid

to waste

3

Dissolve with HC1 Precipitate with Relatine Pass the solution through

Liquid t o waste-

I I

Precipitate with NH

Effluent solution Precipitate with NH

Method 2

Dissolve with HCI Precipitatc with NH4F solution -Liquid I t o waste Dissolve with I l j B 0 3 + HCI Org. phase Extract with TBP I t o waste Precipitatc lanthanoids with HH4F

-

I

I

Dissolve with HCI Precipitate lenthanoids with NHhF at pH 5

Figure 8-7. Separation scheme for the analysis of REE in geological samples (taken from Ref. [77]. with permission)

the anion exchange column. The method is based on the precipitation of the REE hydroxides both at high pH and at pH 9 in ammoniacal solution. SiO, is removed by precipitation with gelatin in acidic solution. Fe is removed by an anion exchanger in the C1- form from 8 M HC1 solution. Zr, Sc and Hf are removed by an anion exchanger in the SCN- form from 0.8 M SCN- + 0.5 M Cl- solution. The only remaining interference is Crt3 which is removed by precipitation of the REE with excess 8 M NaOH. The complete scheme is given in Fig. 8-8. Bishop and Hughes [Sl] used Na202+ NaOH for digestion. To increase REE recovery and recover REE not precipitated in the first step, they added FeC1, and the REE is coprecipitated with ferric hydroxide. The REE is separated using cation exchange column of Biorad AG 50W-X8. The other elements are removed by three elutions (1 M HCI, 2 M HNO,, 1 M H2S04)and REE is eluted in the fourth elution by 5 M HCI. In order to reduce the volume of the eluate (300 ml) but not to zero, 5 g conc. H,SO, is added and the solution is heated until only concentrated H2S04remains. Loron and Ottenlo [82] removed most metals with anion exchange column. However, they add a DEP-celite resin column to remove scandium. REE is precipitated as fluorides together with CaF,. Stosch et al. [83] used the same process and the only change is removal of scandium by precipitation with phytic acid. Koeberl et al. [84] developed a completely different separation process. The irradiated material is digested with 2 M HCl and the mixture is separated to solution and undigested residue. The residue is decomposed with HF/H,S04. It is separated

ZEEV B. ALFASSI

332

.

Sample REE corr~ers

2

Fusion wilh NoOH, No201

-Leach with H$

Supernale Si. AI,

K.

ppl

2' REE. Ni, Fe. Mg. Zr, Sc, HI

Rb. Cs

-NHrCI*NHLOH

Supernole

10

pH9

PPl

Mg. Co. Ba. Sr. Ni

~

PPl

502

REE. Fe, Cr. Zr. Sc. HI

IzFLin Supernote

REE Fe, Cr. Zr, Sc. HI

-6M 200-400

HCI

IWSh

CI-lam -Evopomte

10dryness

100-xx)mesh

SUpQrnOW

PPt RE€ I-HCI RE€

Figure 8-8. Separation procedure flowchart for RNAA of REE in geological samples (reproduced from Ref. (80).with permission)

again, and the residue is dissolved in 7 M HCl, evaporated to dryness and dissolved in 2 M HCl. The two fractions dissolved in 2 M HC1 are added together, brought to 9 M HC1 and then loaded on anion exchange column. The elute is extracted with tributylphosphate,and the REE remaining in the aqueous phase is precipitated with oxalic acid solution. Parthasarathy et al. [SS] separated the REE into two groups rather than to one group, as all other processes. They separated it into a group of light REE and a group of heavy REE. The process is different from all those previously described and the flowchart is given in Fig. 8-9. Other elements besides REE and PGE were also separated in irradiated geological samples, usually 2-3 elements. Garg et al. [86] determined the concentration of Rb, Cs and Ta in mica samples by RNAA. Some other elements were determined without any chemical separation

333

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS HREE

LREE I r r a d i a t e d s o l u t i o n + 20 mg Mn Add 10 m l cc. %NO3 and KBrO.,

+

20

mg La.

&

Iln02 p r e c i p i t a t e 2 3 3 ~ a/ r e j e c t /

S o ht i o n

r

MnOz precipitate 2 3 3 ~ a/ r e jectl Hydroxide p r e c i p i t a t e D i s s o l v e i n cc. H C 1 and c o n v e r t t o c 1' form. Take i n 10 ml of BN ltCl: Add KCIOJ t o o x i d i z e NplIVl to Np/Y/. P a s s t h r o u g h pre-equilibrated Dowex-1 ,column

Add 2 0 mg Mn+KBr03 1 Solution Carry o u t 3 c y c 1 c s Of roll-

Solution Reject

I

Resin 239Np reject

I r r a d i a t e d s o l u t i o n + 20 mg Mn + 20 mg La t 2 0 m Y. Carr o u t t h e s e p a r a t i o n of 213Pa and 1 3 9 N p as shown i n LREE. The e f f l u e n t from t h e Dowex-1 column a f t e r t h e separ a t i o n of 239Np is e v a p o r a t e d t o d r y n e s s and c o n v e r t e d t o NO: form. The f i n a l r e s i d u e i s t a k e n i n 10 m l 2 . 5 8 7H lINO m e t h a n o l m i x t u r e . Pass t h e s o l u t t o n through a p r e e q u i l i b r a t e d Dowex-1 / 2 0 c c / column.

Ef I h e n t

Ef Eluent reject

Eluate Evaporate t o d r y n e s s and precipitate Y-oxalate

Column/Resin/ E l u t e w i t h 50 m l 10%i~ H N O ~ methanol

-

Resin LREE u p t o cd

Evaporate to d r y n e s s . Take t h e r e s i d u e i n w a t e r . Add o x a l l c a c i d and p r e c i p i t a t e La-oxalate

Figure 8-9. Flowchart for RNAA of REE by separation of two fraclions, light and heavy REE (LREE and HREE) (reproduced from Ref. [8S].wilh permission)

(INAA). The irradiated sample was dissolved by fusing with sodium peroxide. Rb, Cs, Cd and Hf were precipitated by sodium tetraphenylborate. Ta was separated by solvent extraction using N-benzoyl-N-phenyl hydroxylamine and cupferron in chloroform. Van der Sloot et al. [87] developed a method for separation for arsenic, selenium and antimony in geological and environmental samples. The method is based on dissolution of the irradiated samples in an acid digestion bomb, evaporation to near dryness, uptake in 6 M HCl and separation of As, Se and Sb from the interfering elements by the formation of the hydrides. The hydrides of these three elements are volatile and can be separated by distillation. The hydrides are formed by sodium borohydride (NaBH,). The hydrides formed are swept out by an inert carrier gas and collected on an active carbon column. To ensure complete volatilization, the elements have to be present as As"', Sb"' and Se'". To achieve this, a reducing agent has to be added before hydride formation. The three elements cannot be determined together due to very different half-lives of the IRNs. For determination of Sb and As the samples were irradiated for one hour and cooled for 24 hr. Reducing to the I11 state was done by 10%ascorbic acid + 0.2 M KI solution. Se (together with Sb also) are determined after 24 hr of irradiation and cooling for 3 weeks. The reduction is done by refluxing with 6 M HCl. Drabaek et al. [88] measured the concentration

334

ZEEV B. ALFASSI

of Hg, As and Se in coal, sediments and biological samples. After digestion, Hg is separated from the solution by distillation as HgCl,. HgCI, is distilled only above 120' C, while HCl would distill off already at lower temperatures. In order to solve this problem, HCI is formed it! sitic at the higher temperature by the reaction of HClO, with glycine. Hg recovery is measured by electrolysis, using gold and platinum electrodes. Hg is electrodeposited on a gold foil, and its amount is determined by weighing. As (111) and As (IV) remaining in the solution is distilled as the bromides after addition of HC1 and HBr. In order to measure recoveries, either Se or As carriers were added, but not both of them. Their recoveries were determined by precipitation with ammonium hypophosphite and weighing of the precipitate. They suggested that if both yields have to be determined simultaneously,it can be done using XRF. Rammensee and Palme [89] used extraction with molten metallic iron from silicates to separate the contents of geological matrices into two fractions,those quantitatively dissolved in iron and those dissolved in silicates. Those which dissolved only partly in iron cannot be determined in this method. Only one separation step was needed in which the geological sample is mixed with iron and with silicates (2:l) and heated to 1580"C. After cooling, the metal spherule is manually separated. The first y-measurement can be made about 3 hr after the end of irradiation (the term pile-out is used many times instead of end of irradiation, in the case of irradiation in a nuclear reactor). All the 24Naactivity, one of the main interferences in geological samples, is concentrated in the silicate phase and siderophile elements (iron-loving) with half-lives from several hours to 100 hr (Cu, As, Sb, Re, Pt, Au, W, Mo) can be determined simultaneously in the iron phase. The main long-lived activities in geological specimen are due to Fe and Co. As these elements are concentrated in the iron phase, litophile (silicate-loving)elements can be more accurately determined. In principle, this is a similar method to the fire-assay technique. However, there are two advantages in that method, according to the authors. More elements than only the noble metals can be extracted, and since the recoveries are quantitative, no yield measurement should be done, simplifying the analysis.

8.3.2.4 Group separation methods Several group separation methods were developed for geological samples. For space reasons, not all of these procedures can be described here, and only a few of them will be given as examples. The main difference between the various separation procedures depends on which geological samples are used. Some methods are good only for some special geological samples, while in other samples some elements with relatively high concentrations interfere with the determination of other elements in the same group. Morrison et al. [90] described a procedure for chemical group separations, which, with the use of a Compton suppression Ge(Li) detector, can determine 45 elements in geological samples. The flow chart of this process is given in Fig. 8-10. The


335

Sample Carriers (Se,Zn,Lo,As,Br and HF+I$S04

1

I

Residue

I

Dlssolvad In 8N HCL

76

82

A s , Br

Solulion HAP botch rxtracllon ond column arlractlon

Passed through o 10cm

-

column of Dowar' 1 ~ 8 . 2 0 0 400 mesh

1

Evaporated

Rekdue ;;s$rd

65Zn, 6 9 mZn,1Z%b,124Sb

In

197

"Fe 239

197n

Hgi

,6oCo :4Cu

Np I

Hg

I87

,%a

W ,"Mo

Extracte with 10 8.20 rnl TBP I

I Organic phase (group 5 ) ] 181

Hf,

97

48

Zr,

40C,47SC,17kf,

Sc "Po

1 Aqueous phase (group 6 11 I

42K, asRb,"4Cs, '7Sc , l"Ea 85Sr, "Ice ,'47Nd!'3Sml15ZE~

'54Eu,1''Gd ,16aTb:46La ,'%o 170Trnr'Yb

,IT7Lu,%r

, *,'Ca

Figure 8-10. Flowchart for chemical separation of 45 trace elements in geological samples into six groups appropriate for simultaneous counting (reproduced from Ref. 1901, with permission)

first group is the one including volatile elements and volatile fluorides. Elements adsorbed to HAP in 8 M HC1 formed the second group. Those adsorbed to anion exchange resin in 8 M HCl formed the third and fourth groups. The separation between these two groups is done by elution with 0.5 M HC1. The elements not adsorbed on the anion exchange in the 8 M HCl are separated into two groups by extraction with tributylphosphate (TBP). Laul et al. [91] developed a RNAA method for trace elements in terrestrial rocks and stony meteorites. They stated that their main emphasis is minimizing chemical procedure and maximizing the -pray spectrometry aspect. However, their separation is longer than that of Morrison et al. [go] and they measure only 16 elements. The flowchart of their separation is given in Fig. 8-11. The scheme involves four different separation techniques: distillation, solvent extraction, ion exchange columns, and precipitation. Allen et al. [92] developed a RNAA method for 39 elements in small or precious geological samples. However, it involves many separation steps, as they

ZEEV B. ALFASSI

336

Exirocfion

6 N HCI

K2H,h,O

&

Oroonic

Pr-i;cipilaie

-Ion -exchonoe 2N H

, r I

-1

Aaueous

Voldtile bromides Preci ilolion

Supernaie

I

@-

b

SuDernote '

Ion - exc hog.

I

E

, Pr+:i:itatiye

Supernole Precioitoiion

c0,sc

Supernole discord

Figure 8-1 1. Chemical separation of trace elements in geological samples, leading to determination of 16 elements (reproduced from Ref. [go], with permission)

separated chemically the sample into 12 groups. For some elements with very low concentrations, they used NaI(Tl) detector. Three chemists can finish the chemical separations and sample preparations within 7 hr after pile out. One NaI(T1) detector and two Ge(Li) detectors were used for counting. The process involves only solvent extractions and precipitation and is given in Fig. 8-12. Keays et al. [93] developed a very detailed method for RNAA determination of 20 elements in terrestrial, lunar and meteoritic samples. In order to determine very small concentrations, the method separates the determinand to many small groups and the power of 7-ray spectrometry is not really utilized. $met et al. [94]developed a group separation for RNAA determination of 24 elements in a wide variety of silicate rocks and minerals. The samples were decomposed in a HF-HNO, mixture in a PTFE-lined bomb, and it was separated into soluble and insoluble fluorides. The soluble fluorides were separated into three groups by sequential elution (0.1 M HF, 3 M HC1+ acetone, 12 M HCl) from a cation exchange column. The insoluble fluorides (Ca, Sr, Ba, REE and part of Rb and Cs) were dissolved and purified from iron and scandium activities by extraction with TBP. The scheme of separation is given in Fig. 8-13. The elements written in bold were determined, whereas the usual type letter elements are those that are in more than one fraction and cannot be determined. In some samples their concentration is too


337

Solid G

A i r m orolnlc

Figure 8- 12. Allen el al. I921 procedure for chemical separation of 39 trace elements in small geological samples into 12 groups (reproduced from Ref. 1921. with permission)

..

IRRAOlAlED SAMPLE 1250 rnpl CARRIEIIS DISSOLVE 111 LO% IIF 111i03

I EVAfQRAIE

- 01 1.1 IIF -

-

-

- 12 1.1 HCI

111 01 1.1 IIF

Zr,Sb,lif,Ta ,W,Pa. sc,cr

T

I

ORG PH

-

3 I 4 llCl ACEIOI*E I10 I 0 v/v

, OISSOLVE

I

-

Cu,Zn,Ga,Fe

-

pH:L,. E D l A ,

1 AQ pH.1,

Sc,Fc

rii s

EOlA.

rmw w t m i l C O

SUPERNAIE

I'IIECIII WllH HJPII'D

[

Na.Cr.Co

-

I - - 1 PR~CIPITATI?

PREClPllAlE

Rb,Cs,i(

R b, C s, I( I C0I.IOIFIE __

I

SUPERNAIE

REE,Ca, Sr, Ba. Na.Cr.Co

Figure 8-13. Chemical separatioii procedure for groups of trace elements in geological samples (reproduced from Ref. 1941. with permission)

high and they might interfere with the determination of the other elements in the group. For these cases, the authors developed a more thorough scheme, in which for each fraction there is a method to remove the interferences. However, these further steps should be done, only if y-spectrometry will not be sufficient. Sun et al. [95] developed a procedure which will enable the determination of all fourteen REE together with other elements (22) in a small geological sample (62.1 mg of lunar sample). The samples were digested with HF + HNO, + HC10, mixture. After heating to almost dryness, the residue was dissolved in 1m19 M HC1. The irradiated mixture was separated into 12 groups, using a HAP column in series

338

ZEEV B. ALFASSI

with an anion exchange column. Two consecutive elutions of the anion exchange column were done (9 M HCl and 0.5 M HCl). The 9 M eluate was extracted with trifluorothenolyacetone in isoamyl alcohol to remove Ti, which is extracted into the organic phase. The aqueous phase was further extracted with cupferron in chloroform. The remaining elements in the aqueous phase are separated into 7 groups by reversed phase chromatography columns of dL(2-ethylhexyl) phosphoric acid (HDEP) on a kel-F support. Two columns of HDEHP were used, first column of 50% HDEHP was eluted successively with 0.05 M HC1 and 9 M HC1. The 9 M HCl eluate was loaded on 10% HDEHP column and eluted successively with five eluants (0.15 M HCl, 0.21 M HNO,, 0.4 HNO,, 1.5 M HNO, and 5 M HNO,). Pietra et al. [76] developed 22 different RNAA procedures for environmental and biological samples. Many of these procedures are for several elements or for single elements; however, some of these are for group separations. The most extensive one (50 elements) was used only for biological samples. The next most extensive (39 elements) procedure was used also for geological samples, and the scheme of 01-05 g sample

DlSSdVQ

in tellan

Wash 50 cm’ CUM H N 4

y’ ccartb- -Sc. Fe. In Hf. NO

!s

-B-N_a &.c_s, !&

a&.%Q

Figure 8- 14. Radiochemical separation scheme for the delermination of 39 elements from geological samples (reproduced from Ref. [76], with permis-

sion)

339


the separation is depicted in Fig. 8-14. The samples were dissolved in acid mixtures in a teflon bomb, and after drying, dissolved in 0.1 M HNO,. 0.1 M HNO, was used also as the only eluant. The trace elements were separated into five groups by the use of five different columns-acidic aluminum oxide (AAO), tin dioxide (TDO), copper sulfide (CUS), cadmium oxide (CDO) and hydrated antimony pentoxide (HAP). The separation time for this procedure is about 2 hr. In another scheme where the geological material is dissolved in 6 M H E HAP column was first used, followed by cation exchange column, and later, anion exchange column to separate the mixture into 4 groups in which 35 elements can be determined. As in the previous scheme, in order to simplify the procedure and to make it easier for automation, they use only one eluant (6 M HF). Vasconcellos and Lima [96] developed procedures which is somewhat between several elements and group separation. It is based on the use of three columns-hydrated atminony pentoxide (HAP), anionic resin column and a reverse phase chromatography column Sample

carriers

( Zn, Co, S c , Sm)

HF H C l O r R t s i d u r dissolved n EN HCI "Na , TaI1"Rb, I n Sb, 1z4Sb,2 3 3 p g , 4 6 s ~

1

Rock solution

#= h=

b= h =

+=

h=

"K,

E I 1I ue nI

"Sc,

4'Sc,131h,

"5,I4'Ce, 147Nd153Sm I 5 2 Eu, Tb, I 40 La 16 6 H ,'

1sgYb,177Lu,' I C r , 4 7 C a , '"HI

Figure 8-15. A simple group separation scheme for geological samples (reproduced from Ref. [96], with permission)

Figure 8-16. A system for rapid separation through a series of column (reproduced from Ref. [96], with permission)

ZEEV B. ALFASSI

340

with tributylphosphate on a kieselguhr support. Several elements appeared in more than one fraction, but all the REE appear in one group and can be measured. The same is true for Na, Ta, Zn, Fe, Co, Cu, Ga, K, Ba, and Cr. The flow chart of the separation is given in Fig. 8-15. Figure 8-16 shows the construction of the three columns in series. The elution is speeded up by suction with a vacuum pump. This series of columns can be used when the eluants is the same for all columns, as in the scheme of Pietra et al. 176). The determination of geological samples was reviewed previously by Laul [97] and Fardy [98]. Chai [99] reviewed the RNAA of platinum group elements.

8.3.3 RNAA of biologicul samples

8.3.3.1 Removal of the main interferences The main activities of irradiated biological material are 38Cl ( t , / 2 = 37.2 m, a = 0.06 6, (T u is the product of the cross section in the natural isotopic abundance which gives the actual probability for formation of the IRN), 24Na ( t I l 2= 14.96 h, . a = 0,53b), 42K( t l I 2= 12.36 h, D . ci = 0.98b) and 32P.32Pis only /3- emitter and its activity can be stopped by a thin absorber which will absorb only a small fraction of the y activity. This absorber should be made of low Z elements in order to diminish the formation of X-rays by the stopping of the p particles. The best absorber will be 5-10 mm thick polyethylene. 38Clis many times removed by decay. In cases where short-lived activity has to be measured, 38Clis removed by one of three methods: (1) dissolution in a mixture of acids, followed by evaporation to dryness, where 38Clis removed as HC1 vapours; (2) precipitation as AgCl with AgNO, solution; and (3) adsorption on an anion exchange column. The main interferences are 24Naand 42K. 24Nais usually the major one as its concentration is larger and its CT a is 5.4 times larger. 24Naand also 42K are usually removed by hydrated antimony pentoxide in high concentration HC1 media [26,39]. Although some other columns [101,102] or composite-HAP columns [103,104] were also used, they are not advantageous to HAP columns. The only disadvantage of HAP columns is that since Na adsorption is irreversible, new resin has to be used after a few separations. The removal of Na by HAP is quantitative and quite fast. Nadkarni and Ehmann [ 1051removed from digested irradiated cigarette tobacco 24Naand 42K on HAP columns. They measured after the removal, the activity of La, As, Br, Se, Cs, Sb, Cr, Ag, Se, Zn, Fe, and Co. Murthy and Ryan [lo21 determined several elements in biological reference material after removal of 24Naby adsorption to a polymer containing cryptand 221B.Piedade-Guei-reiroet al. [ 1061 removed 24Naby HAP column in 12 M HCI media from mineralized biological standard reference material in order to measure Ca, Fe, Zn, Co, and Ba. Maihara and Vasconcei10s [lo71 measured Cu, Zn, Sb, and La in milk powder and bread after removal of 24Nafrom the digested sample in 8 M HCI by a HAP column. D

+

+


34 1

8.3.3.2 Separation of a single element Single-element separation from all other radionuclides in the mineralized irradiated samples allows the determination of very small concentrations of this element. The activity can be measured with a low-resolution high-efficiency NaI(Tl) welldetector. Different single elements were determined in that way: V [ 108,1091, Cr [110],Mn [111],Cu [112-116],Zn [117],As [115,118,119J,Se[63,120-124], Mo [125], Ag [126], Cd [122], I [128,129], Cs [130], La [131], Pt [132-1381, Hg [ 139,1401. Pietra et al. [76]developed procedures for single-elementseparation €or the following elements in biological materials: Ca, Ga, Mg,Hg, I, P,Sn and TI. In the following section, examples of the large number of procedures will be given.

Vanadiuni - Both Cornelis et al. [lo81 and Simonoff et al. [log] used RNAA to determine V in blood serum by solvent extraction with cupferron. Simonoff et al. [ 1091 dissolved the sample in HCl and NH,VO, was added as carrier and 48V02Clwas added for chemical recovery measurement. V was extracted from the aqueous phase by cupferron in chloroform. Chroniiuni - The samples are dissolved in a mixture of HNO, + H2S04 + HF + HClO, (not all simultaneously). The Cr in the solution is in the +6 oxidation state. KMn0, is added to oxidize any remaining Cr to the +6 oxidation state. HCl is added to the cooled solution and Cr is extracted into chloroform containing 5% tribenzylamine (TBA). Cr (VI) is back extracted into the aqueous phase by 2 M NaOH solution.

+ HNO, mixture. The solution was diluted with water, and the solution was neutralized to pH 69 with NH,OH. Addition of ammonium persulfate and boiling the solution lead to precipitation of MnO,.

Murzguwese [111J - Samples were mineralized with HClO,

Copper - Dybczynski et al. [112] dissolved the samples in HN03-HC10, mixture with vanadium (IV) as oxidation catalyst. After pH adjustments the solution was loaded on a LIX 70 column, and the column was eluted with 1 M NaNO, + 0.24 M NH,PO, adjusted to pH N 3.4. Copper retained on tbe column and was eluted with 4 M HC1. Rajadhyaksha and Turel [ 1131separated copper from solution of digested material by substoichiometric extraction. Copper was substoichiometrically extracted with chloroform containing 2-mercaptobenzothiazole. Whitley et al. [1141 extracted Cu from the aqueous phase with neocuproine in chloroform. Cortes et al. [1151mineralized the biological material with HNO, + H202, evaporated to dryness, and the residue dissolved in 6 M HCl. 24Nawas removed by a HAP column. Na2S0, is added to ensure that all Cu is in the +I oxidation state, and KCNS solution is added to precipitate CuCNS.

342

ZEEV B. ALFASSI

Arsenic - Cortes et al. [ 1151 add also H,S04 to the digesting mixture for determination of As (in addition to HNO, + H,O, used for Cu determination). To eliminate excess oxidants, SnO, is added. Then KI is added in order to precipitate arsenic. The arsenic is soluble in toluene, and it was extracted into it. Comparetto et al. [118] precipitated arsenic from the solution of the digested material by reduction with sodium hypophosphite. May and Picot [ 1191used first a cation exchange to remove most of the cations, and then As is precipitated as As$,. Pietra et al. [76] separated As by the formation of the volatile ASH,, by reduction with Zn + SnC1, + K1 in 10 M HCl media, and retaining the ASH, on an AgNO, trap.

Zinc - Shah and Haldah [ 1171 separated Zn by substoichiometric extraction of the Zn-isonitrosoacetophenone complex into chloroform at pH 7.5-8.5. The time required for radiochemical separation and counting is less than 35 min.

Seleniicnz - Damsgaard et al. [120] separated Se by precipitation after reduction with ascorbic acid, followed by extraction with methyl isobutyl ketone from HC1 solution. Itawi and Turel [121] precipitated Se (IV) from HCl solution with 1amidino-Zthiourea. The Se precipitate is dissolved in HNO, and further purified by extraction with chloroform containing 2-mercaptobenzothiazole. Kalouskova et al. [122] also used extraction of Se. To ensure that all Se (VI) is converted to Se (IV) they heated it to dryness in HC1 media. They extracted into toluene the benzoselenodiazole formed in the reaction with o-phenylenediamine. Navarete et al. [124] separated Se by adsorption of its complex with ammonium pyrrolidine dithiocarbamate (APDC) on active carbon at pH 1. A similar method was used previously by Lavi et al. [ 1411 when coprecipitating Se(PDC), with Pb(PDC), pre-irradiation in order to determine Se through its short-lived isotope 77mSe.

Silver - Bowen and Sugari [ 1261separated Ag by precipitation of AgCl. The silver is further purified by dissolution in 18 M ammonia and scavenged with one drop of 5% Fe(NO,), solution. The silver, remaining in the solution, is precipitated with Na,S solution. Ag2S precipitate is dissolved in hot 8 M HNO, and reprecipitated.

Cadnziunz - Itawi and Turel [1271extracted substoichiometrically Cd (11) complex with 2-mercaptobenzothiamle (HMBT) into methyl isobutyl ketone. The HMBT is added in an alcoholic solution.

Zodine- Dermelj et al. [1291determined trace quantities of I in tobacco, by combustion of the sample in oxygen atmosphere in Schoniger technique. This is followed by selective oxidation to I, with NaNO,. The I, is extracted to CCl,, and selectively reduced with Na,SO, and I- is back-extracted into an acidic aqueous phase. The I- is then re-oxidized and re-extracted. The isolation procedure takes 15-20 min.


343

Pietra et al. [76] used combustion distillation to separate I. I, is distilled from the combustion vessel onto a column of hydrated manganese dioxide kept at room temperature. Heating the column to 300° C leads to release of I,, which is trapped on a silver-coated quartz wool. The overall separation time is 20 min. Cessiimi - Biran and Guinn [130] separated Cs by removal of 24Na with a HAP column, and adsorption of Cs on ammonium molybdophosphate (AMP) solid by stirring 50 mg AMP in the solution. Ninety-nine percent of the Cs is adsorbed on the AMP precipitate, removed by filtering.

Plutirzim - Platinum is determined usually as gold due to the chain of reactions: I9'Pt (12, y) 199Pt% IWAu. Although Ig9Aucan be formed also from gold Ig7Au ( n . , ~lg8Au ) (n,y) Ig9Au, its contribution can be subtracted by the measurement of both I9*Au and IwAu. Esposito et al. [132] separated gold by using anion exchange column in HCl media. The chlorocomplex of Au is strongly bound to the column even in 0.5 M HC1 media. The chlorocomplex of Au is strongly bound to the column even in 0.5 M HCl. Au is eluted with 0.1 M HCI containing thiourea. Zeisler and Greenberg [133,134] separated gold by heating the solution of the digested material to above 250"C. At these temperatures, gold compounds decompose and an auto-reduction to elemental gold takes place. The process was done sufficiently slow, to allow the gold to precipitate in the form of fine particles agglomerating to the form of a nugget, or in the form of a leaflet accumulating to a leak-like precipitate, without the inclusion of the matrix activity. Kucera and Drobnik [ 1351 separated gold as metal by coprecipitation with selenium after reduction with ascorbic acid in highly acidic media. Xilei et al. [136] separated gold by electrolysis on niobium cathode (with a graphite anode) at 800 mV. The recovery yield is determined by reirradiation. Czauderna [ 1371 separated gold, for determining Pt, either by precipitation with dimethylglyoxime or by extraction with ethyl acetate, Taskaev et al. [ 1381 determined Pt by Au separation by extraction with Cu(DDC), into chloroform. (DDC = diethyldithiocarbamate).

Mercury -Grimanis and Kanias [1391 separated Hg, after wet-digestion with H2S04/ HNO, mixture, by quantitative extraction of HgI, from 10 M H2SO4-O.025 KI media into toluene. HgI, is back-extracted into aqueous phase containing 0.034 M EDTA + 5% ammonia. Biso et al. [140] separated mercury by combination of two precipitation steps. In the first step elemental mercury is precipitated by reduction with hydroxylamine in the presence of ammonia. The precipitate is filtered and dissolved in HNO,. Hg(SCN), is precipitated in the second step by the addition of NH4SCN solution. Pietra et al. [76] used combustion-distillation technique to separate mercury. The distillate is further purified by precipitation of HgS with the addition of concentrated solution of thioacetamide.

344

ZEEV B. ALFASSI

8.3.3.3 Separation of several elements Many articles have been published on simultaneous determination of a few elements by RNAA. We will summarize a very small part of these articles. They can be divided according to the kind of the matrix, the kind of the elements or the type of the separation. We will not go according to any of these categories, but just report on some of these studies, as examples. Fardy [98] summarized in table form the studies from 1980-1988.

R a w Earth Elements - Siripone et al. [37] used open wet-mineralization with the addition of Mg(NO,), to prevent losses. The solution of the mineralized biological material is adjusted to pH 2.5, APDC (ammonium pyrrolidonedithiocarbamate) is added and the precipitate is refiltered through a 50 mg carbon on filter. The pH is adjusted to 0.2 and refiltered, later adjusting the pH to 2.5, reducing with ascorbic acid followed by APDC + oxine and filtration. Afterwards the pH is adjusted to 5.0, reducing with ascorbic acid, precipitating with APDC + oxine + cupfei-ron, filtering through a carbon filter. Lastly, pH is adjusted to 7.5, precipitating with APDC and filtered through a carbon filter. Before last and the last precipitates contain REE + some other elements. Tjioe et al. [142] determined 8 W E in biological samples together with 15 other elements. Most fractions contain one to two elements and only the REE fraction contains 8 elements. The samples are digested with H$04 + H202in the presence of REE-carriers (50 pg of each of the eight REEs). Br and Cr are distilled as Br, and chromyl chloride. Na and Rb are absorbed on a HAP column (called in this paper - PAA-polyantimonic acid) in 8 M HC1. Loading the eluant on anion exchange column and eluting with 8 M HCl results in the REE with 32P,42K, 47Ca and 47Sc. REE is precipitated with NaOH in the presence of EDTA. Pietra et al. [76] separated REE from digested material in HNO, by heating to dryness and dissolving in 6 M HE The solution is loaded on a column of anhydrous manganese dioxide (AMD) and eluting with 6 M HE REE, Ag and Se are retained on the column. Lepel and Laul [143] separated REE from biological samples by a method similar to that used for geological materials. The sample is fused with N+02 + NaOH, dissolved in H20, acidified and then brought to approximately pH 10 with NH40H in order to precipitate REE hydroxides. NH40H is added in excess to dissolve Ni hydroxide. The precipitate is dissolved in 10 M HCl and loaded onto anion exchange columns. Fe and Np remain on the column while REE is eluted with the 10 M HCl. The REE is purified by three cycles of hydroxide precipitation +dissolution in HCl +precipitation as fluorides ---tdissolution in H,B03 + HNO,. Collecchi et al. [ 1441 separated REE using cation exchange column. The aciddigested material is heated to dryness, dissolved in 0.5 M HCI and loaded on cation exchange column. The column is washed with 0.5 M HCl and eluted by 0.1 M


345

H , q 0 4 followed by elution with 2 M HNO,. Third elution with 6 M HC1 includes the REE.

Orher elenieiirs: Pietra et al. [76] have several procedures for separation of several elements. For example, for As, Sb, Cu, they used digested material in 6 M HC104 and two columns. The first column of copper chloride retains Cu and Sb, whereas the second column of tin dioxide retains As. Rajadhyaksha and Turel [1451 determine Cu, As, Au, Se, Hg, Co, Zn, Ca, Fe, P, Cr, Na and K by a long series of precipitations and solvent extractions. Woittiez et al. [ 1461 used two columns to separate Cd, Mo, Cr. and Co. Cd and Mo are retained together on an anion column while Cr and Co are adsorbed on the chelating resin chelex-100. Vasconcellos et al. [147] developed several rapid separations for determination of a few different elements. As, Hg, Sb and Se were determined by the use of an anion exchange column in 6 M HCI to retain Hg and Sb on the column. The eluate was loaded on a tin dioxide (TDO) column and eluted with 3 M HC1. As and Se were retained on the column. Se and Hg alone were determined by retention of Se on a TDO column and extraction of Hg with Ni(DDC), in chloroform. Yinsong et al. [3,148] developed a rapid separation of As, Cd, Cu, Hg and Zn by extracting Cu and Hg with Zn(DDC),/CHCl, and successively extracted As, Cd and Mn from H2S0,/KI media by methyl isobutyl ketone. Kobayashi [ 1491determined Cd, In and Sn by substoichiometricextraction of indium with NadDC/CHCl,. Indium isotopes are the IRNs for the three elements. 'lSrnInfor Cd, 114mIn for In and 113rnIn for Sn. Fajgelj et al. [150] determined Se, Mo,As and Sb by RNAA by four successive extractions, so that each element was separated alone. Taskaev et al. [ 151developed a extraction method for determination of Hg, As, Cd, Cu, and Zn. Hg and Cu were extracted with dithizone solution in chloroform. Further extractions with trioctylamine/tolueneand NaDDWCHCl, was used to isolate Hg and Cu, respectively. As was reduced to As (111) and extracted with NaDDC/CHCl, from 2 M HC1 media. The same NaDDC/CHCl, was used to extract Cd and Zn from pH 11 aqueous solution. Greenberg [2,152] used hydrated manganese dioxide column to absorb Cr, As, Se, Mo, Ag, Sn and Sb. In order to minimise 32Padsorption, the column was preconditioned with They developed also successive extractions to isolated Cr and Sn in separate fractions. Chatt and coworkers [l] separated As, An, Co, Cn, Fe, Hg, Mo, Sb, Se, and Zn by coprecipitation with CuS + ZnS. Fardy and Mingguang [38] determined Se and Hg by adsorption of their complexes with pyrrolidiiie dithiocarbamate on either C18bonded silica gel or activated carbon. Grimanis and Pertessis-Keis [1531 isolated Hg and Se by solvent extraction with toluene from H,SO,-HBr solution. Firstly, Hg is extracted from 7.5 M H,SO,-O.Ol M HBr, and then Se from 7 M H,SO,-l M HBr. Taskaev [1541 extracted V and Mo by N-benzoyl-N-phenylhydroxylaminein toluene, from acidic solution. Adjusting the pH to 5, Cu was extracted with

346

ZEEV B. ALFASSI

Pb(DDC),/CHCI,. Mn was next extracted with NaDDCKHCl,. Finally Rb and K are precipitated as tetraphenylborates by addition of solution of sodium tetraphenylborate and cooling to 0" C. Dermelj et al. [ 1551extracted Se (after reduction to Se (IV) by heating in 6 M HCl solution) with CCl, containing 4-nitro-o-phenylenediamine. From the aqueous phase at pH- 0.2, Cu, Cd and Co were extracted with NaDDC/ CCl,, while Zn remains in the aqueous phase. Zn was further purified by two extractions. The mixture of Cu, Cd, and Co was separated to each element alone by further extractions. Gharib et al. [156] separated three small groups of trace elements from milk. In the distillate they determined Hg and Br, (separately), Ag and Se were deposited on a copper foil. In the remaining solution they determined Co, Cr, Fe, Mo, Sb, and Zn. Tian and Ehmann [4]determined As, Cu, Cd, and Mo by RNAA. Cu, Cd, and Mo were extracted with NaDDCKHCl, and As was separated from the aqueous phase by tin dioxide column in 10 M HCl-0.2 M HF solution. Dang et al. [ 1571isolated Fe, Co, and Se, each separately, from milk. Se was distilled as SeBr, from HBr solution, collected in chilled water and precipitated as elemental Se with H,SO,. Fe was precipitated as hydroxide in the presence of ammonia. It was further purified by extraction of its chlorocomplex with isopropylether, back extracted and finally precipitated as oxinate. Co hydroxide was precipitated by KOH, and Co was further purified by precipitation as potassium cobaltinitrite. The same group published earlier a single-element separation for Mn, As, Mo, Cu and Zn in milk [ 1581. Nakahara et al. [ 1591 separated As, Cu, Mn, and Zn from human blood serum into two groups: As + Cu and Mn + Zn by extraction with APDCKHC, at different pHs. Meloni et al. [ 1601 adsorbed Na and K by HAP from 1 M HNO, solution, Cu was deposited on a column of grains of metallic Cd. Mn and Br were determined in the eluate from these two columns. After counting, 32Pwas removed by SnO, column and La and Zn were determined in the eluate. Cornelis [161] precipitated together Au, Cu, Zn, As, Sb, Mn, and Hg, as sulfides or oxinates. Samsahl [ 1621 developed a method to separate Br, Hg, Sb, As, and Se by distillation and three anion-exchanges columns. The three columns have the same packings, but were conditioned and later eluted with different acids. This process instead of using one column, and changing the eluants, makes it easier for automation. Br is separated by distillation and absorbed in NaOH. Hg is retained on an anion exchanger in the HSO, form. Sb is retained on the same column in the C1form, whereas As and Se are retained on the column in mixed C1- and Br- forms. A similar system was used by Woittiez and Iyengar [ 1631. They either separated together Mo, As, Cr,Sb, Sn, and Se on hydrated manganese dioxide column (from 1 M HNO, solution) or use three anion exchanger columns with different eluants (1.8 M HSO; for Mo, 0.4 M HBr for Cd and 11.3 M LiCl for Zn and Fe). The final eluant is divided into two fractions containing Co + Cu + Ni and Cr + Mn by the chelating resin - Chelex 100.


347

8.3.3.4 Group separation Many group separations were developed for biological materials. Some of them divide the sample to a few groups measured on a Ge(Li) or HPGe detector, while others made very elaborate separations to many groups to enable activity measurements with a well-type NaI(T1) detector in order to measure very small amounts. Figure 8-17 gives the flow chart of Pietra et al. [76] procedure for the determination of 50 elements. The procedure starts with distillation of the elements (CI, Br, I, O L l g,ramplc

Distillate

5 c d 4 M HCI

Wash 30 c d 4M HCI

30 crr? 6M H C l q

Rb. K

Ts

8'" w Bi

Figure 8-17. Pietra et al. 1761 scheme for radiochemical separation for the determination of 50 elements in biological samples (reproduced from Rcf. [76],with permission)

348

ZEEV B. ALFASSI

biological Ash dissolved in HBr

2 mL 10M HCI + 1% HzOz

18 mt 3M HC1+0.2% HzO2

'-4

As.Cu.Se

I

GmupII

Remaining

on column Y

-

Au. Cd.

Hg, Mo.Zn

MD

Peristaltic puap Sr

A Sample soluiIon/Elumt (LSI IIClO,)

Co, Cs, Fe,

Ilg, Zn

Cmting

vessel

Figure 8-19. Determinationof 16 trace elements in biological samples by separation into four groups using three columns (reproduced from Ref. [164], with permission)


349

-

Wrl ashinp of sample In Iml conc. HzSOt. * l . S m l HZOtSml 6N HCI ( l o t a l volume 7.5 d ; 9 N H' I 5 N SO:-( L N CI-)

Volaliliralim CI Br I 1 (Tell Sc" ,Hg"

Cs' Rb'

RbMP

Ocm)

HAP,

(1.5cm)

Asv Ng: Wf

X)ml 6N NaOH

7.5ml HzO

volume 35,Oml ( includlng 25ml 01

@---Total

,

deod volume

!mm Ihe !hrce columns (2.2N H'11.7N No', 1.7N CI2.2N.

so? 1.

8 i o l l o e A G l x B d t.400:B i o R r r 63

,,Na+

I'

(3Ocm)

Poi-

(6 em)

Sc"'

(Ca) ] R . E .

NaOH/HIO~ (aulomallc lllrallon pH 2 )

---6N

Washlnp solution lSml pH 2-1.9

( 6 d

x

Fe"'

APDC/Char coal

(311 cm) Co"

HCEHP

(6 cm)

R.E.

Chrler lOOd

(18 cm)

Cr"'

(Nil

3

Mntl

1

Effluent (85-90m1, p~ 6 )

K'

Figure 8-20. A half automated procedure for almost complele separation of 25 trace elements in biological samples (reproduced from Ref. [1651, with permission)

0s) and chlorides-bromides (Sb, Sn, Hg, Au, As, Se, Ge, Re, Ru). The chloridesbromides are separated into six groups (not more than two elements in the same group) by the use of two columns (first anion exchanger and then tin dioxide-TDO) with successive elutions. An arrow out of the column means elution. Elements with an-owsto the column are retained on the column. Elements in brackets are present in more than one group. The non-volatileelements/chlorides/bromideswere separated using successive elutions with six columns: anion exchanger, CuS, TDO, acidicaluminum oxide-AAO, and cation exchanger. The procedure of Pietra et al. [76]

ZEEV B.ALFASSI

350

FI, C a , C u , Z n , G o , Mo. Cd. S b . A u , Hg

Grouol

Evoooorole lo new dryness Add I ml I8 M H S O Diilil lwice rim I ml 01 48 Of. ond 1 ml of 30% HpOZ Distil lwicr with Sml of H P

Her

h a , Se

Oissolrc *I ( I +I I HNOj POIS through AAO column Woih rilh 50ml of HNO) W I )

Elllucnl

Sc, CI 80.

,

LO,

CS , , Sm,

Rb.

Ce

Group

Eu

3

Figure 8-21. Yeh et al. [166] scheme for RNAA determination of 21 elements (reproduced from Ref. [166], with permission)

for 39 elements determination by five columns with single eluant described for geological material was used also for biological material (Fig. 8-14). Although 50 elements can be separated and determined, most studies do not need so much information, and most studies measured 10-20 elements in more simple methods. Pietra et al. [76] have several schemes for this purpose. Van Renterghem and Cornelis [23] determined 10 elements in human serum. Their procedure divides 13 elements (Na, K, and Br were not determined in the serum) into three groups by using an anion exchange column in the Br- form with two elutions. The scheme is given in Fig. 8-18. Sixteen trace elements were determined by the same groups using three columns and one eluant [1641as can be seen in Fig. 8-19. Schuhmacher et al. [165] developed an almost complete scheme, in order to enable measuring the activities with well-type NaI(Tl) detector, to measure the contents of 25 trace elements in small biological samples (1-15 mg). The scheme is based on consecutive 14columns divided into three groups (3,7,4).For all columns in one group, the same eluant was used. Between the second and third group the eluate is automatically titrated to pH 2. The scheme is given in Fig. 8-20. Figure 8-21 gives the scheme of Yeh et al. [1661for group separation,measuring 21 elements, not including Na and Br


35 1

which can also be measured. This scheme includes distillation of Br as an element, a HAP column, As and Se distillation as bromides and an acidic aluminum oxide AAO column. Saiiisahl 11671 developed an automatic system for rapid separation. This system was the basis for development [ 1681 of an automatic group separation system for the determination of about 32 elements (other eight can be measured but were not detected in the biological materials). Several later studies were devoted to development of automatic systems (see for example Ref. [46,169-173]), however, it is good only for very specialized laboratories and this trend was neglected in the last decade. The scheme of the separation by ion exchange columns developed by Samsahl and coworkers [ 167,1681 is the basis for most present separations.

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71 Odone, M., Meloni. S., and Vannucci, R., J. Radioanal. Nucl. Chem., 142 (1990) 489. 72 Hoffman, E.L., Naldreu. A.J., van Loon, J.C., Hancock, R.G.V., and Manson, A.. Anal. Chim. Acla. 102 (1978) 157. 73 Parry, S.J., in: Preconcenfration Techniques of Trace Elements. Z.B. Alfassi and C.M. Wai (Eds.). CRC Press, Boca Raton, FL, 1992, p. 401. 74 Hillard, H.T.,J. Radioanal. Nucl. Chem., 113 (1987) 125. 75 Parry, SJ.,Asif, M., and Sinclair. I.W., J. Radioanal. Nucl. Chem., 123 (1988) 593. 76 Pietra, R.. Sabbioni, E.. Gallorini, M.. and Orvini. E.. J. Radioanal. Nucl. Chem., 102 (1986) 69. 77 Zilliacus, R.. Kaislilla, M., and Rosenberg. R.J., J. Radioanal. Chem., 7 1 (1982) 323.

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78 Laul, J.C., Lepel, E.A., Weimer, W.C., and Wogman. N.A., J. Radioanal. Chem., 69 (1982) 181, 79 Meloni. S.. Oddone. M.. Cecchi, A.. and Poli. G., J. Radioanal. Chem., 71 (1982) 429.

80 Wandless, G.A. and Morgan, J.W.. J. Radioanal. Nucl. Chem.. 92 (1985) 273. 81 Bishop. J.G. and Hughes. T.C., J. Radioanal. Nucl. Chem.. 84 (1984) 213. 82 Loron. J.L. and Ottonelo. G.,J. Radioanal. Nucl. Chem., 88 (1985) 259.

83 Stosch. H.G., Helpers. U., and Kotz. J., J. Radioanal. Nucl. Chem., 112 (1987) 545. 84 Koeberl. C., Kluger, F.. and Kiesl, W., J. Radioanal. Nucl. Chem., 112 (1987) 481.

85 Parthasarathy, P., Desai, H.B., and Kayasth, S.R.. J. Radioanal. Nucl. Chem., 105 (1986) 277. 86 Garg, A.N.. Wankhade, H.K., Dogra, R.K.S., and Shanker, R., Sci. Tot. Environ., 67 (1987) 165. 87 van der Sloot, H.A., Hoede, D., Klinkers, TJ.L., and Das, H.A., J. Radioanal. Chem., 71 (1982) 463.

88 Drabaek, I.. Carlsen. V., and Just, L., J. Radioanal. Nucl. Chem., 103 (1986) 249. 89 Rammensee, W. and Palme, H.. J. Radioanal. Chem., 71 (1982)401. 90 Morrison, G.H.. Gerard. J.T.. Travesi, A., Currie, R.L., Peterson, S.F., and Potter. N.M., Anal. Chem., 41 (1969) 1633. 91 Lad, J.C., Case, D.R., Wechter, M.. Schmidt-Bleek, F.. and Lipschutz, M.E.. J. Radioanal. Chem., 4 (1970) 241. 92 Allen. R.O.. Haskin. L.A.. Anderson, M.R., and MLiller, 0.. J. Radioanal. Chem., 6 (1970) 115.

93 Keays. R.R.. Ganapathy, R.. Laul, J.C., KrahenbUhl, U., and Morgan, J.W., Anal. Chim. Acta, 72 (1974) 1.

94 Smets, T.. Hertogen, J., Gijbels, R., and Hoste, J., Anal. Chim. Acta, 101(1978) 45.

95 Sun, Y..Tian, W.. Xiao, J., and Zhou, Y.,Proceedings of Int. Conference on Modern Trends in Activation Analysis, Copenhagen, June 23-27,1986. p. 1273. 96 Vasconcellos, M.B. and Lima, W.F., J. Radioanal. Chem., 44 (1978) 5 5 .

97 Laul, J.C., Atomic Energy Rev., 17 (1979) 603.

98 Fardy, JJ., in: Activation Analysis, Vol. 1, Z.B. Alfassi (Ed.), CRC Press, Boca Raton, FL, 1990. p. 161. 99 Chai, C.F., Isotopenpraxis, 24 (1988) 257. 100 Bowen, HJ.M., CRC Crit. Rev. Anal. Chem., 10 (1980) 127.

101 Kimura, T., Ishimori, T., and Hamada. T., Anal. Chem.. 54 (1982) 1129. 102 Murthy, R.S.S. a i d Ryan, D.E., Anal. Chim. Acta, 144 (1982) 107.

103 Bilewicz, A., Barlos. B., Narbult, J., and Polkowska-Motrenko, H., Anal. Chem., 59 (1987) 1737. 104 Polkowska-Molrenko, H..Zmijewska, W., Bartos, B., Bilewicz, A., and Narbutt, J.. J. Radioanal. Nucl. Chem., 164 (1992) 115.


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105 Nadliarni, R.A. and Ehmann. W.D.. Radiochem. Radioanal. Lett, 2 (1969) 161. 106 Piedade-GueiTeiro. J.. Frcilas. M.C., and Martinho, E.. J. Radioanal. Nucl. Chem., 110 (1987) 531. 107 Maihara. V.A. and Vasconcellos. M.B.A.. J. Radioanal. Nucl. Chem., 122 (1988) 161. 108 Cornelis. R.. Versieck. J., Messe. L.. Hosle. J.. and Barbier, F., J. Radioanal. Chem.. 44 (1978) 247.

109 Simonoff, M.. Llabador, Y., Hamon, C., Berdeu, B., Simonoff, G., Conri, C., Fluery, B., Couzigou. P.. and Lucena. A.. J. Radioanal. Nucl. Chem., 113 (1987) 107. 110 Greenberg, R.R. and Zeisler, R., J. Radioanal. Nucl. Chem., 124 (1988) 5. 11 1 Kucera. J.. Soukal. L.. and Faltejsek. J., J. Radioanal. Nucl. Chem., 107 (1986) 361.

112 Dybczynski, R.. Maleszewska. H., and Wasek, M., J. Radioanal. Nucl. Chem., 96 (1985) 187. 113 Rajadhyaksha, M. and Turrel, Z.R., J. Radioanal. Nucl. Chem., 106 (1986) 99. 114 Whitley, J.E., Stack. T.. Miller. C.. Aggett, P.J., and Lloyd, D.J., J. Radioanal. Nucl. Chem., 113 (1987) 527. 115 Cortes, E., Gras. N., Munoz. L., and Cassorla, V., J. Radioanal. Chem., 69 (1982) 401. 116 Khan, S.Z.. Turrel, Z.R., and Haldar. B.C., Radiochem. Radioanal. Lett., 48 (1981) 109. 117 Shah, P.K. and Haldar, B.C., J. Radioanal. Chem., 72 (1982) 549. 118 Comparetto, G.M., Jester, W.A., and Skinner, W.F., J. Radioanal. Chem., 72 (1982) 479. 119 May, S. and Picot, D., Analausis, 7 (1979) 133; see also Irigaray, J.L., Elmir, H., Pepin, D., and Communal, P.Y., J. Radioanal. Nucl. Chem., 1 13 (1987) 469. 120 Damsgaard. E.. Ostergaard, K., and Heydom, K.. J. Radioanal. Chem.. 70 (1972) 67; Heydorn, K. and Damsgaard. E., Talanta, 20 (1973) 1. 121 Itawi, R.K. and Turrel, Z.R., J. Radioanal. Nucl. Chem., 106 (1973) 81. 122 Kalouskova, J., Drabaek. K., Pavlik, L., and Hodik, F., J. Radioanal. Nucl. Chem., 129 (1989) 59. 123 Polkowsh-Motrenko, H.,Dennelj, M., Bryne, A.R., Fajgelj, A., Stengar, P., and Kosta, L, Radiochem. Radioanal. Lett., 53 (1982) 319. 124 Navarrete, M., Cabrera, L., Descharnps, N., Boscher, N., Revel, G., Meyer, J.P., and Stampfler, A., J. Radioanal. Nucl. Chem., 145 (1990) 445. 125 Hatzistelios, I. and Papadopoulos,C.,J. Radioanal. Chem., 72 (1982) 597. 126 Bowen. HJ.M. and Sujari, A.N.A., J. Radioanal. Nucl. Chem., 106 (1986) 193. 127 Itawi, R.K. and Turrel, Z.R., J. Radioanal. Nucl. Chem., 106 (1986) 71. 128 Allegrini, M., Delfanti, R.. Dicasa, M.. and ONini, E., Radiochem. Radioanal. Lett., 59 (1983) 163. 129 Dermelj, M., Slejkovec, 2..Sorak-Pokrajac, M., and Rossbach, M., J. Radioanal. Nucl. Chem., 144 (1990) 251. 130 Id-Biran, T. and Guinn, V.P., J. Radioanal. Chem., 55 (1980) 61. 131 Katoh, M. and Kudo, K., J. Radioanal. Nucl. Chem., 95 (1985) 55.

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132 Esposito, M., Col1ecchi.P.. Oddone, M., and Me1oni.S.. J. Radioanal. Nucl. Chem.. 113 (1987) 437. 133 Zeisler. R. and Greaibcrg. R.R.. J. Radioanal. Chem.. 75 (1982) 27. 134 Greenberg. R.R.. Fleming. R.F., and Zeisler, R.. Environment Itit.. 10 (1984) 129. 135 Kucera. J. and Drobnik, J.. J. Radioanal. Chem., 75 (1982) 71. 136 Xilei. L., Heydorn, K.. and Rietz, B.. J. Radioanal. Nucl. Chem., 160 (1992) 85. 137 Czauderna, M.. J. Radioanal. Nucl. Chem., 89 (1985) 13. 138 Taskaev. E., Karaivanova, M., and Trigorov, T., J. Radioanal. Nucl. Chem., 120 (1988) 75. 139 Grimanis. A.P. and Kanias, G.D., J. Radioanal. Chem., 72 (1982) 587. 140 Bso, J.N., Cohen, I.M., and Resnizki. S.M., Radiochem. Radioanal. Lett., 58 (1983) 175. 141 Lavi. N., Mantel, M., and Alfassi, Z.B.. Analyst, 13 (1988) 1855. 142 Tjioe. P.S.. Volkers. KJ.. Kroon. J.J., and De Goeij. J.M.M., J. Radioanal. Chem.. 80 (1983) 129. 143 Lepel. E.A. and Laul. J.C, J. Radioanal. Nucl. Chem., 113 (1987) 275. 144 Collecchi, P., Esposito. M., Meloni, S.. and Oddone. M., J. Radioanal. Nucl. Chem., 112 (1987) 473. 145 Rajadhyaksha. M.M. and Turel. Z.R., J. Radioanal. Nucl. Chem., 156 (1992) 341. 146 Woittiez, J.R. and de la c r w Tangona, M., J. Radioanal. Nucl. Chem., 158 (1992) 313. 147 Vasconcellos, M.B.A.. Maihara. V.A., Favaro. D.I.T.. Armelin, MJ.A., Toro, E. Corks. and Ogris. R., J. Radioanal. Nucl. Chem.. 153 (1991) 185. 148 Zhuang, G.S., Yinsong. W.. Min. Z., Wei, Z., and Jianhua, Y.. J. Radioanal. Nucl. Chcm.. 129 (1989) 459. 149 Kobayashi. K., Analyst, 113 (1988) 1121. 150 Fajgelj, A., Dermelj, M., Byme, A.R., and Stegnar, P.,J. Radioanal. Nucl. Chem., 128 (1988) 93. 151 Taskaev, E., Penev. I., and Kinova, L., J. Radioanal. Nucl. Chem., 120 (1988) 83. 152 Greenberg, R.R.. Anal. Chem., 58 (1986) 2511. 153 Grimanis, A.P. and Pertasis-Keis, M., J. Radioanal. Nucl. Chem.. 113 (1987) 445. 154 Taskaev, E., J. Radioanal. Nucl. Chern., 118 (1987) 111. 155 Dermelj. M., Byme, A.R.. F m k o , M., Smodis, B.. and Stegnar, P., J. Radioanal. Nucl. Chem., 106(1986)91. 156 Gharib. A., Rahimi, H., Pyrovan, H., Raoffi, Chem., 89 (1985) 3 1.

NJ., and Taherpoor, H., J. Radioanal. Nucl.

157 Dang. H.S.,Desai, H.B.. Kayaslh. S.R., Jaiswal, D.D., Wadhwani, C.N., and Somasundamn, S., J. Radioanal. Nucl. Chem., 84 (1984) 177. 158 Dang, H.S.. Desai. H.B.. Jaiswal, D.D., Kayasth, S.R., and Somasundararn, S., 1. Radioanal. Chem., 77 (1983) 65.

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS 159 Naliahara, H., Nagame. Y., Yoshizawa, Y., Oda. H.. Gotoh. S.. and Murakami, Chcm., 54 (1979) 183.

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160 Mcloni. S.. Caramella-Crcspi. V., and Fassi. G., J. Radioanal. Chem., 34 (1976) 113. 161 Cornelis, R., J. Radioanal. Chem.. 15 (1973) 305. 162 Samsahl. K.. Anal. Chem.. 39 (1967) 1480. 163 Woittiez, J.R. and lyengar. G.V..Fresenius Z. Anal. Chem., 332 (1988) 657. 164 Xilei. L., van Renlerghem, R.Cornellis, and Mees, L., Anal. Chim. Acta, 211(1988) 231. 165 Schuhmacher, J., Maier-Borst, W.. and Hauser. H.. J. Radioanal. Chem., 37 (1977) 503. 166 Yeh.

S.J.,Chen, P.Y., Ke, C.N., Hsu, S.T.. and Tanaka, S., Anal. Chim. Acta, 87 (1976)

167 Samsahl, K.. Nukleonik, 8 (1966) 252. 168 Samsahl. K.. Wester. P.O., and Landstrtlm. O., Anal. Chem.. 40 (1968) 181. 169 Goode, G.C.. Baker, C.W., a i d Bro0keN.M.. Analyst, 94 (1969) 728. 170 Samsahl, K., Sci. Total Environ., 1 (1972) 65. 171 Elek, A., J. Radioanal. Chem., 16 (1973) 165. 172 Liverns, P.. Cornelis, R., and Hoste, J., Anal. Chim. Acta, 80 (1975) 97. 173 Gaudry, A., Maziere, B., and Comar. D., J. Radioanal. Chem., 29 (1976) 77. 174 Wester, P.O. Brune, D.. and Samsahl, K.. Int. J. Appl. Radiat. Iso.. 15 (1964) 59.

119.


CHAPTER 9

Determination of trace elements by electron spectroscopic methods

MICHA POLAK Departmetit ofhiaterials Eiigiiieeritig,Beti-Gurioti Utiiversity ofthc Negev, Beer-Sheva84105,Israel

Contents 9.1 9.2

9.3 9.4

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .359 General review of X-ray photoelectronand Auger electron speclroscopies . . . . . . . 362 9.2.1 Basic principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 9.2.2 Surface sensitivity and depth profiling . . . . . . . . . . . . . . . . . . . . 369 Chemical-slate information . . . . . . . . . . . . . . . . . . . . . . . . . . 374 9.2.3 9.2.4 Quantitativeanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 XPS/AES applications in trace element determination . . . . . . . . . . . . . . . . . 384 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389

9.1 Introduction A great variety of modern spectroscopic methods of surface analysis are now available, which in contrast to classical techniques, can furnish quantitative, atomiclevel chemical information about elements composing the surface region. Either electrons, ions or photons are used to excite the surface atoms, typically under ultra-high-vacuum conditions to avoid surface contamination. Each technique is usually named after the emitted particle which is analyzed and detected by it. Thus, in electron spectroscopy electrons carrying information on the quantum states of surface atoms are measured. Using electrons has the following advantages

1. Most important in surface analysis, the short mean-free-path (A-0.4 -3 nm) of slow electrons in solids (energies up to -2 kiloelectron volts), make them suitable for probing surfaces, i.e. obtaining surface-selective information. Slow electrons emitted from atoms composing deep layers loose energy by collisions, and do not contributeto the characteristicspectrum lines;

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2. Relatively easy determination of electron energy and angular distributions;

3. Electrons disappear from the test chamber after analysis;

4. Efficient detection and counting. Since their advent before a quarter of a century the two most widely used techniques for surface chemical analysis, X-ray Photoelectron Spectroscopy (XPS 01‘ ESCA) and Auger Electron Spectroscopy (AES), have been applied successfully in numerous material issues in metallurgy, thin-film semiconductors, ceramics and polymers. In addition to basic research on well-defined samples, such as mechanisms of molecular adsorption and reactions on atomically-clean single-crystal surfaces, both techniques have been very useful in ‘real world’ surface analysis, addressing practical problems in coiTosion, catalysis, adhesion, microelectronics, etc. Probably this category of industrial applications, which includes also trouble-shooting and quality control, has contributed the most to overwhelming developments, experimental-instrumental as well as theoretical, in these methods during the years. Two other powerful method, which together with XPS and AES constitute the so-called “Big Four” surface analysis techniques, use ions for probing the surface chemistry: SIMS (Secondary Ion Mass Spectrometry) and ISS (Ion Scattering Spectroscopy). Part of the many available methods can provide information on physical properties, such as atomic-level structure of surfaces and thin films (adsorbate geometry, for example), surface vibrations and electronic states. While structural information cannot be obtained in conventional XPS/AES experiments, the high “chemical surface sensitivity” inherent in these techniques enables the detection and analysis of a small fraction of a monolayer on a solid surface or a few atomic layers below it. Although new developments in spectrometer technologies have also included improved sensitivities, under normal operation, both can’t be considered as trace analysis techniques. In particular, depending on spectrometer performances and the specific element analyzed, it is not possible to detect less than 10-2-10-4 impurity concentration in the bulk. However, if a trace analyte originally present in a solid, gas or a solution is deposited as a very thin film on a solid surface (see Fig. 9- l), these surface sensitive methods with their intrinsic high absolute detection power, become suitable for trace analysis with good accuracy and precision. (The absolute detection limit can reach lo3-lo4atoms using AES with highly-focused and relatively intense electron-beam, see Section 9.2.4.3). Obviously, since the information comes from the outermost few atomic layers, for best results, diffusion of the deposited or adsorbed material into the bulk of the carrier should be absolutely prevented. Then, detection limits can be comparable to those achieved with more conventional trace analysis techniques, such as those described in other chapters. Indeed, reported detection sensitivities in the parts-per-billion (and in some cases N

N

DETERMINATION OF TRACE ELEMENTS

36 1

Figure 9-1. Illustration showing the principle of applying surface-sensitive techniques, such as electron spectroscopy, in trace element analysis: the analyte, which originally is homogeneously distributed in the bulk (a), should be deposited as a very thin layer (preferably in the monolayer range) on a suitable solid surface (b). While the sensitivity of XPS/AES may be insufficient to detect Ihe aiialyle atoms in case (a), a bulk-sensitive technique (having several orders of magnilude larger sampling-depth) is expected to give the same signal intensity for (a) and (b)

even higher) range, make XPS and AES attractive tools in environmental, biomedical or geochemical applications. While in most cases the analyte is “artificially” trapped from solution on a suitable surface, in atmospheric environmental studies, for example, elemental or compound surface accumulation occurs “naturally.” In any case, these techniques can’t detect less than about 0.1-1% of a monolayer. The number of publications concerning the use of these powerful techniques in trace analysis is quite limited. (This does not mean that there are not unpublished works done routinely in analytical laboratories!). Therefore, a principal objective of this chapter is to draw the attention of a wide audience to the merits of XPS/AES in trace analysis applications. It should be noted that conventional XPS seems to be advantageous in trace analysis since electron beam damage in AES (e.g. by electron-stimulated desorption) can usually be much more severe than damage caused by X-ray irradiation, especially in cases of organics and non-metallic thin layers. Obviously,the issues of XPS/AES quantificationand characteristicdetection limits deserve special emphasis in the context of trace element analysis. Other useful aspects of these methods, such as analysis of elemental chemical-state and in-depth distributions, are discussed as well. Electron spectroscopic instrumentation, which is beyond the scope of this review, and additional spectroscopic issues can be found

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in numerous books specific to these techniques [ 11. Briefly mentioned are some nonconventional electron spectroscopic experiments with improved sensitivity, which were proposed in the literature. Before the concluding section of the chapter, the performance of conventional XPS/AES in trace analysis is demonstrated, and procedures for analyte separation and deposition/trapping on solid surfaces of appropriate carriers are reviewed. 9.2 General review of X-ray photoelectron and Auger electron spectroscopies

These techniques are based on energy analysis of slow electrons which are emitted from atoms excited by X-rays and an electron beam in XPS and AES, respectively. All elements, except H and He, can be detected, and besides their qualitative and quantitative analysis, AES and XPS can furnish specific chemical-state information. Since in both spectroscopies, peak overlap is quite rate, simultaneous multielement analysis is straightforwardly feasible. Microprobe analysis, namely, analysis of small areas and determination of the lateral distribution of elements across an inhomogeneous surface, is another advantage. Using a focused electron beam, AES can provide much higher spatial resolution compared to XPS, although recently, remarkable progress has been made also in microprobe XPS. Furthermore, combining XPS/AES with controlled inert ion bombardment provides quite detailed information on in-depth concentration distributions (“sputter depth-profiling”). Together with the former capability, they furnish what can be called a “three-dimensional chemical analysis.” Table 9-1. Characteristics of X-ray photoelectron spectroscopy and Auger electron spectroscopy Elements detected

Quant. ChemicalSampling Spatial Detection limits’ Quant. depth resolution3 [conc.] anal. anal. state [monolayersl [mono~ayers] accuracy’ precision information

XPS 2 > 2

2-10

5-50 ,urn

10-2-10-4 10-’-10-~

20-30%

f5%

Directly from “chemicalshifts” in core levels

AES 2 > 2

2-10

5-100 nm

10-2-10-4 10-1-10-3

20-30%

55%

Mainly from CVV lineshape and fine structure

’ The range represents variations in elemental detection sensitivity. It can change depending on the spatial resolution and the spectrometerperformance. Detection limits can be improved by a factor

of 10-100 with non-conventionalelectron spectroscopicexperiments mentioned in the text.

* Achievable by using standard procedures based on published elemental sensitivity factors. Higher

-

accuracy is possible by using locally produced standard specimen, for example. In conventional XPS 2 mm.


363

For all these reasons, both methods have become very popular for practical surface analysis. Table 9- 1 summarizes principal characteristics of the two methods. Other advantages include the ease of use and operation, and the ability to analyze “real surfaces,” without the need for any special sample preparation (in comparison, trace analysis involves the “artificial” preparation of surface layers). Thus, vast amounts of support data have been accumulated during the years and are available in various handbooks [2,3], textbooks [ 13 and specialized journals [4]. Before introducing the two electron spectroscopies in some detail, it should be noted that for achieving a correct and comprehensive solution of a surface problem, often combination of several complementary techniques is desirable. 9.2.1 Basic pr-iiiciples

There are three basic stages in an XPS or AES experiment (see Fig. 9-2): 1. Excitation of sample atoms resulting in the emission of characteristic (relatively slow) electrons from the surface region;

2. Energy analysis of the emitted electrons by an electrostatic analyzer; 3. Detection and amplification of the electron signals to produce an “electron spectrum.”

Figure 9-2. The three principal stages in XPS/AES experiments: (a) excitation and electron emission. A is the electron mean-free-path; the effective probing (“escape”) deplh, &a, can be decreased by choosing lower electron take-off angles (Y), thus enhancing surface sensitivity, see Section 9.2.2.2;(b) electron energy analysis; (c) electron signal detection

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Figure 9-3. The basic atomic mechanism involved in photoelectron (photoemission) spectroscopy. Electron binding energies are usually referenced to the Ferrni-level (Ef) of the solid (metal in this case); Ev - vacuum level.

The fundamental well-known physical phenomena behind the first stage are the photoelectric effect in case of XPS (Fig. 9-3) and the Auger process in AES (Fig. 97). In particular, in XPS, the energy of a photon (hv) is transferred to an electron in an atomic orbital having binding-energy E,. If hv exceeds E, (plus a few eV work-function, q5), the electron is emitted outside the solid surface with characteristic kinetic energy, E , = hv - E, - 4 (9.1) Since core-level energies are unique to each element, and hv is fixed (typically Mg or Al-Ka), measurements of photoelectron energies constitute means to identify elements composing the surface. This includes all elements with 2 > 2 (Fig. 94), and since line overlappings are quite rare, qualitative analysis with XPS is frequently an easy task. Figure 9-5 shows an example of XP spectra measured for air-exposed Na P-alumina [6]. Besides the photoelectron lines of Al, 0, Na and C (contamination layer), characteristic Auger lines appear in the spectra. These X-ray induced Auger electron spectral lines, denoted XAES, often contain useful and complementary information on chemical states [7],and can be used in highsensitivity quantification [8]. The photoelectron spectrum shows in Fig. 9-6 is taken from one of the early studies demonstrating the applicability of XPS in trace analysis [9]. In spite of the quite low bulk concentration (1 ppm) of the trace ions, after ion-exchange absorption decent signal-to-noise ratios have been observed for all the elements tested except cadmium and barium. (The Ba line obscured by the nearby oxygen Auger lines, can be resolved by using AI-Ka instead of Mg-Ka). Electron emission in AES (or in XAES) involves a somewhat more complex process compared to photoemission. The Auger transition is shown schematically

365


I

1

1

10

20

30

I

I

1

40

50

60

I

I

I

100

110

I20

3 >-

ZW

104

z W

(3

z 0

lo3

z, m Io20

70

80

90

ATOMIC NUMBER, 2

Figure 9-4. Binding energies of core-level electrons vs. elemental atomic number

BINDING ENERGY, eV

Figure 9-5. Binding energy, eV

in Fig. 9-7. After an initial ionization, induced by a few kV electron beams (or Xrays) with energy Ep > E l , the excited atom can relax by the transfer of an electron from higher levels (e.g. E,) to fill the El core-hole. In the final stage, the extra energy (= E2 - El) can be released as a photon (X-ray emission) or radiationlessly by ejection of an electron from E, or from higher orbitals. The kinetic energy of this so-called Auger electron is given approximately by,

(In case of a solid, the kinetic energy is slightly reduced by the work-function, 4). All elements with 2 > 2 can emit Auger electrons and are quite easily identified since line overlapping is not common and several principal Auger transitions occur for most of them (Fig. 9-8).

MICHA F'OLAK

366 c u2 p

Pb4f

Blndlnq Energy, r V

Figure 9-6. X-ray photoelectronspectra of a mixlure of ions (1 ppm in solution) adsorbed on acrylic acid grafted polypropylene[91

Figure 9-7. Illustration of the mechanism leading to Auger electron emission: I -electron bombardment (X-rayin XAES), I1 - atomic ionization, I11 - intra-atomic relaxation, and 1V ejection of an Auger electron: (schematic energy-level diagram of a free atom)

Since Auger signals are relatively weak (but sharp) and superimposed on a large, but more gradually varying background (see Fig. 9-9 and Section 9.2.4.3), a standard approach for data acquisition is to take the derivative of the Auger energy spectrum (using modulation and phase-sensitive detection). AES data acquired in a study of single-crystal Na @-alumina[ll] are shown in Fig. 9-10. In this special case of a highly anisotropic superionic conductor (but an electrical insulator) the electrical field due to the impinging electron beam, triggers fast Na ion migation from the bulk towards a certain surface (Fig. 9-1Oc). The surface-sensitivityof AES enabled


367

90

85 80 15 70

65 60 55

50 45 40 35 30

25 20 15 10

5

0

200

400

600

800 1000 1200 1400 1600 ELECTRON ENERGY (eV)

1800 2000 2200 2400

Figure 9-8. Auger elcclron energies of thc cieinenis [3]: The speclroscoDic nolalions KLL. LMM and MNN refer to the three shells involved in the Auger innsition ( E l ,Ez and E3 respectively, see Fig. 9-7)

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Kinetic

Figure 9-9. Schematicrepresentation of the energy spectrum of back-scatteredsecondary and primary electrons emitted from a solid surface which is subjected to bombardment by an electron beam with energy Ep(taken from Ref. [lo])

quantitative characterization of this unusual diffusion process. However, it should be noted that such perturbation caused by the electron beam is usually unacceptable in AES quantitative analysis of surfaces. The next example presented here is relevant to trace analysis since it deals with the common phenomenon of the spontaneous accumulation of bulk impurities at the surface (or interface) layers of metals and alloys ('surface segregation'). It has wide technological implications [12], such as in the case of sulfur, which is a common impurity in nickel and exhibits extensive surface and grain-boundary segregation at elevated temperatures, often leading, in the latter case, to embrittlement of the metal. AES has been used to study the kinetics of segregation of sulfur at the surfaces of a Ni-9% A1 alloy containing about 200 ppm S in the bulk ( [13] and Fig. 9-11). While without annealing, no sulfur signal could be detected, as can be seen, after about 20 min of 700" C anneal, the S intensity starts to exceed the Ni signal. Argon sputtering revealed that the sulfur is confined to the outermost surface layer, and intensity analysis (next section) can be used to estimate its coverage. Thus, a spontaneous (or artificial) accumulation of a low-concentration element as a very thin layer enables its characterization by AES (or XPS).As already mentioned, this forms the basis for utilization of these techniques in trace element analysis.


1

1

1

200 400 600

369

1

800

1

I

I

loo0 lzoo 1400

ELECTRON ENERGYeV. Figure 9-10. AES measurements of single-crystalNa P-alumina demonstratingelectron-inducedNa ion migration to the surface: (a) spectrum of the as-inserted sample acquired with relatively low beam current density: (b) spectrum acquired after short Ar bombardment ('sputtering') removing mainly carbon surface contamination, and (c) the spectrum resulting from several minutes of relatively high current electron bombardment done after (b) [I 11

9.2.2 Surface sensitiviry and depth profiling 9.2.2.1 The probing depth A key factor in the electron spectroscopy sensitivity to surface atoms, namely,

its shallow probing depth, is the use of slow electrons with relatively short inelastic mean-free-path (IMFP, A) as the information carriers. Thus, the residual number of electrons ( N ) , left with their initial energy after traveling a distance d in a solid is given by N = Noexp(-d/A) (9.3)

370

MlCHA POLAK

--.---.-.-LA--

__._------

AN--

0

60

120

180

240

300

Annealing time (rnin.1

Figure 9-11. Studying sulfur surface segregation in Ni-9% A1 alloy [13]: ratios of sulfur to nickel AES line intensilies as a function of time at three leinperalures

where N o is the initial number, such as that produced inside the solid by the photoelectric and the Auger processes. This attenuation is due to energy losses by inelastic collisions of the moving electrons with the matrix (A is the average distance between successive events). Consequently, these electrons do not contribute to the characteristic signal. As electrons are emitted deeper within the sample, the probability of escaping outside the surface is reduced exponentially. (95% of the detected signal intensity originates from atoms within the so-called informationdepth, 3 4 . Depending on the nature of the solid matrix (metal, insulator, etc.), X values vary significantly with the electron kinetic energy (Fig. 9-12). In particular, it exhibits a minimum value of several angstroms at several tens eV kinetic energy, so that for highest surface sensitivity, lines around this energy range should be measured. (In photoemission this can be readily achieved with a tunable excitation source, i.e. synchrotron radiation). Recently, advanced theoretical approaches [ 151, some of which include also effects of elastic scattering [16], have been introduced, giving more accurate estimations for the attenuation of Auger or photo electrons in solids. In particular, the “attenuation length” (AL) or “escape-depth” (ED) to be used in quantification of thin overlayers (Section 9.2.2.2) is equivalent to the average net distance traveled by an electron between inelastic as well as elastic scattering events. However, it has been claimed that in quantitative AES analysis (Section. 9.2.4) IMFP should be used rather than AL [ 171. The rather complex issue of accurate definitions and comparison of these three parameters has been addressed by Jablonski and Ebel [ 181.


371

Energy (eV1

Figure 9- 12. Dependence of the inelastic mean-free-path (in monolayers)on the electronenergy; dots -experimental values for elements; curve -a least square fit to an analytical expression often used in the literature [ 141

9.2.2.2 Quantification of thin overlayers and composition gradients In many cases of practical importance, modifications in chemical composition and bonding are not confined to the outermost surface layer (monolayer), but extend to depths of several nm or even much more. Air oxidation products, corroded metals or multilayered electronic materials are just a few examples. (Predeposition for trace analysis can result in a several atomic layer-thick analyte, although better accuracy is expected for the monolayer range). The inherent shallow probing depth of XPS/AES obviously limits the information-depth obtainable. But even for very thin films (< 3 4 one often wants to know the thickness and the detaileddistribution of elements and chemical states as a function of depth. Equation (9.3) forms the basis for estimation of overlayer thickness from measured substrate line-intensities before and after deposition. Likewise, this simple attenuation law can be readily applied to derive expressions for XPS/AES intensities from elements composing a thin overlayer or a submonolayer, by which thickness and coverage, respectively, can be evaluated from experimental data. For the case of signal intensity (I) from a submonolayer, one obtains,

I = OIo[1 - exp(-u/X sin Y ) ] ,

(9.4)

where I"is the intensity from the same element as a bulk sample, 6 is the coverage (monolayer fraction), u denotes its thickness, and 4' ' is the angle between the surface

372

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and the electron take-off direction (XsinY is the “effective escape depth,” see Fig. 9-2). The linear relationship expected between the element signal intensity and its coverage (which, obviously, is linear with the deposited weight) can be used in trace analysis in coiljunction with calibration curves. When the ratio between line intensities from deposited element or adsorbate ( I Aand ) substrate element (In) is measured (a procedure that usually yields higher precision), its dependence on coverage is generally not linear (except at low coverages) [19]:

--8, 1 - OA

- IAIg { 1 - exp[-u/X,(E,)

sinY]}

IBIi{ 1 - exp[-u/XA(EA) sinY]}

(9-5)

Equations pertinent to quantitation of coverage on rough (non-polar) surfaces with complex structures are given in [21]. Non-destructive depth profiling can be accomplished through angle-resolved measurements of spectral intensities obtained by titling the sample with respect to the analyzer entrance, as illustrated in Fig. 9-2. Enhanced sensitivity for surface elements can be achieved by using low electron take-off angles (“grazing-angle”). Another non-destructive procedure for extracting in-depth information (such as layer thickness or relative location of an element) compares intensities of low and high energy lines of the same element since they reflect different information depths. As a reference, ratios of intensities measured for the pure element (or for a homogeneous surface region) are used. In this case too, one is limited by the electron information-depth, and for deeper concentration gradients sputter etching by inert ions is commonly used. The sample surface is eroded by a 0.5-5 kV ion beam and the newly exposed atomic layers x e analyzed by XPS or AES (see Fig. 9-10 above). Approximate sputtering rate calibration can be accomplished by measuring the time it takes to sputter, under the same conditions a chemically similar layer of known thickness. Alternatively, the sputter depth can be calculated from the measured ion-beam current (J,Am-’), the sputtering yield of the sample (5’ in atoms/ion, from literature) and the sputtering time, t [22], z = SJMt(lOOOepNAn),

(9.6)

where M ,n,N, and p are respectively, the molecular weight, the number of atoms per molecule, Avogadro’s number and the layer density (kg m-3). Several factors limit the practical depth resolution during depth profiling and result in a certain broadening of interfaces compared to the true profile. This limited depth resolution is associated with bombarded-induced changes of topography (roughening of polycrystalline metal layers) and of chemical composition of the eroded surface due to preferential sputtering of an element [23]. Another smearing effect is associated with the finite probing (escape) depth of the photo or Auger electrons (see above). An example for very high interface sharpness is the case of Ta,O, on Ta developed as


373

I

1

Depth. z .nm

Figure 9-13. Oxygen AES depth profile of a 28.4 nm TaZOs layer on Ta, used as a reference material for testing depth profiling resolution qualily. The depth width (oblained with 2 kV Ar+) over which the signal intensities change from 84 to 16%of their plateau values, 1.40 nm, marks the resolution [22].

v)

w

k cn z w !-

z w

-z -I

v)

0.

x

No ( KLL) 0

I00 200 SPUTTERING TIME. ( S C )

Figure 9-14. XPS peak area vs. spultering time (500 V Art) measured for Na /3-alumina [6]

a reference material for depth profiling (Fig. 9-13). More typical depth resolutions are in the range of several tens nm. Finally, Fig. 9-14 presents an XPS/Ar depth profile measured for air-exposed Na-b alumina (see the initial spectrum in Fig. 9-5), from which the existence of carbon overlayers and extensive sodium segregation from the bulk towards the surface are inferred (the segregation of the highly mobile Na' is attributed to its preferential interactions with atmospheric H,O [6].)

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374

9.2.3 Chemical-state irformation One of the principal advantages of XPS, realized already during its earliest days, is the relatively simple characterization of chemical bonding of surface atoms. Thus, oxidation states of ions, for example, are manifested as “chemical-shifts” in corelevel binding-energies, typically 1-5 eV relative to the energy corresponding to the pure element. In AES (or XAES) such information is somewhat less straightforward, although when valence levels are involved in the Auger process, the remarkable lineshape features observed can be used as fingerprintsfor chemical-state identification. Figure 9-15 shows 3d photoelectron spectra measured for arsenic in three known chemical states together with a spectrum of a black precipitate which appeared under certain conditions during the preparation of arsine for trace-analysis [24]. Clearly, the relatively wide range of XPS chemical-shifts enables in this case to easily identify the cationic valences, and the black precipitate is definitely elemental arsenic. Likewise, anions exhibit characteristic line chemical-shifts, as demonstrated by Briggs et al. [25] in trace analysis of IO;/I- (Fig. 9-16). Considering the observed

-

1

48

1

1

1

46

1

44

1

1

42

1

1

1

40

1

1

38

Binding Energy ( e V )

Figure 9-15. XPS Chemical-shifts - application to cations: arsenic 3d spectra for As2S04, Asz03, elemental arsenic and black precipitate formed during preparation of arsine for trace analysis [24]

375

DETERMINATION OF TRACE ELEMENTS I-

Figure 9-16. XPS “Chemical-Shifts”- application to anions: iodine 3d5/2 spectra measured for 1 p g 10; and I- in 10 pI of KIO3 and KI solution [25]

signal-to-noise ratios, much lower quantities of these ions can be detected by XPS. In many cases the shift is proportional to the ionic valence, with higher binding energy corresponding to larger positive charge. In other cases contributions from electronic relaxation (hole screening energy) lead to deviations from this regularity. In the Auger process, with a two-hole final-state, extra-atomic relaxation can be dominant, leading in cases of conductors, to shifts significantly larger than those observed in photoemission (if the initial hole is in the inner shell). Sodium, for example, is characterized by a very narrow range of photoelectron chemical-shifts. Na Auger lines. on the other hand, exhibit a relatively wide range of kinetic energies reflecting different chemical environments which induce diverse relaxation. Spectra of metallic sodium vs. Na+ on Na &alumina are shown in Fig. 9-17 [ 111. Other examples are summarized in Table 9-2. In such cases, examination of shifts in the energy difference between photo- and Auger electrons in the same XP spectrum (“Auger parameter”), which depend only on extra-atomic relaxation, is sometimes more useful in chemical-bonding identification than either spectroscopy alone [7]. 9.2.4 Quarrtitative analysis Two basic approaches to quantification of XPS and AES, namely relating line intensities to composition, have been established. One is based on first-principle calculations, while the more common and practical second approach uses empirical elemental sensitivity factors. A major problem in quantitative analysis is the accurate measurement of the number of photoelectrons or Auger electrons contributing to characteristic spectral lines [26,27]. In particular, it is necessary to separate these electrons from background electrons appearing in the raw data, which are produced by various processes discussed in Section 9.2.4.3.

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Figure 9-17. Chemical effecls in AES Na (KLL) spectra measured for a Na @-aluminasurface [ 1 I]: (a) Na': (b) metallic Na produced by intensiveelectronbombardment(nole the different line shape and energy, and the plasmon loss structures)


377

Table 9-2. Photoelectron and Auger energy shifts' in selected elements [7] ~

Element Mn Zn Ga Ge As

cd In Sn

~~

Photoelectron2

Auger electron

BE shift (eV)

KE shift (eV)

-1.2

6.2

-0.5 -2.0 -3.2 -3.0 -0.4 -0.8

4.2 6.2 6.7 6.4

-1.4

3.9

5.5

2.6

Shift corresponds to energy in the elemental fonii less that in the oxide fomi. 2The average is taken of two values: 21)2/3and 3d-jl2, or 3d-j/2 and 4d.

9.2.4.1 First principles (theoretical sensitivity factors) The photoelectron and Auger currents which vary linearly with atomic concentration can be expressed by means of a product of several terms related to the excitation process, to electron transport in the solid and its emission outside, and to instrumental parameters. For the case of a homogeneous surface region (with 12 as the number of atoms of the element of interest per unit volume of sample), the detected photoelectron signal current, corresponding to a particular line, reads

I = n@u4!~Xsin Y T ,

(9.7)

where CP - the X-ray flux on the sample (replaced by I,, primary electron beam current,

in AES); (T

- total photoionization cross-section (the corresponding term in AES is the product of three terms: (i) the cross-section for direct ionization by the primary electrons; (ii) primary electron backscattering factor which adds to I , and increases with the matrix atomic number [28], and (iii) the probability for Auger process following ionization which competes with X-ray emission);

4 - angular factor dependent upon the angle between photon and detected electron directions (irrelevant in AES);

T - instrumental collection efficiency related to characteristics of the electron energy analyzer and the detector.

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The problem of several unknown parameters in this equation is partially overcome by taking relative intensities of lines in the spectrum correspondingto two elements ( A , B): ll,A/lzg = ( 1 A / 1 5 ) ( s 5 / s A ) (9.8) where S, defined according to Eqn. (9.7), is the “Atomic Sensitivity Factor” usually expressed relative to a certain line from a particular element (S = 1). In estimating S, various approximations have been made, assuming invariance of certain terms. For example, one XPS handbook [2] uses S values calculated from theoretical photoelectric cross-sections (Fig. 9- 18) multiplied by energy correction terms only. The latter factor has been modified in a subsequent study [29], and compared to empirically derived sensitivity factors (see below). Likewise, first principle calculations have been made in quantitative AES [30].

9.2.4.2 Empirical sensitivity factors In deriving a set of sensitivity factors empirically, intensity ratios of spectral lines either from pure element standards or from various compounds have to be carefully measured. The first approach is more common in AES with the strongest

10

100 BINDING ENERGY, *V

1000

Figure 9- 18. Photoelectric cross-sections and theoretical sensitivity factors vs. electron bindingenergies [291


379

1“““‘“”’””1

Ep :SkeV

Alomlc number

Figure 9-19. Empirical “Alomic Sensitivity Factors” based on measured peak-to-peali intensities of the differential Auger spectrum for pure elements relative to Ag (MsNsN45) intensity ( [28], compiled from Ref. [3])

I

10

I

I

I

I

I I l l

I

100

I

I

,

,

,

,

1000

BINDING ENERGY. eV

Figure 9-20. X P S empirical and theoretical “Atomic Seiisilivil y Factors” for 3d5/2 transitions [29]

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MICHA POLAK

Ag line chosen as primary standard (SAg(MsN45N45)=1, Fig. 9-19), while in the second approach XPS intensity data from 135 compounds of 62 elements have been used with SFls= 1 1291. The significant discrepancies noted between the XPS empirical and theoretical values (Fig. 9-20), indicate the existence of fundamental errors in implementation of the latter approach [29]. When pure element standards are used as a basis for first-order approximation in quantitative analysis, it is desirable to include “matrix-correction-factors” as second-order approximation calculated from first principles. For the general case of a multi-element sample, the molar fraction of each element is given by [28],

where in XPS, the matrix dependent F factor consists of two contributions, namely, atom densities and attenuation lengths, while in AES it includes electron backscattering contributions as well. Calculated matrix factors for pairs of elements published in the literature [28,31] show that usually this correction cannot be ignored in both XPS and AES. It should be noted that measuring the pure element intensities (I”) in the same spectrometer (and under the same experimental conditions!) is preferable to using published values. In this third route to quantification, a relatively high degree of accuracy can be obtained by the use of locally produced “real” internal standards, namely, measuring relative intensities from adequately prepared sample (scribed under UHV, for example) with a chemical matrix similar to that of the test specimen. However, since such samples are often not available, application of one of the above-mentioned quantification methods is quite common. Of particular importance in quantitative analysis is the accurate measurement of photoelectron or Auger current, namely the correct line intensities in a spectrum. Therefore, severe efforts have been made to develop appropriate procedures for background subtraction, deconvolution of overlapping lines (“curve fitting”) and other data processing methods [l, 26,271. Recently, the validity of traditional methods for background subtraction from XPS lines has been studied by comparison to first-principle intensity calculations [32]. In particular, a much higher accuracy has been achieved using a novel method based on a detailed description of the physical processes involved. Finally, it should be recalled that all the above equations for XPS/AES quantification apply only to homogeneous compositions (at least within the information depth, 3X). Otherwise, modifications should be made in accordance with expressions for overlayer (or “underlayer”) intensities such as reviewed in Section 9.2.2.2. For example, Wagner [33] introduced a set of “monolayer sensitivity factors” derived in a simple way from the corresponding (empirical) bulk factors [29].

-


381

9.2.4.3 Detection sensitivity Despite its importance, this somewhat complex issue is rarely addressed in XPS/ AES textbooks or review articles. However, in the context of trace analysis, it deserves a detailed discussion, based here primarily on theoretical studies by Cazaux [8,34,35]. Thus, this section describes the principal factors affecting detection sensitivity in XPS and AES, highlights means for its enhancement and presents some estimations of detection limits. A pulse-counting signal (common in XPS) is confidently detectable in the presence of background if its amplitude is three times larger than the background noise. Thus, the relative detection limit is given by 2,

= 3(J?BG/t)”210-’

(9.10)

where IBG and I” are the numbers of counts of the background and the pure-element signal, respectively, and t is the measurement time per channel. Clearly, to optimize (minimize) z ,, the signal intensity and the acquisition time should be maximized as much as possible, and the background intensity minimized. However, due to intrinsic limitations in both XPS and AES (see below), x,, turns out to be about at best. Suggested modifications to conventional XPS/AES experiments, which can enhance overall sensitivity, will be mentioned too. Before discussing the complex issues of background and signal intensities, possible improvements in data acquisition are briefly addressed. Increasing the duration of the experiment enhances sensitivity (by t’/*),but there are limitations due to X-ray (or electron) induced surface modifications (desorption, dissociation, etc.). Furthermore, surface contamination by residual-gas adsorption can also limit data acquisition time, depending on the gashrface reactivity. Simultaneous multi-channel detection instead of the more conventional sequential acquisition is another means to improve sensitivity. Finally, it should be mentioned that various mathematical manipulations of raw data (e.g. “digital smoothing,” see Ref. [36]) aimed to reduce noise levels and enhance sensitivity, have been developed. For example, in a recent AES study of phosphorus in silicon, a detection limit of 0.04 at. 9% P was reported, compared to 0.09% achieved without mathematical treatments [37].

9.2.4.4 XPS and AES signal intensities First-principle expressions for the signal intensities were given before (see Eqn. (9.7). Increasing the X-ray flux (higher voltage on the anode is more effective than higher current), or the primary electron-beam current in AES, improves the sensitivity. As already mentioned, radiation damage sets physical limitation to the highest possible dose of photons (or electrons) incident on the sample suiface (see below). Furthermore, 5, is inversely proportional to the photoionization crosssection which varies quite considerably from element to element (see Fig. 9-18), depending on the photon energy to binding-energy ratio. Similar effects on x , in AES are caused by the ionization cross-section and Auger transition probability.

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9.2.4.5 Background intensity Besides instrumental electronic effects there are three distinct, matrix-related contributions to the background of an XPS line: 1. Photoelectrons (excited by characteristic hv) which are inelastically scattered before leaving the surface. This includes also contributions from neighboring photoelectron lines at higher kinetic energies, and therefore can depend also on composition;

2. Elastically scattered photoelectrons excited by non-characteristic X-rays. (The use of X-ray monochromator to improve spectral resolution, reduces the overall sensitivity since it attenuates the principal radiation but eliminates only this contribution to the background); 3. Photoelectrons excited as in case 2, but which have undergone inelastic scattering (as in case 1). In conventional XPS, this contribution is negligible compared to the other two. All three contributions have been evaluated quantitatively by Cazaux [8], and the derived expressions together with that for the signal intensity allow estimation of detection limits (via Eqn. (9.10) in XPS and XAES. Considerable improvement in sensitivity is predicted for an XAES experiment with continuous synchrotron radiation (CXAES). In particular, the signal can be maximized and the background reduced to its physically lowest level (only from inelastically scattered valenceband photoelectrons), by using intense, tunable X-rays with adjustable bandwidth. Then, calculations give detection limits of for boron and indium, and respectively, in silicon [8], i.e. more than two orders of magnitude higher sensitivity than estimated for conventional XPS. Clearly, such performance makes CXAES a potentially more attractive tool for trace element analysis. In AES, excitation is done by electron bombardment, and the background accompanying a particular line is much more intense than in XPS. It consists of three contributions (see Fig. 9-9 in Section 9.2.1):

1. Backscattered primary electrons which were inelastically scattered; 2. Auger electrons coiresponding to higher kinetic energy transitions, which were inelastically scattered;

3. The “true” secondary electrons.


383

Figure 9-21. Variation of Auger signal-to-noise with electron beam energy (E,) for (a) thin unsupported carbon film, and (b) the same film on copper (bulk) [34]. U is defined as E&b, where E b denotes the binding energy of the level initially ionized in the Auger transition.

All three contributions increase with decreasing primary electron energy. Their quantitativeestimation together with the signal behavior give the variation of signalto-noise with primary energy, which is compared to experimental results for cases of supported and unsupported thin film in Fig. 9-21. Clearly, sensitivity is improved in the latter configuration, and can reach a factor of 10 at higher Auger electron energies [34]. Nevertheless, the estimated detection limit in conventional AES (w is obviously not sufficientfor bulk analysis of traceelements. Considerable suppression of noise can be achieved by correlating AES with Electron Energy Loss Spectroscopy [38], as suggested recently [34,39] and verified experimentally for Fe on Si [40]. This coincidence spectroscopic technique, which under certain experimental conditions may have sensitivity improved by about two orders of magnitude, seems to be another attractive new tool for trace analysis. Finally, the optimization of the incident beam spot size in microanalytical techniques, such as AES and spatially resolved XPS,is briefly reviewed. Generally, for a given electron (or photon) dose received by the sample, the minimum concentration detectable (znl)is obtained by using the largest achievable spot size, while for detecting the minimum (absolute) number of atoms, ym = N a , (N is the number of atoms in the analyzed volume), the smallest achievable spot must be used [34,35]. Thus, considering only beam parameters, yn1is proportional to 6*/IiI2, where 6 is the diameter of the incident beam spot and I, is its intensity. Obviously, one has to take into consideration also the critical dose received by the sample. All these factors affecting AES sensitivity, including comparison to XPS performance,

MICHA POLAK

384

A

\

\

Y

\ I

\ I

..

-

4C-2

40-~

(i

....-

YPP . and ....... ...... ." A F P

Eioiirp T m and ahcnliitp *I I ' a " . " , 0-33 , . a . Rdativp "m.1.Y ""Yv.".ym detertinn limitc " in . . I . . . " . .

Y

z

nc fiinrtinn nf Puritatinn v-

I " Y"

* " . . . I . . . . -I.--."..-..

doses received by the sample for different incident spot sizes (6). The calculation assumes simultaneous data acquisition and signal-lo-baclrgroundintensity ratio of 10 for XPS and 0.1 for AES, while the same values have been chosen for olher parameters: atomic density = 5 x 10'2at./cm', X = 3 nm, cross-section = 5 x lo-*' cm' and (see Eqn. (9.7) in collection efficiency of the spectrometer/detector system T = Section 9.2.4.1). For the same spot size in X P S and AES there are three different situations depending on the relative critical dose received by the sample (D" in XPS,P in AES) which is the limiting factor of sensitivity: (i) Da = ff: the advantage of XPS over AES is its better signal-to-backgroundratio:(ii) D' > Dc: the detection limits of XPS correspond to an abscissa to the right of thal corresponding to the Auger critical The advantageof X P S can reach 2-3 orders of magnitude; (iii) Dx / (IAS/~c)max).

1. The capability of simultaneous multielementanalysis (see Fig. 9-25b). Chemically similar species, which can be volatilized simultaneously, often exhibit distinct spectral features;

2. The relatively simple identification of the analytechemical-statevia core-level “chemical-shifts’’(see Fig. 9-15). On the other hand, attempts to perform As trace analysis by Auger Electron Spectroscopy faced major difficulties (see Table 9-3). As already mentioned, the number of publications dealing with XPS/AES applications in trace element analysis is quite small in spite of its long established promising potential. Table 9-3 summarizes procedures, capabilities and some problems reported in those studies.

387


60-

I

I

-

0

40X

a

L,

20-

I

I

200

I

I

1

600

4 00

I

I

800

I

I000

P p b As

Figure 9-24. Calibration c w e for quantitative trace arsenic analysis. Since one ml of sample has been used, the concentrationcan also be read as nanograms of arsenic [241.

A53d

b

I

45

I

I

43

1

41

EB(eV) Figure9-25. (a) Enhancement of detection limit in arsenic trace analysis: As 3d spectrum recorded for arsenide trapped from a solution 300 parts per trillion in arsenic using large (165 ma) sample volume: (b) Multielement trace analysis; simultaneous deiection of As, Se, Sb and Sn (100 ppb each in 1 ml sample) using the XPS/Volatilization method [24].

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MICHA POLAK

Table 9-3. Summary of developments in trace element determination by XPS and AES‘ Element Carrier and analyte Detection Comments (ref.) depositioii technique limit2 As [24] Mercury-chloride Elemental arsenic produced at high arsenic impregnated paper concentration; problems with AES: due to (deposit mercury charging, metals had to be used as carriers arsenide); instead of paper, but As was not trapped Volatilization at trace levels Se, Sn, Mercury -chloride Simultaneous analysis Sb, [24l impregnated paper (deposit mercury arsenide); Volatilization Controlledelemental selectivity by chelate Pb, Cu, Glass fiber surfaces chemistry; cation specific; use of TLC plates Hg. ”4 coated with chelating failed due to inward diffusion. ditiocarbamate; Reaction 1411 with ions in solution (“two dimensional ion-exchange”) Inward diffusion of Ag suggests the use of Pb. Ag, Acrylic-acid grafted Cu, Fe, polypropylene; 2D ionthinner grafts: Ba not detected due to Ca, Cd, exchange from solution competitive reactions among the ions and overlapping with Ox Auger lines; cation Hg PI specific. Complication of quantitative analysis, partially Pb, Ba Cleaved calcite (scratched surface); adsorption from because of carbon contamination. Wetting [201 solution (evaporation problems solved by light scratching the calcite in air). surface or soaking it in water for long time. Pb, Bi, Hg-coated Pt electrode; Cation specific; multicomponent trace analysis of solutions in a single experiment Cd [42] electrochemical even when intermetalliccompounds are deposition formed. Problems with quantification because of preferential layering. A1 strip etched by HCI; Large chemical shifts for I- vs. 10; 1 (I-/ identification; Some decomposition of 10; to Adsorption from solution 10;) (evaporation in air) I-; small volume sample (lo-* ml); Uneven 1253 deposition on untreated Al, but longer etching (rougher surface) led to higher detection limits; no competition effects in multielement analysis of both cations and anions. F Soft analysis conditions (low electron and ionA1203, MgO, ZrOz or sputteringcurrents and energies to prevent F 143,441 Er2O3 vapor deposited thin films on glassy carbon desorption;best performance with 50 nm Volatilization as evaporated MgO; precision: 10% (CH3)3 S F S Soft analysis conditions to prevent S Ag vapor deposited on segregation; precision: 10% W3.441 glassy carbon; volatilization I

The studies in the last two rows were done with AES including sputter depth profiling; all thc rest were done using XPS (no sputtering). Lowest concentrations measured. Actual detection limits can be lower, and in some cases they were estimated.


389

Finally, it should be noted that in the characterization of environmental materials, such as atmospheric particulate air pollution material, surface analysis techniques can be applied “naturally,” i.e. without the need for any preconcentration or deposition procedures. Representative examples are a Scanning Auger Microprobe (SAM) study combined with argon sputtering of the surface layers of single particles of a coal fly ash [45],and XPS studies of the chemical state of sulfur in atmospheric aerosols, revealing the presence of sulfate, sulfite and chemisorbed SO, and SO, [46]. Actually, application of XPS or AES in studying surface predominance of elements and organic compounds on the particle surface [47], circumvents drawbacks inherent in the more traditional procedures of solvent leaching followed by trace analysis, using, for example, atomic absorption spectrometry for metals. In particular, the latter approach lacks important features characteristic to SAM, namely, the high spatial and depth resolutions (see Table 9-1), which are crucial for detailed and meaningful analysis of such heterogeneous systems having microscopical dimensions. When this is combined with chemical-state information, easily obtainable in XPS/AES, understanding of fundamental mechanisms for complex environmental materials becomes possible.

9.4 Summary and conclusions The main objective of this chapter is to elucidate and demonstrate capabilities and merits of modern electron spectroscopic methods applied to trace analysis. In particular, if a trace element is trapped as a very thin deposit (preferably in the monolayer range), the two most widely used surface analytic methods, Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy, can provide quantitative determination with high sensitivity (detection limits < ppb), as well as good accuracy and precision. Continuous efforts in manufacturing more advanced instruments, have resulted in better overall performance, and improvements in data acquisition and manipulation have led to enhanced sensitivity. Various parameters, affecting the signal-to-background relative intensities (and hence the detection limit) have been reviewed above, with emphasis on its optimization including minimum detectable mass considerations. In this context, it has been claimed that what can be considered as the ultimate goal, namely the chemical detection of a single atom (!), will become possible with Auger analysis using a very highly focused (and intense) electron beam. Surface selectivity can be enhanced by analyzing lines corresponding to lowenergy (several tens eV) electrons collected in small take-off (“grazing”) angles. All this can serve the technique user as guidelines for getting the best performance out of a given spectrometer (purely technical/instrumental considerations, which can help in designing a spectrometer, are beyond the scope of this review). Two non-conventional electron spectroscopic experiments, which were proposed in the literature as potentially yielding improved sensitivity, were mentioned. This can make electron spectroscopies even more attractive tools in trace element analysis.

390

MICHA POLAK

However, one should be aware of potential problems. Surfaces should be treated very carefully, especially when submonolayer analysis is concerned. Thus, contamination from surrounding gases usually affects XPS/AES line intensities used for elemental quantification and may modify composition and chemical-state (this sets limits on acquisition times even under ultra-high vacuum). Several procedures used for trace analyte deposition on surfaces were described above. While suitable c m i ers were usually chosen, i.e. substrate materials that do not allow inward diffusion, none of these experiments avoided exposure of the surface to atmosphere before analysis. Definitely, for improved accuracy and precision, direct interfacing of the spectrometer with the deposition (preparation) chamber is desirable. Another factor to be considered when dealing with surfaces is the stability of the layer under photon or electron irradiation, which also limits the possible duration of data acquisition time. Material specific, stimulated processes such as decomposition and desoi-ption, are more likely to occur in AES than in conventional XPS, which was the technique chosen in most of the reported electron spectroscopic studies of trace elements. Prominent advantages of these methods include also multielement simultaneous analysis via commonly well-spaced spectral lines, and chemical-state information accessible via small, but characteristic and measurable shifts or shape changes in the lines. In this context, the existing vast amount of support data, as well as availability and relatively easy use of equipment, are further justifications for choosing these techniques for quantitative analysis of trace elements. It seems that the full promising potential of XPS and AES in such applications has not yet been fully recognized. Hopefully, this review will contribute to this goal.

References 1 Seah, M.P. and Briggs, D. (Eds.), Practical Surface Analysis, Wiley, New York, 1990.

2 Wanger, C.D., Riggs, W.M., Davis, L.E., Moulder, J.F., and Muilenberg, G.E. (Eds.), Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer (Physical Electronics), Eden Prairie, MN, 1979; Moulder, J.F., Stickle, W.F., Sobol, P.E., Bomben, K.D., Handbook of X-Ray Photoelectron Spectroscopy,Perkin-Elmer (Physical Electronics), Eden Praire, MN, 1992.

3 Davis,L.E., MacDonald, N.C., Palmberg,P.W., Riach,G.E., and Weber, R.E. (Eds.), Handbook of Auger Electron Spectroscopy, Perkin-Elmer (Physical Electronics),Eden Prairie. MN, 1978. 4 I n particular. Journal of Electron Spectroscopy and Related Phenomena; and Surface Science Spectra. 5 Feldman, L.C. and Mayer, J.W., Fundamentals of Surface and Thin Film Analysis, NorthHolland, Amsterdam, 1986. p. 223.

6 Grinbaum, Y,Livshits, A., and Pol&, M., Appl. Surface Sci., 25 (1986) 203-212.

7 Wanger, C.D. and Joshi, A., J. Electron Spectrosc. Relat. Phenom., 47 (1988) 283-313. 8 Cazaux, J. Appl. Surface Sci., 10 (1982) 124-140. 9 Czuha. M. and Riggs. W.M., Anal. Chem.. 47 (1975) 1836-1838.


39 1

10 Langeron, J.P., Surf. Interface Anal., 14 (1989) 381-387. 11 Polak, M. and Livshits, A.. Appl. Surface Sci., 10 (1982)446-454. 12 Polak, M.. in: Surface Segregation Phenomena. PA. Dowben and A. Miller (Eds.). Chapter 11, CRC Press, Baco Raton, FL, 1990. pp. 291-325. 13 Arkush, R.. Talianker, M.. and Polak, M., Scr. Met. Mater.. 24 (1990) 297-300. 14 Seah, M.P. and Dench, W.A., Surf. Interface Anal.. 1 (1079) 2-1 1. 15 Tanuma, S., Powell, C.J., and Penn. D.R., Surf. Inlerface Anal., 17 (1991) 91 1-926. 16 Werner, W.S.M., Gries, W.H.,and Stori. H.,Surf. Interface Anal., 11 (1988) 627632. 17 Jablonski, A., Surf. Interface Anal., 15 (1990) 559-566. 18 Jablonski, A. and Ebel, H., Surf. Interface Anal., 17 (1988) 627-632. 19 Deviation from linearity is sometimes ignored in calibration curve analysis (e.g. Ref. [20]). 20 Bancroft, G.M., Brown. J.R.. and Fyfe. W.S.,Anal. Chem.. 49 (1977) 1044-1047. 21 Fulghum, J.E. and Linton, R.W., Surf. Interface Anal., 13 (1988) 186-192. 22 Seah, M.P.. J. Vac. Sci. Technol. A, 3 (1985) 1330-1337. 23 Hoffman, S., in: [ll, Chapler4, pp. 143-200. 24 Carvalho, M.B. and Hercules, D.M., Anal. Chem., 50 (1978) 2030-2034. 25 Briggs, D., Gibson, V.A., and Becconsall. J.K., J. Electron Spectrosc. Relat. Phenom., 11 (1977) 343-347.

26 Grant. J.T..Surf. Inlerface Anal.. 14 (1989) 271-283. 27 Powell, CJ. and Seah. M.P., J. Vac. Sci. Technol. A, 8 (1990)735-763. 28 Seah. M.P., in: [l], Chapter 5. pp. 201-256. 29 Wagner, C.D., Davis. LE., Zeller, M.V., Taylor, J.A., Raymond, R.H., and Gale, L.H., Surf. Inlerface Anal., 3 (1981) 21 1-225.

30 Mroczkowski. S. and Lichtman, D., Surface Sci., 127 (1983) 119-134. 31 Hall. P.M. and Morabito, J.M., Surface Sci., 83 (1979) 391-405. 32 Tougaard, S. and Jansson, C., Surf. Interface Anal., 19 (1992) 171-174. 33 Wagner, C.D., J. Electron Spectrosc. Relat. Phenom., 32 (1983) 99-102. 34 Cazaux, J., Surface Sci., 140 (1984) 85-100. 35 Cazaux, J., Scann. Electron. Micros., 111 (1984) 1193-1202. 36 Seah, M.P. and Cumpson, PJ., Appl. Surface Sci., 62 (1992) 195-198. 37 Procop. M. and Weber. E.-H., Surf. Interface Anal., 15 (1990) 583-584. 38 Electron Energy Loss Spectroscopy (EELS) is based on Ihe excitation of core-level electrons to empty electronic States by means of a beam of electrons which loose the corresponding energy. (See Fig. 9-9 in Section 9.2.1). Expressions for detection limits in EELS and AES are similar, but the numerical values differ [34]. 39 Cazaux. J., Jbara, O., and Kim, K.H., Surface Sci., 247 (1991)360-374.

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MICHA POLAK

40 Gerely. G., Surf. Interface Anal., 19 (1992) 1 4 . 41 Hercules, D.M.. Cox, L.E.. Onisick. S.. and Carver, J.C., Anal. Chem.. 45 (1973) 1973-197s. 42 Brinen, J.S. and McClure, J.E., Anal. Chem., 5 (1972) 737-743; Brinen, J.S. and McClure, J.E., J. Electron Speclrosc. Relat. Phenom., 4 (1974) 243-248. 43 Macho. K., Garten, R.P.H.. Klockow. D., and Tolg. G.. Surf. Interface Anal., 12 (1988) 574-575. 44 Macho, K.. Garten, R.P.H., Bubert. H.. Auffarth, J., Klockow, D., and Tolg, G.. Surf. Interface Anal., 9 (1986) 5 16.

45 Hock, J.L. and Lichtman. D., Environ. Sci. Technol., 16 (1982) 423. 46 Clark, W.E.. Landis. D.A.. and Harker, A.B., Atmos. Environ.. 10 (1976) 637. 47 Adams, F. and De Waele, J., Surf. Interface Anal., 12 (1988) 551-564.

CHAPTER 10

Trace element determination by electrochemical methods

RAY VON WANDRUSZKA Department of Chemislry, Uiiiversity ojldaho, Moscow, Idaho, USA

Contents 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Anodic and cathodic striQping voltammetry . . . . . . . . . . . . . . . . . . . . . . . 10.2.1 Electrodes.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Stripping waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3 Film stripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.4 Cathodic slripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.5 Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Non-stripping methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Poteiitiomelric stripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Adsorptive stripping voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

393 394 395 397 399 400 400 405 405

408 416

10.1 Introduction Electrochemical methods of analysis encompass a wide variety of techniques, divided among the realms of potentiometry and voltammetry. Potentiometric methods (with one important exception) usually do not have limits of detection much below the micromolar level, so in the area of trace analysis, voltammetry generally holds the edge. The sensitivity of voltammetric methods can be greatly enhanced by the inclusion of a preconcentration step in which the analyte is accumulated at the electrode by either faradaic or non-faradaic processes. The analytical signal is generated by a subsequent stripping step.

394

RAY VON WANDRUSZKA

This type of stripping analysis, in the form of anodic, cathodic, or adsorptive stripping voltammetry, is undoubtedly the most successful method for electrochemical trace analysis of the elements. However, potentiometric stripping analysis, which combines analyte accumulation with a potentiometric stripping procedure, gives excellent results for some analytes and is gaining popularity. In addition to the stripping methods, “traditional” voltammetry (not involving preconcentration), including some polarographic techniques, can also give the low limits of detection that classify it as a trace method in a number of cases.

10.2 Anodic and cathodic stripping voltammetry Anodic stripping voltammetry (ASV) [ 11 involves the cathodic deposition of an element on the working electrode, constituting a preconcentration step, followed by anodic redissolution (Fig. 10-1). The current arising in this oxidative stripping I I

I

l a .

Time

Figure 10-1. Applied potentials in anodic stripping of a metal at a mercury electrode

process is the analytical signal, which is linearly related to the original concentration of the analyte in the sample solution. The reductive preconcentration

0 + tie +R

(10.1)

when carried out at a cathodic potential that is sufficiently large to lead to immediate reduction of 0 to R upon arrival at the electrode surface, and hence a resident surface concentration of 0, that is effectively z r o , produces a limiting electrolysis current, i, given by

i, = nFAD,,-c,.

(10.2) 6 where n is the number of electrons exchanged, F is the Faraday constant, A the electrode surface area, Do the diffusion coeffficient of the oxidized species, C: its bulk concentration, and S the thickness of the diffusion layer around the electrode. The electrolysis current reaches a maximum value because of diffusion limits of 0

TRACE ELEMENT DETERMINATION BY ELECTROCHEMICAL METHODS

395

and R in solution. To increase i, the diffusion layer thickness, 6,may be decreased, and this is easily achieved by vigorously stii-ring the solution. If analyte deposition proceeds for a sufficiently long time to measurably effect C;, the electrolysis current displays a time dependence of the form [2] i(i) = kIe-k”I

(10.3)

where k’ and k” are constants that depend on the experimental parameters. 10.2.1 Electrodes

The nature of the anodic stripping process is highly dependent on the type of electrode used. The most widely used electrode material for ASV analyses is mercury, and it is commonly employed in one or two configurations: the hanging mercury drop electrode (HMDE), or the thin mercury film electrode (TMFE). These two electrodes are primarily distinguished by their shape and their volume. The HMDE is spherical and has a relatively large volume, while the TMFE is flat and has a small volume. It is customary and permissable in analytical practice to ignore the curved nature of the HMDE surface, effectively presuming an infinitely large mercury drop, and treat it as if it were flat. With this assumption in place, a fast anodic scan proceeding from the deposition potential produces a stripping current peak, i, given by i, = knin213ADK 112I / 112 C:t (10.4) where k is a constant, 172 is the mass transfer coefficient, v is the potential sweep rate, and t is the electrolysis time. Eqn. (10.4) shows the linear dependence of the analytical signal on the analyte bulk concentration and on the preconcentration time, affirming the need of careful control oft. The typical thickness of a TMFE is 10 pm, produced by electrolytically plating mercury from a solution of its salt onto a substrate (usually glassy carbon) [3]. Electrodes of greater thickness may be used, but if the mercury layer is much thicker than the diffusion layer, the TMFE response becomes virtually identical to that of the HMDE. At a thin TMFE, the diffusion of the reoxidized species into the solution is entirely linear, but the finite volume of the mercury layer now exerts an effect. If a mercury layer of 1 pm or less is used, two conditions that simplify the dependence of the stripping current are met. The first of these is that the preconcentration ratio be large (i.e. C,/C,’ is large, where C, is concentration of analyte in the electrode), and the second is that diffusion of amalgamated analyte in the electrode be negligible, If, in addition, a fast sweep rate is used, then the following stripping current dependence applies [4,5].

i, = kn2ACRlv

(10.5)

RAY VON WANDRUSZKA

396

where k is a constant and 1 is the electrode thickness. Equation (10.5) can be written [6] in terms of the bulk analyte concentration, the electrolysis time, t, and the mass transfer coefficient, 772:

i, = k'nin2ACztu

(10.6)

Solid electrodes are not widely employed in stripping voltammetry, but in some instances, especially with metals that do not amalgamate, glassy carbon electrodes can be used to good advantage. Wax impregnated carbon electrodes are a somewhat cheaper alternative, while carbon paste electrodes are being increasingly applied in specific analyses. These latter electrodes are cheap and offer possibilities for modification for special purposes [7].Their construction is further discussed in the section on adsorptive stripping voltammetry later in this chapter. Various new developments of electrodes for voltammetric analysis include claymodified electrodes for ion-exchange voltammetry [8]. The large surface area of these electrodes allows them to provide limits of detection in the picomolar range. The similar zeolite-modified electrodes are noted for their selectivity based on size exclusion [9]. Solid carbon electrodes can be modified by the attachment of polymer films. Examples include a glassy carbon electrode onto which a polysalicylic acid film was electropolymerized, giving a 40-fold improvement in stripping response for copper [lo]. Wang et al. [1 11 have constructed an interesting device consisting of a mercury-coated carbon-foam composite. This electrode gives low background currents and offers the large surface area usually associated with zeolites. Short accumulation times in quiescent solutions can therefore be used. A novel development for ASV in highly resistive media without supporting electrolytes [12], consists of a thin mercury film electrodeposited on a glassy carbon electrode inteifaced to an ion-exchange membrane. The other surface of the membrane, which may be regarded as a solid polymer electrolyte, is in contact with an electrolyte solution containing the reference and counter electrodes. Screen printed electrodes (SPEs) must count among the most interesting and promising developments in modern electrode technology. Tliese devices do not constitute a new and different class of electrodes based on their mode of operation, but rather on the way in which they are produced. They are small, cheap, and disposable, and they may be adapted to most existing electroanalytical techniques. Their importance lies in their suitability for field studies, decentralized environmental monitoring, clinical testing, and food analysis. SPEs consist of thin carbon or metal films printed on an inert support such as PVC or alumina. They may have any desired shape and can be chemically modified, not unlike traditional electrodes. Wring et al. [ 131and Craston et al. [ 141have published accounts of SPE fabrication in the laboratory, which allows the experimenter full


397

control of variables such as size and shape. In circumstances where this is not important, the basic device may be purchased in a drug store. Figure 10-2 shows a diagrammatic representation of a type of SPE. Insulating layer

/

Sobpate

Working electrode

/

Conlacls

Re&ce

electrode

Figure 10-2. Diagram of screen printed electrode

The first application of SPEs to voltammetric trace metal analysis was recently reported by Wang and Tian [15]. They used inexpensive commercial carbon and reference electrode strips printed on PVC. The carbon strips were coated with mercury, in analogy with the production of TMFEs. Scanning electron micrographs showed that this coating in SPEs, as in TMFEs, consists of individual spherical microdroplets of 1-2 pm diameter. The electrodes were used for both voltammetric and potentiometric stripping analyses of trace amounts of lead and cadmium. Limits of detection of 30 ppt Pb and 50 ppt Cd were obtained with ASV, and 1 ppb Pb and 0.5 ppb Cd with potentiometric stripping (see discussion of potentiometric stripping analysis below). 10.2.2 Stripping waveforms The use of a differential pulse waveform is recommended for anodic stripping, because it minimizes the contribution of the double layer charging current to the measured stripping current, thereby increasing the sensitivity of the technique. As shown in Fig. 10-3, a regular series of square potential pulses (30-50 mV amplitude) is superimposed on a voltage ramp. The current is measured both before and during the latter part of each pulse, and the difference of the two measurements is recorded. The double layer charging current decays during the early portion of the pulse, leaving a relatively pure faradaic current. If the potential pulse, AE, has a magnitude less than about 120/n mV, then a reversible electrode reaction at a HMDE gives a maximum current difference of [ 161

n2F2

DkI2 (10.7) 4RT (7F t)'/* where t is the time since the application of the pulse. For pulsed voltammetric stripping from a TMFE, the electric charge in the deposition step Qnl has been invoked [ 171: (2, = I z F CAI~ ~ (10.8)

Ai,,,,= -AACRAE-

RAY VON WANDRUSZKA

398

SO ms f------)

-E

I

time Figure 10-3. Potential waveform and current response in differential pulse voltammetry

giving a maximum stripping current

Ai,,, = 0.138Q,/tP

(1 0.9)

where t, is the pulse duration. A technique that is gaining popularity in stripping analysis is square wave (sw) voltammetry, which was developed from square wave polarography [18,19]. The waveform consists of a square wave superimposed on a staircase potential (Fig. 10-4), and it is carried out at substantially higher frequencies (10&300 Hz) than differential pulse stripping. The technique is especially useful for adsorptive stripping voltammetry (vide irfiu), since this involves species present in a surface monolayer and does not require diffusion from the electrode interior. This aspect makes very high sweep rates possible, resulting in greater stripping currents. The diffusion of the reaction product away from the electrode remains a salient feature, of course. Another attractive attribute of sw stripping is the short analysis time - the shipping step is usually complete in little more than a second. A final advantage of the high frequencies used in sw stripping is that interference by dissolved oxygen becomes less of a problem when reductive analyte stripping is employed. The reduction of oxygen, which depends upon diffusion of the dissolved gas to the electrode, contributes little to the overall stripping current [20-231 The sw differential peak current, i, is linearly related to the surface concentration, r, of the analyte. If this adsorbed species provides the major portion of the sw current, the expression is

i,= IiAr

(10.10)


399

I I

:-E I

cumnt MeaSurcd

#-.

Differential cumnt

Figure 10-4.Potential waveform and current response insquare wave voltammetry

I' is related to the bulk concentration, C*,of the analyte [24]

r = 0.739DLf2C8t'l2

(10.11)

where t is the preconcentration time. To be determined by ASV, metals must be soluble in mercury, have a lower reduction potential than mercury, and have a higher reduction potential than water or other reducible species present in the sample. The first of these requirements eliminates nonamalgamating metals such as Fe, As, Si, Ni, Mo, Ti, Te, Sb, and Au. The alkali and alkaline earth metals, rare earths, aluminum, and transition metals such as Ti(II), Mn(II), V(II), Th(IV), and Zr(1V) have standard reduction potentials that are too low. Conversely, the reduction potentials of Au, Pt, and Pd are too high, making their reoxidation from mercury impossible. The possibility of interference because of (near) equipotential stripping is very real in ASV, and analysts should consult standard potential tables such as those published by Bard et al. [25].

10.2.3 Film stripping Elements that do not form amalgams at the HMDE or TMFE clearly cannot be determined by the ASV methods based on these electrodes. A related technique, termedfilm stripping, may be used for these species [26]. Nonamalgamating metals are deposited cathodically [27] and stripped anodically in film stripping voltammetry for metals (FSVM). Alternatively, the preconcentration step involves

RAY VON WANDRUSZKA

400

an electrochemical change of oxidation state of the metal, followed by precipitation with an added agent, A [28].

The insoluble compound is stripped in the opposite electrochemical direction. 10.2.4 Cathodic stripping

In cathodic stripping voltammetry (CSV), anionic species are deposited anodically and stripped cathodically. This usually involves oxidation of the electrode material (mercury) and its subsequent reaction with the anionic analyte: M -+ M + + e M’+A- 2 MA Cathodic stripping of MA completes the analysis. The distinction between CSV and adsorptive stripping (discussed later in this chapter) is often only semantic, although the term “adsorptive” is not usually applied when compound formation as described above, is involved. 10.25 Inteiferences

The most widely encountered interference in stripping voltammetry is the overlap of stripping peaks of different elements present in the sample. Many of the examples quoted in Tables 10-1-3 address this problem for individual analytes, mostly through judicious choice of accumulation potential, stripping sweep range, and solution conditions. Two techniques used in ASV/CSV and adsorptive stripping deserve special mention. The first is irradiation with UV light [29-311, which is useful for the destruction of organic surfactants that block the electrode surface by adsorbing onto it. The second is referred to as medium exchange and involves removal of the electrode from the sample solution after the accumulation step [32-361. The stripping process is canied out in a blank solution, free of potential interferences. The removal of oxygen from the analyte solution is always an important practical consideration in stripping voltammetry, especially in cases where sfiipping is not carried out with a high frequency (sw) waveform. Oxygen is usually driven from the sample solution by purging with pure nitrogen, but chemical methods can also be used. A recent report on flow systems notes [37] that citric acid under UV radiation is an effective oxygen scavenger, giving better results than conventional removal with nitrogen.


401

Table 10-1.Species determined by anodic aid cathodic stripping voltammetry aid other voltammetric inethods A. Metallic and Seniimetallic Species Analyte Supporting electrolyte NHJOH, NH3NOj, N b O A c (pH 5 or 7). KNO3 0.2 M KNO3

0.1 M HzSOj

As

0.5 M NaOAC + HN03, pH 4 0.2 M H2 0.1 M NHOj + 0.9% NaCl HCI or HClO4 with NazSO3 HCI 7 N HCI 0.36 N H2W4 + 40mg/ml Se 1 NHCl

Au

0.1 M HCI 1 M HCI

Bi

Acid medium wilh excess mecumus ion Seawwatet filtered through 0.45 pm sieve acidified with 0.05 M HCI pH 5.6 acetate

Bi, Cd, Co. Cu,Fe, Ni, Pb, Sn, TI

Various media, i d . CI-, OAc-, ClO,-. NH;

Cd

Acetate buffer pH 4.64.7 HNO3: acetate buffer HCI-sodium dodecylsulfate

0.05 M N&OAc 0.1 M tartaric acid 0.1 M acetate buffer

Cd. Cu. Pb

0.02 M HCI Acetate pH 4.8; ammonia pH 8 5% HCI

1:1 benmne/niethanol, 0.1 M NaC104 Acetate buffer, pH 5.7 HCI, pH 2 Unmodified low salinity

solutions

HNo3 PH < 2 HCI or HOAc Cd, Cr, Ni. Pb,

Ref.

ASV at W E E

39

FSVM with LS on graphite. C P E lod 2.5 x 10-9M GC and F’I ring electrode: lod M, no intermetallic compounds FSVM with Ison graphite PI electrode DPASV at CPE ng/ml range ASV and DPASV ASV, codeposition with copper ASV at Au-film electrode CSV at HMDE lod 2 ppb DPCSV at HMDE in presence of CuCl2; Ir 0.2-20 ppb DPASV at CPE nghol range Trioctylphosphineoxide-modified GCE 50 lod 0.6 ppb I n situ Hg deposition on Pt RDE to 5 x M Bi Ir 2.5 x lit situ Hg depos. on G C E ASV; ppb range

40 41 42

43 43 44 45 46 47 48 43,49 51 52

HMDE: adsorptive wave of benzoylphenylhydroxyamine complex: lod 2 x lO-’M ASV a! CPE

53

Interactions with humid acid Preconcentrationonto controlled-pore glass chronidtogrdphyc o h i n . eluted with HNOj; DPASV at GCE Adsorptive wave of iodide brilliant green complex; lod 0.16 ppb ASV at Ni-MFE, range 5 x ID-‘” lo 10-7MF%.2x10-10to 10-7MCd ASV at cellulose acetate coated electrode lod 7 x 10-toM pb, 1.3 x loA9Cd carbon SPE lod 30 ppt Pb,50ppt Cd DPASV on TMFE; no deaeration needed

55 56

d 0.5 ppb ASV at HMDE; I ASV: lod 1 x lo-’ M DPASV at W E E interference by Cu and Zn in ppb range DPASV at HMDE ASV at HMDE; ppb range; medium exchange with high salinity samples DPASV at HMDE lod 0.1 ppb ASV at TMFE 50-min deposition for ppb levels; samples UV irradiated

Ammonia buffer, pH 9.6

DPASV; lod Cd, Ni. Zn 5 x lo-* M, pb 3 x IO-’M; Ir Cr 6.6x lo-* to 1 x lo-’ M

0.1 M HOAc + 0.1 M NaOAc

Ce”

Zn Ce

Comments

+ 4H10 = Ce(OH)J + 4H+ + e

54

57 58

15 60

61 62 63 64 36 65 31

66

67

RAY VON WANDRUSZKA

402 Table 10-1 (Continued) A. Metallic and Semimetallic Species Analyte Supporting electrolyte

co

Cr

Cu

1.25 M NaSCN + 2.5 M KOH 0.4 M N k O H t 0.05 M NQCI pH 4.5 acetate buffer

2.5 M NaOH + 0.2 M Na2H2EDTA + HNO3,pH 8 1.3 M niangsnese sulfate 0.1 M NaF, 0.01 M NaCIO7. 10-3M NH7. -. 10-4 MN H ~ F H2SO4. H20,0.9% NaCl 6.8 phosphate buffer: 0.03 M acetylacetone Solid polymer electrolyte

0.5 M NaCl Seawater 0.1 M H N q + 10-sM Hg(N03)2 Seawater, pH 4.9 1 0 - 3 ~HCI 0.07 M acetate buffer pH 4.0 0.05 M NaCl pH 5 acetate buffer Filtered seawater, 0.45 pni niillipo~tfilter 0.1 M acetate buffer pH 5.8-6.0 Seawater at natural pH; pH 5.8 acetate M e r Seawater at natural pH orpH 1 Seawater Cu, Cd. Pb, Bi,

pH 5.5-5.8 acetate

Zn Cu, Co,a, Hg, Ni, Pb, 2n

0.1 N HCIOJ

Fe

H3BO3 + NaOH, pH 8

Ga

1.5 M KOH + 0.006M H2 tafl tO.001 M NazS 0.5 M NaSCN + 4.2 M NaCIO4

pH 5.3 acetate buffer 0.1 M KSCN + 0.025 N HCI +20 ppb Cu (PH 3) 1 M HzS04

Comments

Ref.

LS on graphite; lod 2 x lo-' M Forms CoR3;R = 2-nitroso-I-naphthol

68 69 70

2-(3.5-dibromoZ-pyridy)azo-5-(diethyl. amino)phenol complex: single sweep phenol complex (1 2);lod 5 x lo-" M M LS on graphite; lod 1 x

71

TMFE lod 1 ppb ASV at TMFE and HMDE: electrocatalyzed reoxidation of Cu by C102-

72 73

DPASV at CPE and HMDE Single sweep adsolptive wave of acetylacetone complex Ion exchange membrane replaces electrolyte; lod Cu 0.02 ppb, Pb 0.05 ppb /n$fu Hg deposition on GCE: electrode awg DPASV at rotating Hg-coated GCE FSVM by Isv at graphite electrode

43 74

76 77

Automated ASV

78

ASV at Hg-coated Nafion electrode

80

ASV at Hg-coated GC tubularelectrode; potable, battery-powered equipment ASV at RDE lod Cd and Pb 0.002 pgA, Cu 0.004 pgfl ASV at HMDE

81

82

ASV and DPASV at WIGE

84

Some UV radiation; compare in sibr Hg-film GCE to HMDE

85

Subtractive mode DPASV at twin Hg-coated GCE disks; lod Cd 0.025 ppb. Cu 0.067 ppb DPASV; lod < I ppb in working soln., < 3 ppb in effluent ASV at GCE; deaeration not necessary

86

Fez* + 30H = Fe(OH)3 + e; iod 1 x 1 0 - 7 ~ Is on graphite; lod 2 x lo-' M

12 75

83

87 88 89 68

SW stripping of Zn-Ga intermetallic mipound at HMDE lod 1 x lo-* M Adsorptive wave of eriochrome blackT complex; lod 0.42 ppb DPASV at W E

91

ASV at PI riiig and GC disk with Au film:

93

90

92

lod lOppb pH 2 buffer Various elearoly~es; NaOH, HCI, KNO3 + HNO3. NaC104 t HC104. KSCN + HClO4

Subtractive-mode DPASV; twin rotating Au electrodes; 0 . 0 4 4 0 p p b range Rotating GCE: KSCN + HCIO, pH 2 preferred

94 95


403

Table 10- 1 (Continued) A. Metallic and Semimetallic Species A~ialyte Supporting eleclrolyte

Seawater + HNO3. pH 2.5; 0.005 M HClO4 stripping solution 0.1 M KSCN HNO3

Neutral elec~rolyte

In

Acetatebromide pH 4.65 Chlorofumi/EtOH/H~O (1:4:1),0.005 M NaOAc, 0.06 M KBr, 0.06 M HCI

Mo(V1)

0.1 M HClOj 3 M NaCl in pH 5 acetate pH 5.8-6.7 phosphateborate buffer Acetate buffer

Ni

ammonium taltrate buffer

Mn Mo

SCN media

0.03 M KOB + 10-6M H2D

Pb

HOAc-KNOj Seawater 0.01 M H ~ S O J 0.1 M NaN03 + HCIO, 0.01 M HNO3 0.1 M KNO3

Pu Rh(II1)

HN03 pH 3.0 buffer

Ru Sb

Se(IV)

S e w ) , Se(W

1.5 M KCI + 0.5 M H2SO4 +4 x lo-' MR pH 2.2-2.3 HzSO4 NaBtlJ, 0.05 M NaOH HCI, HCIO4, Hzso4, HNOJ 0.2 M HCI

pH 8.25 ammonia buffei Various electrolytes: N € ~ C I Obuffer J pH 8

Comments

Ref.

ASV at WlGE with medium exchange lod 0.005 ppb

96

~ M ASV at WIGE lod 4 . 0 lo" DPASV at WIGE and G C E lod at WlGE 10 ppb, at GCE lppb Constant current stripping from Au electrode coated with Hg; lod 40 ppb Oxidn. peak of tetraphenyl borate monitored; lod < 8 x 10-9M DPASV at HMDE and ASV at TMFE, bod 0.5 pptr Liquid extraction followed by ASV at HMDE

97 98

at pt; iod I ~ 1 0 - 7 ~ ASV at HMDE ASV a1 HMDE

102 103 104

Catalytic current of Mo-ferron complex with oxidant DPP sensitized by ethanediamidoxime lod 2 ppb ASV at Pt and Au electrodes: lod 5 x 10-* M FSVM of Ni(DH)2; H2D = dimelhylglyoxinie; lod 2 x 10-9 M Soil digest sample ASV at TMFE CSV of PbO2 film on conducting.~ glass electrode; low ppb level Pb/fulvic, huniic complexes pH 6; filtered CSV at conductive tin oxide electrode; lod 2 x lo-* M ASV at ultramicroelectrode; no deaeration Hg on silica-carbon deposit; Iod 10 nM Hg film on poly(3-methylthiophene) electrode; lod 0.05 ppni DF'SV at AS Rh(NH3)g: ASV at ultramicroelectrode; to M level Oscillopolarographic;catalysis of Ce(IV)/AS(III) reaction; lod 7.9 x lo-" M

105

99 100

101

62

106

107

108 109 110 111 112 113

114 115 116

117 118 119

(sbcl6y- + R* = R(SbCb) + 2e: R = diodamine C ASV at HMDE DPCSV a1 HMDE; lod 1 ppb

120

CSV at HMDE either as adsorbed hexahaloselenous acid or HgSe film; Iod adsorption: 7~10"Mlodfilm5~1O-~M After Dentvl alcohol extraction: lod 1 nM M SW C'SV it HMDE; I d 3x DCP. NPP and DPP ASV and CSV at graphite and GCE

123

121 122

124 125 126 127

404

RAY VON WANDRUSZKA

Table 10-1 (Continued) A. Metallic and Semimetallic Species Analyte Supporling electrolyte

Comments

Ref.

~~

Sn

Sn, Pb

n

1.2 N HCI. ascorbic acid, 0.1 M KCI HBr. HzSO4, EtOH, hydraziniuni hydroxide 1 M HCI + 0.0015 M [(Cz Hs 0hPSz INi Heniatein, dioxane, N h O H . pH 2-3 Acetate buffer + EDTA

ASV at HMDE no R intelference

128

sirrr Hg deposition on G C E lod 0.05 ppb Lod 5xlO-*M

129

CPE; Sn done anodically, Pb calliodically

131

CSV at TMW; in presence of Cu,bi,

132

fti

130

Pb,cd TI

U

Acetate buffcr + EDTA pH 4.6 acetate + EDTA 2.5 M (Nk&S04 + NH40H pH 8.5 0.001 M PIPM buffer. sodiuni sulfite, 0.002 M catechol (pH 8.6)

V W'

0.06 M HCI

Zn

pH 1-2 HNOj 5% HCI with N h O H , pH 4 KBr

Zn,Pb. Cd, Cu

B. ANIONIC SPECIES 0.1 M HCIO,, 1:l EtOHIH20 Br-, CI-. IBr-. CI-

HCIO, 1.8 M HzSO4

CI-

0.1 M K N a + 0.002 M H N q 0.1 M KNO3 + 0.1 M H3Cit 0.08 M HNO3,80% EtOH

CN-

0.1 M K N q , CuSO4

Cr04-

pH 4 HNO3

1-

4 x lo-, M ascorbic acid; 0.01 M HNO3. 0.01 N HNO3.0.05% ascorbic acid pH 4.7 acetate buffer MdNOoh 0.1 M NaOAc, 0.1 M HOAc

DPASV at HMDE and Hg-plated GCE ASV at HMDE anion exchange preconc. M LS on graphite; lod 2 x

133 134 135

CSV at twin CFEs

136

DW; catalytic current with bmiate; lod 5 ppb Catalytic Hz wave of p-acetylcarboxylazo complex; lod 8 x lo-'' M DPASV at HMDE: ppb level ASV at HMDE; lod 0.5 ppb GCE semidifferential; lod ppb range

137 138 139 61 140

I-. l40 pg/l Br-, CSV at HMDE; 8 @ 177 @gA C1- detected CSV at HMDE CSV at TMFE

141

HMDE (PI); lod 5 x M Mercury-pool electrode; lod 5 x M CSV at HMDE (pl); temp = 2 'C ASV in presence of Cu at HMDE lO-'OM level M CSV at HMDE (Pt); lod 2 x

144 145 68

DPCSV at HMDB; many interferences; Sub-ppb level DPCSV at SMDE

148

ASV at Agniicroelecrode; lod 4 x IO-*M lod 5 X lo-* M CSV at HMDE (R); LS a1 HMDE; lod 4 x lO-'M

150 151 68

142 143

146

147

149

mi-

pH 4-10.6 (HNO3: KOH) Potasium antimony taretrate; ammonium heptamolybdate; HCI

CSV of Cu-phosphate film Adsorptive wave of ternary heteropoly acid

152 38

S2-

1 M KNO3.1 M NaOH

CSV at HMDE after HgS formation; LodIppb Indirect method. excess Hg; lod 8 x lo-* M CSV; low ppb level

153

0.4 M KlO3 0.2 M NaOH

68 154

Abbreviations: ASV - anodic stripping voltammetry; CFE - carbon fiber electrode; CPE - carbon paste electrode; CSV - cathodic stripping voltamnietry; DASV - differential anodic stripping voltammetry; DCP - d.c. polarography; DPP differential pulse polarography; DPASV - differential pulse anodic stripping voltammetry; DPCSV - dilferential pulse cathodic stripping voltammetry; FSVM - film strippng voltammetry for metals; GC - glassy carbon; GCE - glassy carbon S -linear scan (sweep); MFE -mercury film electrode; RDE - rotating electrode; lod 41limit of detection; Ir -linear range; L disk electrode; SMDE - static mercury drop clectrode; SPE - screen printed electrode; TMFE -thin mercury film electrode; WIGE - wax-impregnated graphite electrode.


405

10.3 Non-stripping methods Although accumulative preconcentration followed by stripping is by far the most effective electrochemical method for trace element analysis, single-sweep voltammetric techniques that do not involve analyte accumulation have also been able to reach low limits of detection. Adsorptive wave voltammetry (AWV) is a non-stripping method that has been reported in some recent trace analyses. The technique should not be confused with adsorptive stripping voltammetry, which is discussed later in this chapter. AWV involves a normal voltammetric sweep without preconcentration, often in the form of a polarographic procedure. The analyte is treated with a complexing agent that renders it surface-active, and its adsorption to the electrode occurs during the sweep. In the course of this sweep, and immediately following the adsorption, analyte oxidation or reduction takes place. The resulting adsorptive faradaic process can be recognized by having a maximum in the currenthemperature curve and a minimum in the electrocapillary curve [38]. Examples of adsorptive wave analyses are included in Table 10-1, which also lists particulars of metal and nonmetal determinations by ASV and CSV. 10.4 Potentiometric stripping Potendometric stripping analysis (PSA) was developed as a method for the determination of trace metals by Jagner and GranCli [ 1551 and has been reviewed by Jagner [156] and by Hansen [157]. The technique shares with ASV a cathodic faradaic deposition process, which preconcentrates the analyte in the working electrode. The TMFE is the customary electrode in PSA. The stripping procedure (Fig. 10-5), however, is carried out without the impression of an external potential, and the electrode potential, rather than the reducing current, is the measured quantity. This spontaneous potential, which arises as the amalgamated analyte is reoxidized by a solution borne oxidizing agent such as oxygen, provides a qualitative measure for its identification: n n M(Hg) + -A M"' + -Amm m where A is the oxidizing agent. Besides oxygen, Hg(I1) is also frequently used as the oxidizing agent, in which case it is added as a mercury salt to the analyte solution. This has the added advantage that it can also serve to form the TMFE mercury film by codeposition during preelectrolysis. Some analytes require strong oxidizing agents for the chemical reoxidation step. For instance, Hg(I1) can be analyzed by PSA, when KMnO, is added as the oxidant [158]. The characteristic potential in PSA is given by the Nernst equation __t

(10.12)

RAY VON WANDRUSZKA

406

1 I

Em. Potcntiostatic accumulation

potentiornetric stripping M(Hg)--t M * + C'

Time

Figure 10-5. General potential-timecurve in potential stripping analysis

where {M"'}, is the steady-state surface activity of the metal ion at the electrode. In some instances, PSA can be used for compounds that are anodically deposited and reductively stripped. Examples of this are selenide and sulfide, for which an ingenious re-reduction process was devised by Christensen et al. [1591. They used a mercury drop electrode which contained amalgamated sodium as the reducing agent. This was produced electrolytically by subjecting a sodium salt solution to -1.94 V vs. SCE prior to the experiment. The analyte (Sz- or Se2-) was then deposited from the sample solution as the mercury salt at the electrode surface. This obviously required a more anodic potential (e.g. -0.3 V vs. SCE) and resulted in some reoxidation of sodium. However, a sufficient quantity remained in the mercury drop to effect reduction of the surface bound chalcogen after electrolysis ceased:

-

H++ 2Na(Hg) + HgSsUfiace

2Na'

+ Hg + HS-

The electrode assumed the characteristic potential of the analyte couple and maintained it until the surface was stripped clean. The quantitative measure in the potentiometric stripping step is the time required for the reoxidation of the analyte. This time is usually short, ranging from milliseconds to seconds, by virtue of the small volume of the TMFE (and hence the s m d quantity of amalgamated analyte), and because the the electrode is rotated to minimized the thickness of the diffusion layer. The latter expedient promotes transport of A to the electrode, and removal of M"' from it. If a constant rate of rotation (or stirring) is used, which is identical for deposition and stripping, if the sample temperature remains constant, and if the concentration of A does not change significantly during the experiment, the stripping time, t, depends only on the original analyte concentration and on the deposition time, t,:

t, = k[M""]td

(10.13)

Potentiometric strippingcurves thus obtained therefore consist of a series of plateaus (Fig. 10-6), positioned at potentials characteristic of the analyte, and of a length


407

Figure 10-6.Potentiometricstrippingcurves obtainedafter2 min (A) and 4 min (B) of potentiostatic deposition at -1.10 V vs. S.C.E.for a sample containing 1 ppm Cd2', Pb2+,and Cu2+, and 80 ppm Hg2' in 1 M HCl. [From Jagner. D., Analyst, 107 (1982) 593. With

permission.]

linearly related to its concentration. The measured quantity in PSA is time, while in ASV it is current. This imparts an analytical advantage of PSA, since time is inherently more accurately and precisely measurable than current. Some serious interferences in ASV are less so in PSA, making the latter technique more robust in certain environments. The presence of other electroactive species is a case in point, notably oxygen. While deaeration is routinely practiced in voltammetry to eliminated O2 interference, oxygen is the common reoxidizing agent in PSA and there is no call for its removal. Interference by low concentrationsof surface active agents is also less serious in PSA than in ASV. These compounds have a tendency to adsorb onto the electrode surface, effecting a partial blockage that impairs the voltammetric stripping current. It does, however, affect electrode access by analyte (deposition) and by oxidizing agent (stripping) in a similar fashion, resulting in virtually unchanged stripping times. Lastly, PSA stripping times are independent of electrode area (see Eqn. (10.13), dispensing with the need to control this parameter. On the negative side, Eqn. (10.13) is only valid if diffusion layer thicknesses during deposition and strippingare constant and equal. This imposes rather stringent hydrodynamicdemands on PSA and necessitatescareful control of electroderotation or stirring rates. It should also be recognized that redox couples of which both members are soluble can produce a signal in PSA if the concentrations are high enough. A case in point is the H,BO;/BH; couple, which gives rise to an untidy strippingplateau between ca. - 1.4 V and - 1.28 V vs. SCE23'.

408

RAY VON WANDRUSZKA

The instrumentation used for PSA must be able to accommodate analysis times of the order of milliseconds and should provide the appropriate sample rates and data storage facilities. Sampling around 40 kHz is sufficient for most applications, but this clearly lies beyond the realm of stripchart recorders. However, these instrumental demands can easily be met by modern electronic data capture devices. PSA has wide application in elemental analysis, as shown in Table 10-2. Very low limits of detection, (down to 0.01 ppb) can be achieved, and linear dynamic ranges that extend from the 1.o.d. to the ppm level are common. The relative insensitivity to interference by electroactive species and surfactants makes the technique eminently suitable for complex matrices such as seawater. The above discussion of potentiometric stripping should bring to mind the older technique of chronopotentiometry, in which electrode potential and time are also the measured quantities, but which proceeds under the influence of an externally imposed constant current. This method has suffered some bad press [179,180], and probably justifiably so, because its accuracy is severely limited by double-layer charging effects inherent in all electrochemical techniques that involve the flow of current in the analytical step. Unlike in PSA, dissolved oxygen constitutes a real interference in chronopotentiometry. Despite these reservations, stripping chronopotentiometryhas seen some analytical use, especially the cathodic variant (for a further discussion of cathodic stripping voltammetry, see Section 10.5). For further details see Table 10-2.

10.5 Adsorptive stripping voltammetry Adsorptive strippingvoltammetry (AdSV) is a relatively new analytical technique that takes advantage of the tendency of many compounds to adsorb to electrode surfaces. This may happen under open circuit conditions, or may occur more extensively at certain applied potentials. In neither case, however, is the process a faradaic one and no charge passes across the electrode-solution interface. The adsorption constitutes an effective preconcentration process that lays down the analyte in (usually) a monolayer on the electrodesurface, where it is readily available for the subsequent analytical stripping step. As in related methods, stripping may in principle be anodic or cathodic in nature, but in AdSV of the elements it is usually a cathodic process. As a consequence of its usual electrochemical direction in elemental analysis, AdSV is sometimes referred to as adsorptive cathodic stripping voltammetry (ACSV). Recent reviews of the technique provide considerable detail [181,182]. Adsorptive preconcentration of analytes leads to vastly improved limits of detection, making AdSV a true trace technique. Reported limits of detection usually lie in the 10-7-10-8 M range, although values down to the picomolar level are not uncommon [183,184]. Not surprisingly, it has found most extensive use in the


409

Table 10-2. Species determined by potentiometric strippinganalysis Element

Cominents

Ref.

Ag, Au, Hg As(II1)

ASC, graphile eleclrode; NH4CI/NH40H; 30 ppb Au lod PSA: dep. direct on GCE; Au codeposited for conductivity;2 mn preelectrolysis @ -0.05 V; 4.3 nM lod PSA; Mg(OH)2 coprecipitation;8 min preelectrolysis @ -0.9 V 0.003 nM lod PSA; 32 rnin preelectrolysis;0.04 nM lod PSA; Hg(I1) oxidant; 30 rnin preelectrolysis @ - 1.3 V; ng/l lod CSC; Hg-coated C-fiber electrode CSC: DMG complex; pH 8.5 borate buffer; 60 s adsorption 0.1 nM lod PSA; 4 min preelectrolysis; 20 nM lod CSC; quinolin-8-01 complex; 1.8 nM lod PSA; in beer and wine; acidified with HCl; 8-16 rnin preelectrolysis @ - 1.2 V PSA; pH 9 tartrate; sub-ppb lod range PSA; KMn04 oxidant; 5 nM lod PSA; in urine; KMnO4 trap; preelectrolysis @ -0.7 V; 2 ng/l lod; rsd 4-896 PSA; reductive stripping; 5 min preelectrolysis @ 4 . 6 V,GCE and Pt electrode; MnO2 deposited; hydroquinone reductant; 0.1 pM lod CSC; 3 pA constant current; DMG complex; Hg-coated C-fiber electrode; 40 ng/l lod CSC; DMG complex; pH 8.5 borate buffer; 60 s adsorption 0.1 nM lod PSA; 32 rnin preelectrolysis; 0.06 nM lod PSA; in urine; Hg(I1) oxidant; 0.2 M HCI; 16 rnin preelectrolysis @ -0.95 V; 1 &Ilod PSA; complexometric titration PSA; in urine; 2 rnin preelectrolysis @ - 1.25 V; 1 nM lod PSA; in whole blood, serum; 2 rnin preelectrolysis; 25 nM lod PSA; reductive stripping; 1-2 rnin preelectrolysis; Na(Hg) reductant CSC; quinolin-8-01 complex; 1.6 nM lod PSA; 16 rnin preelectrolysis; 0.25 nM lod PSA; sample sparged with Nz; Hg(I1) oxidant; pH 4.7 lod (ppb); Zn 0.03, Pb 0.03. Cd 0.01 PSA; 16 rnin preelectrolysis@ -1.4 V vs Ag/AgCI; 0.25 nM lod PSA; biological tissue; HNO3 digestion; Hg(I1) oxidant

160

Bi(II1) Cd Cd, Pb co

cu Cu, Cd, Pb Cu, Pb, Zn, Cd

Hg Mn(I1) Ni

Pb

Pb, Bi(II1) Pb, Cd

s2-,Se2U Zn Zn, Pb, Cd

Zn, Pb, Cd, Cu

161 162 163 164 163 165 166 165 167 168 158 169,170 171 172 165 163 173 174 175 176 177 165 163 166 159 178

ASC anodic stripping chronopolentiomelry;CSC: cathodic stripping chronopotentiornetry; DMG: dimethylglyoxime;lod: limit of detection;potentials quoted vs. SCE, unless otherwise stated; GCE: glassy carbon electrode

RAY VON WANDRUSZKA

410

voltammetric analysis for compounds of pharmaceutical and medical interests, because these generally have more surfactant character than small ions. Their greater tendency to adsorb onto the electrode makes them more suitable for AdSV. Anson and Brown have described the adsorption and strippingprocesses in AdSV and have identified five classes of adsorbates that interact with electrodes in different ways [ 185,1861. For the purpose of this chapter, which deals with elemental analysis, the discussion will be confined to the classes that include adsorbed elements and some other small inorganic ions. In all cases, adsorption from aqueous solution involves an initial dehydration of the adsorbate. This entails an energy expenditure which must be compensated to render the overall process energeticallyfavorable. As a consequence, small ions such as alkalis and alkaline earths, which have high hydration energies, have little or no tendency to adsorb on electrodes. Larger ions, including Cs', R4N+,R3NH+,H,PO;, NO, PF;, and ClO;, are only weakly hydrated and easily adsorb onto oppositely charged electrodes. Some sulfur and nitrogen containingions such as S2-, HS-, SO:, S,Oi-, NCO- ,and NCS-, adsorb onto mercury electrodes through strong covalent interactionsinvolving donation of electrons to empty mercury orbitals. This type of bonding can even occur to slightly negatively charged electrode surfaces. Large metal ions such as Cd(II), Hg(II), In(II), Pb(II), Tl(II), and Zn(I1) do not adsorb by themselves, but can do so through the intervention of anionic bridging ligands. In this mechanism, ions such as I- and NCS- are the primary adsorbed species on a mercury surface, but can act as d'' anionic bridges anchoring the cation in a secondary fashion. Transition metal ions that form strong isocyanato complexes experience especially strong adsorption to mercury surfaces. With an overall negative charge, species such as &s-Cr(NH,),(NCS); have a strong tendency to adsorb even onto positive electrodes. The interaction is assumed to occur chiefly through the sulfur atom [ 1871. Complexes in which the metal has the possibility to undergo intermetallic bonding with a mercury surface constitute a final class of adsorbed compounds. Reactions are comparableto those between dissolved species, leading to similar surface complexes such as [188]; Co(CN):-

2 Co(CN)!-

+ CN-

Theoretical considerations in AdSV assume monolayer coverage or less in the adsorption process. While adsorbate-adsorbentinteractions are obviously dominant, lateral interactions between adsorbed species cannot be ignored when the monolayer is approached. If is the surface concentrationof analyte and r,,,is the concentration


411

conesponding to monolayer coverage, then a fraction surface coverage 0 = r/T,, can be defined. The simple Langmuir adsorption isotherm then becomes

0 BC' = 1-0

(10.14)

where B is the adsorption coefficient and C' is the bulk analyte concentration. No account is taken for lateral interactions in this case. The introduction of an interaction parameter, 9,in the Frumkin isotherm addresses this problem [1891:

(10.15) AdSV mostly utilizes hanging mercury drop electrodes (HMDE), which have the advantage of being easily and reproducibly renewable. This renewable nature is important because the stripping step is usually reductive, and in the case of metal ions, the products may amalgamate and contaminate the electrode. An entirely different type of electrode that has been given increasing attention for elemental analysis by AdSV is the chemically modified electrode (CME). It is especially important for the analysis of metals that are incompatible with mercury. In CMEs, the analyte is entrapped at the electrode surface by chemical means involving a modifying agent such as a ligand or an ion exchanger. This agent is made part of the electrode surface by incorporating it in a polymer coating attached to the electrode surface [190] by dipping, spin coating, or electrodeposition. The modifying agent may be a part of this polymer backbone, or be introduced to it through subsequent functionalization. An interestingapproach to the production of CMEs has been pioneered by Guadaloupe and AbruAa [1911, who used bifunctional and multifunctional polymer films containing electroactivecenters and coordinating groups. The former induces precipitation of the polymer on the electrode surface, and the latter binds strongly and selectively with the metal ion of interest (Fig. 10-7). The stripping current is generated by reduction of the immobilized metalhigand and is related to the bulk concentration of the analyte. Functionalized polymer films permit synthetic variations to taylor the electrode for particular applications. There are two general approaches to the preparation of such films [192]. In the first, the ligand is part of the polymer backbone itself [1931, which is formed by the copolymerization of, for instance, vinylferrocene (the redox center) and vinylpyridine (the ligand). In the second approach, the ligand is incorporated by ion exchange. A pyridinium group, for instance, may be used to attract an anionically charged ligand such as sulfonated bathophenanthroline. This ligand has the added advantage of producing a multi-use electrode suface [ 1941, since it has a high affinity for Cu+,but readily releases Cu2+. Cleaning of the electrode therefore requires merely a number of anodic scans.

RAY VON WANDRUSZKA

412

+xe-

+

- xe-

+

.X)+

Figure 10-7. Electrochemical reduction reduction involving complexed metal and ligand bound to polymer backbone at electrode surface. [From: Guadaloupe, A.P. and Abrufla, H.D., Anal. Chem., 57 (1984) 142. With permission.]

Carbon paste electrodes have been popular in voltammetry because of their wide applicability, ease of preparation, and low cost. Their working surface consists of a paste of graphite powder mulled with a hydrophobic agent such as light oil or a-bromonaphthalene. They have seen most use in the determination of organic species, but chemically modified versions are also suitable for elemental analysis. Wang et al. [ 1951 have incorporated a cation-exchange resin directly in the carbon paste, and used the electrode for the determination of Cu(I1). Baldwin et a]. [ 1961 have reported a nickel specific CPE which contains dimethylglyoximein the carbon paste. They found a limit of detection of 50 nM Ni, linearity over 3 orders of magnitude of concentration, and little or no interference from other metal ions. Exposure to acid regenerated the surface, making this a multi-use electrode, Modification of electrode surfaces with microorganisms, specifically algae, has recently been introduced by Wang and coworkers [197,198]. The organisms bioaccumulate metal complexes through sorption of functional groups in the cellular matrix and are generally of greater interest for their selectivity than their sensitivity. Gold(II1)and copper(I1) were determined with limits of detection of 2.4x lO-'M and 2 x 10-6M, respectively. Fairly sensitive determination of H202 with a horseradishroot-modified carbon paste bioelectrode, based on hydrogen peroxidase activity in tissue, was recently reported [ 1991. A limit of detection of 3 x 10-7M was achieved. The usual sequence of reactions in AdSV involves the formation of a metal-ligand complex which has increased surfactantproperties relative to the aiialyte alone. The complex adsorbs onto the electrode, and this accumulation step may be allowed to


413

Table 10-3. Species determined by adsorptive stripping analysis Analyte Electrode

Ligand/Reagent

Comments

LOD

Ref.

A1

Hg

Solochrome Violet RS DSA DMG

5x M 1 x lo-% ppb level

204 198 205

As

Hg Hg cme/algae

Calmagite Copper Chloride Rhodamine B Beryllon 111 Solochrome Violet RS

Ligaid reduction Ligand reduction Triethanolamineammonia buffer Borate buffer

15 PPb 3x10-9~ 2.5 x M 5 PPb 5 x lo-'@M -5 x lO-'@M

206 207,208 209 210 21 1 212

8.2 x M 1 x IO-'@M

213 214,215 216 217 218 219 220 203 22 1 191,194, 222

cme Be

Hg

Ca,Mg, Hg Sr. Ba Cd,Pb Hg Co Hg

Cr

Hg

Cu

Hg,cme

Medium exchange Ligaiid reduction Ligand reduction

8-Hy droxy quinoline Dimethylgloxime Nitroso- 1-naphthol Bipyridine Nioxime 1 ,lo-Phenanthroline Phase selective ac Dimethylglyoxime DTPA/nitrate Catalytic current (2-pyridy1azo)resorcinol pH 5.1 acetale Catechol. S-Bathocuproine. Diethyldithiocarbamate 8-Hydroxyquinoline Thiourea Thiocyanate Tropolone Salicylideneamino-2thiophenol

0.2 ppb 5 x lO-*M 5 x 10-loM 0.1 ppb 1 x lo-" M

4x

M

Open circuit Accum.

cme crnelalgae cmdpeat moss

Fe(I1)

2~10-~M

195 197 226

Reusable Solochrome Violet RS

Ligand reduction

5-14x lO-'OM

227

Salicylate Catechol,

swv

Hg,cme

1 x lo-* M 6 x 10-I"M

228 191,214, 229

cme

1-Amine-2-naphthol4-Sulfonic acid, S-Bathophenanthroline, Solochrome Violet RS 1-Nitroso-2-napthlol 2,2-Bipyridyl

2 x 10-yM M

22 230

Dy,Ho, Hg Y,Yb Eu Hg

Fe

213 223 224 22 225

Ligand reduction

RAY VON WANDRUSZKA

4 14

Table 10-3 (Conlinued) Analyte Electrode

Ligand/Reagenl

Comments

LoD

Ref.

Ga Ge Hg H2O2

Hg Hg cme cme

SolochromeViolet RS Calechol 18-Crown-6

Ligruid reduction

~XIO-~M

23 1 224 232

Hg

Copper

< 1 x 10-'M 2~10-~M 3~10-~M 3x M 6 x 1O-IoM 2.6 x M 2~10-'~M 6 x lO-'OM 1x lO-'OM

I

H&+ In Hg La, Ce, Hg Mn Mo

Ni

Pb

Morin Cresolphthalexon Eriochrome black T 8-H ydroxyquinoline Tribulyl/tripropylphosphate Tropolone Mandelic acidhhlorate 3-Methoxy-4-hydroxymandelic acid Dimethylglyoxime,

Ligand reduction Ligand reduction

Catalytic current Catalytic current

-

Bipyridine Histidine Tributyl/tripropylphsphate 8-H ydrox yquinoline Dime1h ylglyoximc

Formazone/H+ Calechol

Catalytic current

Copper

Si Sn Tc

Molybdale Tropolone Thiocyanate

pH 4.5 pH 1.6 Ligand reduction

Te Th

Copper Mordant blue Solochrome Violet RS Mandelic acidlchlorate Diisopropylmeth yl Mandelic acid Cupferron Pyrocatechol 8-Hydrox yquinoline

U

4 x lo-" M 1 x lO-'OM

Pd PI Sb Se

Ti

1 x lO-''M 1 x lO-'OM

Ligand reduction Ligand reduction Catalytic current

50 ppb

3 x lO-'OM 2.1 x 10-'oM 4x M 1.8 x lo-'" 3~10-~M 1x lO-''M IXIO-~M 5 x lO-'M 50 pg/ml IxlO-'M 4 x 10-'oM 7~10-'~M 7 x lo-" M 3 x 10-IoM 5x M 2x10-9~

Diisopropylmethylphosphate Tribut y l/lripropy lphosphate Morband blue Ligand reduction

4-(2-pyridylazo)resorcinol

0.2 nM

199 233 234 235 236 237 238 239,240 24 1 200 242 196,214 243,244 245 246 243 247 248 249 250 25 1,252 253 254 214 255 25 1 23 1 256 257 239,240 258 259 260 26 1 262 239,240 23 1 263


415

Table 10-3(Continued) Analyte

Electrode

Ligand/Reageiit

Comments

V

Hg

W Zn

Hg Hg HI3

Catechol Tributyl/tripropylphosphate Beryllon I11 Ligand reduction Tribut ylhipropylphosphate Ammonium pyrrolidinethiocarbamate

Zr

Hg

Diisopropylmethylphosphate Solochrome Violet RS Ligand reduction Tribut ylltripropylphosphate

LoD

Ref.

1 x l0-loM

264 23 1 265 23 1 266

2x 10-loM 3 x IO-IOM

l00A

> 107p/mm2

A

> 1010atoms

1P

> 1013

ZandA

> loloatoms

Specificity

Limit of sensitivity

Prompt Reaction Analysis Rutherford Backscattering Spectrometry Radioactive Ion Microscopy Ion Micro-Absoqtiometry Particle Induced X-ray Emission

Lateral resolution A few p m Current density required for high sensitivity > 1013p/mm2 paaicles (p)/mm2

density

Ion transmission sensitive to electron

Mass sensitive in principle applicable to a l l elements

RIM3

Nuclear nactions limited number of isotopes detectable

RBS?

Principle 19scope

PRA'

Table 124. Synopsis of some features of microprobe and microscopic techniques (from Ref. [Z]

Verylow

> 1013p/mm2

Depends on ion energy and Z of target, N 1 pm

> 100 A A few pm

> 1010atoms

Z

Characteristic X-rays from ion-atom collisions

PEE4

(?I

Density

Like RIM

IMA~

SPECIATION OF TRACE ELEMENTS

467

Table 12-5. Examples of typical results for some common charged-particle activation analyses (after Ref. [ 3 ] )

Element determined

Nuclear reaction

Product half-life (min)(MeV)

Particle kinetic energy @pm)

B

"B@,n)"C "B(d, 2n)"C

C

12C(p,7)13N 12C(d,n)13N

'2C(3He, a)' C N

20.3 20.3 9.96 9.96

Detection Limit for 10 pA

beam @pm)

10-15

0.001

10-15

0.0001

10-15 10-15

0.01 0.001

20.3 20.3

10-20

0.001

W ( a ,an)"C

40

0.01

14N(p,a)"C

20.3

10-15

2.05

I60(a, 4°F

Typical results Concen- Matrix tration

0.0005

10-15

0.0002

109.7 109.7

10-15 10-20

0.01 0.001

109.7

40

0.00 1

'

or

% Abundance

0.0014

Reaction

He

Elem.

Table 12-12. Summary of neutron depth profiling reactions. After Ref. [98]

cross

12.3

4.4

0.49

0.19

31000

0.24

1.83

3837

4800

940

5333

section (barns)

1.4~10'~

3 . 8 10l6 ~

3.4x 1017

1.2x 1018

4.7x 10'2

7.1 x 1017

9.1 x 10l6

4.3~ 1013

3.5x 1012

1.8x 1014

3.1 1013

Detection limit (atoms/cm2y *

x 9


487

Table 12-13.Reported parameters for neutron depth profiling facilities. After Ref. [98]. ~

InstitutioxVreactor

Reactor

Neutron

power

beam flux

(MW)

density (cm-* s-')

Institut Max von 57 Lauepaul Langevin High-Flux Reactor [14,15] BrookhavenNational 60 Laboratory/High-flux Beam Reactor 1111 Hahn-Meimer5 Institut f. Kernforschung Berlin/ Ber I1 I151 6 Czechoslovak Academy of Sciences/VVR/S CsKAE Research Reactor [16] UniversityofMichigan/ 2 Ford Nuclear Reactor [171 National Bureau of 10 Standards

NBS Research Reactor

1x109 10

Beam diameter (mm)

Cadmium ratio

Gamma dose rate

00

30

Neutron

Beam filtering

Guide tube

2.3 x lo8

3x107

20

?

?

>20 (variable)

>3x104

3xld

?

20 cm single

crystal Bi

1.6~ lo8

1.4~10~

4x108

65

?

?

30 cm single crystal Si

13x12

2.8

?

None

9.5

> 104

200

20 cm single crystal sapphire

Figure 12-11.Side view of the reactor neutron sources and the neutron collimator and filter. After Ref. [981.

488

H.A. DAS

Nculron berm Mooitor

Vacuum Chamber 7-

Beam

Figure 12-12. Neutron depth profilingchamber. After Ref. [98]. Borosilicate Glass Film on a Silicon Wafer

l7----Particle Energy

Figure 12-13. Borosilicate glass film on a silicon wafer. Courtesy of NIST.

12.3.4 Depth profiling by activation analysis [IOI-I49] Activation analysis on the (off-line)measurement of irradiation-inducedradioactivity in small aliquots. In case of charged particle activation, spatial discrimination can be obtained by changing the energy of the incident beam. Usually protons, deuterons or a-particles of L 20 MeV and a beam current of 1 pA are applied. The reactive 3He-particlesare attractive but limited by their high costs. Activation with beams of 6Li and 14N at 14 MeV and 5 1 pA has been applied in the determination of surface carbon [ 1191. Discrimination with respect to depth is obtained by sequential etching at a carefully controlled rate. Yields as a function of depth have been determined by observing the activation N

-


489

of stacks of foils of 25 pm each [104]. The mathematical description of charged particle activation is somewhat complex due to the concurrent effects of decreasing energy and changing concentrations with penetration depth. For a homogeneous material the reactivity (flux x cross-section)decreases about exponentially until the particle energy reaches the threshold of the (endothermic) reaction. This effort can be suppressed by using high initial energies (- 20 MeV) on thin targets ( 5 200 pm) [ 1201. The influence of the incident particle energy is determined by activating ‘infinity thick’ aliquots in which all particles are absorbed. Standardization of charged particle activation is based either on knowledge of the particle range as a function of the (major) composition or on the excitation curve, the relation between cross-section and energy [ 1151. If the surface layer features a markedly different composition form the bulk this implies an iterative calculation procedure. Application of depth profiling by charged particle activation analysis are met in oxygen distribution and diffusion studies [120,150]. Depth resolution is 5-10 pm. N

12.35 Proton induced X-ray analysis (PIXE) and proton induced 7-ray spectrometry (PICE)[72,73,82,84,90,91,151-166,166-1721 Probably the most popular ion-beam method is PIXE. It is applied in the determination of lateral spatial distributions, mainly in thin samples of 5 0.5 mg.cm-2 thickness. It is often considered as a complement to XRF. Figure 12-14 gives the outlay [91] while Table 12-14 summarizes the main parameters of two facili-

Figure 12-14.Schematic outline of the experimental set-up for P I E measurements at Eindhoven Technical University. After Ref. [91].

ties [91,168]. Table 12-15 and Fig. 12-15illustrate the difference in X-ray spectrum between P E E and XRF [1721. The lateral resolution is effectuated by mounting the aliquot on an X-Ytable [84]. The sensitivity of PIXE is a marked function of Z, with an optimum around Z = 30. Figure 12-16presents a PIXE X-ray spectrum obtained for human tooth enamel with a 3 MeV proton beam of 1 mm diameter [82]. With

H.A. DAS

490

PRIMARY BEAM (protons

or photons)

-

CHARACTERISTLC Background:

-

x-rays

-*

SIGNAL

Bremsstrahlung, scattered photons or y -rays

ELECTRONIC PULSE HANDLING SYSTEM

t

1

Cwnpton scattering

I

PARTICLE EXCITED SPECTRUM

PHOTON EXCITED SPECTRUM

Figure 12-15. A pictorial comparison between PIXE and XRF. After Ref. [162].

microbeams of 5 25 pm, inclusions at the surface of minerals can be scanned [ 1521. The simultaneous use of proton induced (prompt) y-rays (PIGE) is based on the reactions given in Table 12-16 [73]. Obviously, only lateral spatial distributions can be obtained.

12.3.6 Depth profiling by radiotracer methods Radiotracer measurements combine high sensitivity and specificity with poor spatial resolution. This implies either on-line counting with strong collimation or off-line measurement on many subsequent aliquots. The first approach reduces the sensitivity while the second is cumbersome, When abrasion of the radioactive aliquot is feasible, sampling may be done stepwise, using a specially designed polishing device, or continuously on a ribbon of abrasive paper. Examples of both procedures are considered below. 12.3.6.1 Stepwise sampling [ 1731 The diffusion of phosphorus in oligocrystalline silicon is measured by way of 32P(t,,z = 14.5 d; E, = 1.7 MeV). To this end, the radiotracer is diffused into the sample. A droplet of H,PO, containing 1 mCi 32P is put on the (previously polished) surface; this is followed by diffusion and annealing. Thin layers are

-

49 1


Table 12-14. Comparison of the PIXE facilities at the Central Bureau for Nuclear Measurements (CRNM), Gel. Belgium, and Eindhoven Technical University (ETU) for the analysis of aidust CNBM

ETU

~

-200

sample mass,pcm-2

Membrane 0.45 pm

Filter Proton energy, MeV

2

Beam cross-section. mm2

0.75

100-3000

Suprathin foil 1.4

30

Beam current, nA

2-3

20

Current density, nA.mm-2

3 4

Angle of incidence

90"

0.7 60"

Energy loss in the sample, KeV

30

20-500

Irradiation time, min

25

5-6

4

6-7

Total dose, pC Pressure in sample chamber, Pa Beam sweeper? Increase in temperam=. C O

Filter in front of detector Dosimetry

10-3

no N

100 -

80 pgcm-2 Au foil monitor in primary beam and surface banier detector

0.2

Yes -50 12.5 rngcrn-' Be Bremsstrahlung specuum after absorption correction

Angle between primary and secondary beam

157.5"

90"

Detector

Si (Li)

Si (Li)

polished off by a precision polishing equipment with a slowly rotating copperdisk [ 1741. This permits the stepwise removal and collection of layers down to 0.5 pm thickness. The radiotracer is then measured in two complementary ways. The aliquot is counted before and after polishing with a solid anthracene scintillator to find the amount removed. Concurrently the abrased silicon on the copper disk is collected and its Cerenkov-light emission is determined. Quenching is corrected for by internal standardization.

Not possible by microbeam N

Can use y source instead of x-ray tube Relatively simple technology No coating needed

Microbeam can be produced 10-~-10-~

Can use a source instead of Accelerator Relatively complex technology Require coating with conducting

Microbeam?

Lowest detectable concentration (fractional)

For portability

Degree of complexity:

Non-conducting specimens? Necessitate bringing proton beam into air.

film Volatile specimens - no problems

Spatial scanning not possible

Spatial scanning possible

Scanning

Volatile specimens?

Thicker samples needed (often 2 mm or more).

Samples need by only 10-15 mg thick

Thickness N

Gram quantities may be required

Only mg required

Specimen quantity

Table 12-15. A comparison between PIXE and XRF. After Ref. [ 1621


493

Figure 12-16. PIXE spectrum of human enamel on the distal contact area of a central incisor. Kapton absorber thickness 0.76 mm. After Ref. [82].

12.3.6.2 Continuous sampling to abrasive paper [ 1751 This procedure was applied to graphite AAS furnaces loaded with radiotracers and subjected to various temperature regimes. A pressure rod with 0.5-3 kg load keeps the specimen on the abrasive paper which is pulled at a constant velocity of 1 cm-s-’. The spatial resolution depends on the quality of the paper, the load and the pulling speed. It reaches down to N 0.5 pm.

-

12.4 Phase speciation and the use of radioanalysis 12.4.1 Survey

Speciationof trace elements by separation of coexisting phases and their analysis is often met in environmental and geoprospecting investigations. Examples are the study of the concentration pattern around the contact plane of two mineral concretions,the dismbution of trace elements over the constituents of a sediment or solid waste and that in aqueous systems, thus between water, colloids, particulate

H.A. DAS

494

Table 12-16. Nuclear reactions, y r a y energies and detection limits of the PIGE technique for a collected charge of 60 pC. After Ref. [73] Element

Reaction

Li

7 ~(p,i pr)'~i

478

B

loB @, cyp)'Be "B (P,p-y)"B

429 2125

F

I9F(P, PY)'4:

110

0.0 1

Na

23Na @, py)23Na

440

0.03

0.015 0.5 0.6

1.o 0.2

0.1

Si P

31P @, py)3lP

1779

0.25

1268

0.05

matter and sediment. Radioanalysis is often used in this respect. Radiotracers can yield information on distribution and availability under equilibrium conditions or on the kinetics of reaching that equilibrium state. Activation analysis is applied in leaching studies, either by using an activated aliquot or in analyzing leaching effluents. Within a system in apparent equilibrium, elements often Occur in different physical phases. Eventually these phases can be intermixed to a high degree of homogeneity. The interest of the environmental or prospecting scientist is usually focussed on the availability of some elements as a result of a natural or man-induced leaching process. Examples are met in hydrogeochemistry, lixiviation procedures in the mineral industry and standardised environmental leaching tests. Current literature abounds with more or less well defined procedures for physical speciation. They refer to trace elements in soils and sediments [176-1851, particulate matter and colloids in natural water [186-1981 and availability of trace metals from granular solid wastes [ 199-2011. Eventually this pertains to the Occurrence of radionuclides from the natural series or nuclear bomb tests and waste reprocessing [202-210]. Quoted references give some examples only. The quest for well-defined and generally applicable speciation schemes has been successful in a few cases only. The separation of colloids from fresh water samples by the hollow fiber technique [191,196] is an example. The standard leaching test on coal fly ash is another [199] although a recent survey of various standardized


495

on coal fly ash is another 11991 although a recent survey of various standardized procedures for this material revealed still discrepancies [211]. In this section, the four most frequent physical speciation procedures and their application of radioanalysis are summarized: (a) Sampling and separation of sea- and surface water and the determination of trace elements in the isolated fraction; (b) Determination of exchangeable phosphate in fresh water sediments; (c) Leaching tests on granular solids by means of radiotracers and a previously radioactivated aliquot; (d) Measurement of the in sitir diffusion coefficient and distribution constant in (partly) wetted soils and granular wastes. These topics adequately reflect the importance of radioanalysis in physical speciation. Whereas numbers one and three refer to the measurement of net mass transport by radiotracers and to activation analysis, applications two and three emphasize the use of isotopic exchange at equilibrium conditions. 12.42 Sanipling and separation of sea- and surface water arid the deterniiiiutioii of truce elements in the isolated fractiotis

12.4.2.1 Sampling Water samples are collected by samplers which are shut at the desired depth or by pumping. Ideally, the sample should then be separated into a soluble fraction (particle diameter > pm), a colloidal fraction (particle diameter 10-3-10-’ pm) and a particulate fraction (0.1-50 pm) [212]. Often the soluble and colloidal fractions are taken together and the distinction is between “clear water” and a “suspended particulate matter” fraction [186]. The sampler may interfere by adsorption or contamination by erosion, pump and bucket behaving somewhat differently. Separation between “water” and “suspended matter” should be effected speedily to avoid changes in the trace element distribution. It can be effected by: 0

Gravitational settling or sedimentation.

0

Filtration.

0

Centrifugation.

H.A. DAS

496

12.4.2.2 Gravitational settling Sedimentation is based on the size and the density of the particles. When a sedimentation tank of one meter height is used, it takes 10 days to have particles with a diameter of more than 1 pm removed completely [213]. Consequently, sedimentation is used almost exclusively in model experiments on transport phenomena and hardly for the preparation of analytical samples.

-

12.4.2.3 Filtration Membrane filters with a definite pore-width are applied widely. A pore diameter of 0.45 pm is most common 1186,2141, although occasionally other values are used 11841. Eventually the filter makes part of a standard equipment. For the filtration of sea water a 10 litre vessel of fibre glass, an overpressure of 5 5 atm was used [186]. At a pressure drop of 0.9 atm, a specific filtration velocity of 65 ml cm-2s-1 was observed. The effective pore-width changes over the filtration procedure. Some elements are partly bound to the filter.

12.4.2.4 Centrifugation As in gravitational settling, separation of suspended matter by centrifugation is based on the size and the density of the particles. Continuous flow centrifuges of various types are available. At the ECN-laboratory, "Junior- 15000", centrifuges manufactured by Heraeus-Crist are used. The maximal attainable revolution rate is N 15000 rpm. The corresponding flow-rate is 1.2 1 (min)-'. To minimize contamination by the metal of the rotor, parts not subjected to friction are coated with teflon. Up to 300 ml of sediment can be collected. A centrifuge mounted in gimbals serves separations on board a ship 11861. Comparison between filtration and centrifugation was performed on a large surface water sample. The concentration of suspended matter was determined either by filtration through a pre-weighed 0.45 ,urn membrane filter or by centrifugation at > lo4 g and collection from the rotor. Comparativemeasurements on aliquots of one large sample gave the following results:

Filtration Centrifugation Subsequent filtration

4 3 f 2 mg.1-l 32f2 mg-1-' 4f0.5 mg-l-' 3 6 f 3 mg-l-'

(3035%organic material) (26f4%organic material)

+

Contamination by the centrifuge rotor may be serious and has to be investigated separately [186].


491

An elegant in situ procedure for trace element isolation from natural waters in equilibrium is membrane dialysis [192,196,2091. A small dialysis cell is placed in the water and left there to reach equilibrium. This takes from 24 to 72 hr [ 1961. The cut-off molecular weight of commercially available membranes is from N lo3 to 2.104Dalton. This enables the separation of colloids and humic acid complexes from free ions, small organic complexes and the usual inorganic complexes from free ions, small organic complexes and the usual inorganic complexed forms. Eventually, hollow fibres are used through which the water is pumped [220]. Obviously the behaviour of as much complexes as possible has to be checked in laboratory experimentswith selected membranes. Here radiotracers can be of great help [1921. A special case of samplingis met in rainwater. It is difficult to sample for at least three reasons: N

(a) The amounts available are often small; (b) Wet and dry deposition should be separated preferably;

(c) The concentrationsof trace constituents may be very low, i.e. /v

Y CY (12.35) + ([Hly>/(II'~,,y)[Hxl~~

In a similar way it can be shown that the extraction efficiencies Q can be written as

and

From this, it is obvious that the calculation of all qy values is possible at any pH for given values of Cy, CR,V, V,, and nAVOxprovided II'Ex,R and Ii'Ex,Y are known. The use of radiotracers to determine the conditional extraction constants was investigated in a laboratory for the case of the diethyldithiocarbamate anion, (C2H& NCS-, further denoted as DDC. Two procedures are used to determine for chloroform as the organic solvent [332]:

512

H.A.DAS

(a) Measurement of the distribution of the metal between the two phases at various pH-values and equilibrium concentrationsof HDDC in the organic phase; (b) Based on the displacement reaction. 12.5.2.2 Experimental Chemicals and equipment

A digital pH meter with glass electrode, a shaking device and a 3 x 3 NaI well-type detector connected to a 400 channel analyser and read out are used. All chemicals are of analytical purity; the radioisotopes 65Zn, 'lSmCd '03H g, mBi and 212Pbare of radiochemical purity. *l6'"In and 64Cuare obtained by irradiation of small amounts of In(NO,), and Cu-foil respectively at a thermal neutron flux of 5 x 1013C ~ - ~ . S - ~The . crystallized reagents Zn(DDC),, Pb(DDC), and CU(DDC)~ are prepared according to the procedure described by Wyttenbach et al. [333]. Analysis of these products for their metal content is carried out both by complexometrictitration after their destruction in HNO, and by radiometric titration of their organic solutions: 3

Procedure I This procedure is applied only in the determination of Twenty ml of a solution of Zn(DDC), in chloroform are shaken with 50 ml of an aqueous phase, containing no metals able to react with DDC. In several experiments the pH was adjusted at different values. About 5 min is ample time for equilibrium to be reached. After separation of the phases, the distribution of Zn is determined radiometrically, measuring the activity of Zn in 5 ml aliquots taken from both the aqueous and the organic phase. The determination of [HDDC], is carried out by shaking 10 ml of the organic extract with 25 ml of an aqueous solution of Cu(I1). From Ref. [333] it is known that HDDC as well as Zn(DDC), will react quantitatively with a suprastoichiometricamount of Cu(I1). So, from the distribution ratio of Cu, determined radiometrically too, [HDDCJ,, present after the previous reaction with Zn(DDC), can be deduced. Then the extraction constant Kka is calculated according to Eqn. (12.28). Procedure XI

An aqueous solution of Y(y') was shaken for at most 30 min with 25 ml of a solution of R(DDC), in chloroform which does not contain any free HDDC. In equilibrium state the distribution ratio(s) of one or both metals were determined radiometrically. Knowing either the value of KEx8or Kh,y (one of them determined previously) the unknown extraction constant I(k,y or I(,,, was calculated from Eqn. (12.27).

SPECIATIONOF TRACE ELEMENTS

513

With a masking agent present in the aqueous phase, Eqn. (12.38) has to be used:

due to the formation of the non-extractable complexes M L , , M L , . .. ML, with the masking anion L-. 12.5.2.3 Results The experimental conditionsas well as the K h values ~ are recorded in Table 1220. For Cu, Pb, Cd and Zn, the present results are in good agreement with other data from the literature.

125.3 Trace element speciation in human serum 12.5.3.1 Survey With improving limits of determinationsof trace elements, their role in biochem-

istry has come within range [335-3401. Based on this work, the relative distribution of 15 elements can be tabulated in terms of the following three parameters: (a) The total blood volume elemental content. Data are obtained by multiplying whole blood elemental concentrations [343]with the total blood volume; (b) The amount of element in serum of 1 ml blood related to the elemental content per ml blood, based on a haematocrit value of 0.44 [343];

(c) The amount of element bound to serum or plasma-proteins to the serum concentration, as reported by various authors. Results are given in Table 12-21 [385]. It is seen that Na, C1, Cu, I and V are present in serum primarily while K, Zn, Fe, Rb, Mn, Co and Cs mainly occur intracellularly. The elements Mg, Se, Sb and Br are about equally divided. The information on the function of elemental associations with proteins is still scarce. Albumin seems to be the usual carrier of Cu and Zn [360]. The elements are loosely bound and easily transferred to binding sites in various tissues. The situation is less clear for other elements, notably Fe and I.

514

H.A. DAS

determined mble 12-20. Values of conditional extraction constants of metal diethyldithiacarbamakxcarbamates. for the system CHCI~HZO. Composition of

Metal ion aqumus phase

Zn(I1)

V,, = 50 ml 5 spike ~ (1) pH=2.97 (2) pH4.01

a

log I ~ E ~ , M

organic phase

E N

V, = 20 ml Z~(DDC ~ x 10-4~ = 2.5

r3311

literature Ref. W31

After the first exrractions, second ones are 2.3f0.02

2.6

carried out to determine HDDC. 2.39i0.02 V,=ZSml

acu

pH=3.76 Cu2+=1.78~10-~M C d g Vq=50ml llSrnCdspike pH=3.67 Cd2+= 1 . 7 8 ~ 10-5M WII) V., = 50 ml 212Pbspike pH=2.97 PP+= 1 . 0 ~ 1 0 - ~ In(II1) V,, = 50 ml 1 1 6 a n h

Vml = 10 ml conmns extracts of the former exrractions V,. = 25 ml Pb(DDC)2 = 2.0 x

M

pH=2.97 (1) Pb” = lO-’M (2) pd“ = 2x 10-zM Bi(I1I) Vap = 50 ml 207Bispike pH=l28 Bi3+= 4 . 9 6 ~10-7M

4.4

5.6

5.77f0.05

V, = 25 rnl Zn(DDC)Z = 1.0~1O-~M

7.0f0.1

6.8 7.94f0.09

~

V, = 25 ml W(DDC)~= 2 . 0 ~1 0 - 4 ~

pH=3.335 (1) In3+= 1.49~10-’M (2) In3+=1.49xlO-’M

WZ‘=5.44~10-~M Cu(II) V,, = 50 ml

5.50f0.05

9.95f0.08

V, = 25 ml aCu in Cu(DDC)z CU(DDC)~ = 9.81 x 10-6M

12.81f0.07

13.2

V, = 25 ml = 4.29 x 1 0 - 4 ~ CU(DDC)~

15.7f0.2

-

V, = 25 ml Hg(II) V,, = 50 ml z03Hgspike (1) Cu(DDCh =4.29xlO-’M pH=2.76 (2) CU(DDC)~ = 1.72x lo-’ M Hgz+= 4 . 4 9 ~ l O - ~ M Cu2 = 2 . 7 8 ~ 10-5M C1- = 8 . 9 7 ~ lO-’M

26.92M.05

515


Table 12-21.Distributionof elements in blood and blood fractions Element

Ref.

Total blood

Ratio whole serum/

volume amount

whole blood

0.5

92

15.4

71

8.6

7

0.2

44

0.04

9

5-4

66

Protein-bound amount None None None

< 10%

0.3

65

0.9

40

0.01

4

2.5

46

0.05

4

0.3

70

0.024

30

0.0.15

17

Complete Complete Complete ca. 90% 2 80% None ca. 2% Complete Not known Not known Not known

2.1

32

Complete

2.4

0.1

12.5.3.2 Elements

Na,C1, K,Mg The great majority of Na is in the extracellular fluid. In serum, it is found in ionic form exclusively. There is no evidence for any protein-bound Na [342,347]. Chlorine is known to be in ionic form in serum. Negligible amounts may be proteinbound [350]. Potassium is concentrated intracellularly. A minority of blood-K of 7%is found in the serum in ionic form. Both K and Mg are eluted in the albumin fraction during chromatography on DEAE-Sepharose C1-6B [348]; Mg may occur protein-bound in serum [342,349,350].

-

Zn About 80% of whole blood Zn occurs in the red cells, 17% in plasma and 3% in leukocytes [337]. The binding proteins are q-macroglobulin (3040%) and albumine (60-70%) [337,351,352]. By incubating serum, single protein solutions

H.A. DAS

516

and amino acid solutions with 65Zntheir relative binding capacities and competition for Zn were observed [354]. By starch block electrophoresis, human serum proteins were fractionated; extraction and analysis by AAS revealed Zn in the a,-macroglobulin and albumine fractions [349].

cu Most of the blood-Cu is present in serum where > 90%is bound to caeruloplasmin at 7 atoms per molecule. The remaining fraction is either loosely bound to albumin or to ultrafilterable amino acids. Gelfiltration on sephadex G-100 and subsequent AAS of the eluate fractions yields (93f3)%bound to caeruloplasmin and the rest to albumin [356]. Deficiencies in protein bound Cu are observed in Wilson’s and Menke’s disease. The determination of Cu in isolated serum protein fractions by gel electrophoresisand AAS implies rigorous blank precautions and determinations 1349,3621. Fe

About 60-70% of all Fe is bound to haemoglobin [337]. The Fe content of serum is that of the erythrocytes serum-Fe bound to transferrin at 2 atoms per molecule [337,363]. Less than 1% is bound to transferrin; the distribution over the two proteins varies [337,342,363,364]. The distribution may be studied by adding 59Feto plasma and performing gel filtration [355,356,365]. I

-

About two-thirdof the I is in the serum of which a few percent are inorganic. The majority is protein bound thyroxine while 10%is present as iodinated compounds, di-iodo and tri-iodo thyronine and thyroxine. Usual analytical techniques are ion exchange and NAA [342,366-3681. Se

About 4 0 4 5 % of all blood Se occurs in the serum [370], bound in S-containing amino acids and proteins [337,371,378]. The kinetics of Se-uptake has been studied with 75SeO;1injected intravenously [372,373]. Gel filtration on sephadex G-200 is used to fractionate proteins, followed by hydride generation AAS [370] or NAA [350]. The combination of ultracentrifugation and NAA has been used too [376], as well as electrophoresis[377]. Rb

Most Rb is found in erythrocytes,like K [375,379]. Combination of gel filtration on sephadex (3-25 and NAA does not reveal any Rb [347,350].


517

Br

No or less than 5% is protein bound [347,375]. Small amounts have been observed by gel filtration and NAA [350] or protein precipitation and NAA [380].

Mn The element occurs in serum, bound to /3-globulines [337]. This has been confirmed by 54Mnradiotracer experiments. The usual analytical procedure is gel filtration and NAA [350,38 13. V

Most V is found in serum, probably transfemn-bound [337,382,383]. Gel filtration andNAA are applied. Radiotracer experimentswith 48Vallow the measurement of the uptake rate.

Sb Trivalent Sb is mainly found in the red cells [337]. Serum gel filtration on Sephadex G-25points to some protein-bound Sb. Both NAA and '24Sb-radiotracer have been used.

cs The radiotracer '"Cs has been applied to detect the very small fraction of Cs that is found protein-bound after gel filtration on Sephadex-25 [347].

co The Co-containing vitamin B-12 is mainly found in red blood cells. In serum it is bound to trmscobalamin [384].

12.5.3.3 Radiotracer experiments in trace element speciation in proteins Separationof serum proteins is mostlyperformedby gel filtrationor electrophoresis. The cross-contamination by, originally inorganic impurities as well as the interaction of the metal-bearing proteins with the gel can be studied by radiotracer experiments. Three types of sample solutions are used: (a) Serum spiked with 50 pl aliquots of carrier-free radionuclides; (b) A 0.9% NaCl solution spiked in the same way; (c) Buffer eluent with carrier and spike. Recovery in the eluate fractions is measured as a function of contact time. Table 12-22presents the results for Na, Zn and Se obtained at ECN [385].

H.A. DAS

518

Table 12-22.Recoveries in percentages after gel filtrations of Zn,Se and Na in serum, 0.9% NaCl and carrier solution. Total bed volume = 8 ml Biogel P-2. How rate is 14 cm.h-'. Numbers in parentheses are incubation times before desalting. Protein is collected in fractions 3 + 4,salt in fractions 6 + 7. Element

Sample

Concentration of element

Na

Carrier solution 0.9% NaCl Serum (0.6hr) Serum (2.5hr) Serum (20hr) Carrier solution 0.9% NaCl Serum (0.6hr) Serum (2.5hr) Serum (20hr) Carrier solution 0.9% NaCl Serum (0.6hr) Serum (2.5hr) Serum (20hr)

50ppm 0.36% 0.32% 0.32% 0.32% 20 ppm

-

-

-

1PPm 1 PPm 1 PPm

71 68 68

-10PPm

-

zn

Se

0.1 ppm 0.1 ppm 0.1ppm

Recovery in protein fraction

-

0.5

Recovery in salt fraction

101 100 101 94 103 41.5 1.2 2 3 3 105

Total recovery

101 100 101 94 103 42 1

73 71 71 105

84

84

100 99 104

100 99 104

125.4 A case in point: Arsenic speciation in aqueous samples by selective As(III)l As(V) preconcentration and hydride evaporation AAS 12.5.4.1 Survey

Arsenic undergoes extensive chemical and biochemical transformations in the aquatic environment as reviewed by Lemmo and Faust [386]. Speciation, i.e. qualitativeand quantitativedetermination of specific chemical forms is necessary as environmental hazards vary widely with molecularform. Thereforedetermination of the total arsenic concentration is no longer satisfactory, but differentiation between arsenic species is required. The prevalent dissolved arsenic species are arsenite [As(III)], arsenate [As(V)], monomethylarsonicacid (MMAA) and dimethylarsenic acid (DMAA). Before aquatic samples are speciated some manipulation is carried out. Usually samples are filtrated to remove suspended matter; this is done with a 0.45 ,urn membrane filter. For the preservation of the samples acidification (to prevent the loss of methylated arsenicals) and frozen storage (to slow down the oxidation of As(II1) to As(V)) is recommended [387]. In the acidified samples free from suspended matter the soluble species are further speciated. Characterizationof dissolved arsenic species in the aquatic environment has been perfomed by different analytical techniques:


519

1. Determination of inorganic arsenic (arsenite and arsenate) by solvent extrac-

tion [389-3941. Only arsenite can be extracted from acidic media in the presence of a complexingagent (pyrrolidinedithiocarbamate,sec-butyldithiophosphate). Arsenate can be extracted after preliminary reduction to the trivalent state. When the extraction is done once with and once without addition of reducing agent (KI, thiosulphate, hydrogen sulphitehhiosulphate),the As(II1) and As(V) contents can be differentiated. To mask interferences at extraction, generally EDTA is used. Either direct analysis of the organic extract (GFAAS) or indirect analysis after mineralization (HGAAS) or back-extraction (GFAAS, NAA) has been performed; 2, Column preconcentration techniques for the preconcentration of single arsenic

species. The eluted species have been subjected to off-line detection by different detectors. Persson and Irgum [395] preconcentrated dimethylarsinate on a strong cation-exchangeresin and and detection was performed by GFAAS. Howard [396] used a mercapto-modified silicagel for the selective preconcentration of As(II1) and detected As(II1) by HGAAS. Terada et al. [397] preconcentrated As(1II) and As(V) - after education - on a thionalide-loaded silicagel; the arsenic was determined by silver diethyldithiocarbamatespectrophotometry. Yu and Liu [398] used a “thiol cotton” for preconcentration of As(II1) and As(V) - the latter after reduction to As(II1) - followed by HGAAS; 3. Conventional ion-exchange chromatography for the separation of several arsenic species. Faust et al. [399] speciated heavily contaminated watersheds by using a cation-exchange column and were able to differentiate between inorganic arsenic, MMAA and DMAA; not all peaks could be identified. Detection of arsenic in the eluted fractions was by flameless AAS. Off-line detection of arsenic in eluted fractions was also used in anion-exchange chromatography: Ficklin [400] separated As(I1I) and As(V) and detected both species by GFAAS; Aggett and Kadwani [401] used a two-stage anionexchange method for the speciation of As(III), As(V), MMAA and DMAA followed by HGAAS; 4. More sophisticatedchromatographic techniques like HPLC for improved resolution of arsenic species were recently reviewed by Irgolic [402]. HPLC is used in different modes like ion-pairing chromatography or ion-exchange chromatography with either isocratic or gradient elution, while even column switching techniques have been used. For on-line detection the HPLC separation device is interfaced with an element-specific detector (GFAAS, ICAP, HGAAS). Several examples of speciation of arsenic in aqueous samples have been reported. Tye et al. [403] and Haswell et al. [404] speciated As(III),

520

H.A. DAS

As(V), MMAA and DMAA in soil-pore waters and commercial bottle waters by ion-exchange HPLC with isocratic elution and detection by HGAAS. Bushee et al. [405] employed ion-pairing HPLC with isocratic elution and detection by HG-ICAP for speciation of As(III), As(V) and DMAA in drinking water supplies. Fish [406] demonstrated the speciation of As(III),As(V), MMAA, DMAA and PAA (phenylarsonic acid) in process water samples by anion-exchange HPLC with gradient elution and detection by GFAAS. Complete identification of all arsenic species in these process water samples was impossible; only fingerprintsfor different water samples were register&, 5. pH-selective hydride generation is a frequently used technique to differentiate between As(II1) and As(V). At pH 5 only As(II1) is converted to ASH, (arsine) and at pH 1 both As(1II) and As(V) are converted As(V) can be determined by difference. Yamamoto et al. [407]determined As(III) and As(V) in sea water by using a gas sampling vessel for collection of arsine; detection was by AAS with a nitrogen-entrained air-hydrogen flame; 6. Extension of the pH-selective hydride generationmethod can be accomplished by connecting the HG device to a cold trap in which not only ASH, is collected

but also monomethylarsine (MMA) and dimethylarsine (DMA), generated at pH 1 from MMAA and DMAA respectively. Selective heating of the trap will result in sequential volatilization of the arsines. Braman and Foreback [414] originally introduced this technique. On-line detection is with various detectors like AAS, ECD and FID [408]; 7. Several gas chromatographic methods used on the conversion of the arsenicals into volatile compounds have been reported. Daughtrey et al. [409] presented a gas chromatographic method for total inorganic arsenic, MMAA and DMAA after preparation of their diethyldithiocarbamates. Fukui et al. [410] described the gas chromatographic separation of As(III), As(V) and MMAA as their 2,3-dimercaptopropanol (BAL) complexes. Dix et al. [411] used capillary gas-liquid chromatographyfor separation of arsenicals as their methylthioglycolate derivatives. Andreae [a81 used hydride generation for derivatization of the arsenicals and trapped the arsines in a cold trap; rapid heating of the trap resulted in instantaneous volatilization of the arsines and introductionof a plug flow into the gas chromatographfor further separation.

In evaluating the described techniques, one learns that solvent extraction procedures, column preconcentration techniques and pH-selection hydride generation methods lack specificity. Column chromatography,HPLC and most gas chromatographic methods add to the specificitybut are not suitable for the speciationof arsenic in natural waters because they are not sensitive enough as a result of introduction of


521

small sample volumes onto the colunn. For this reason only contaminated waters with a relatively high arsenic concentration can be speciated in this way. Chromatographic methods interfaced with an effective preconcentration are therefore the obvious choice. The usually applied technique is selective reduction by NaBH, followed by cold tapping in a U-tube at liquid nitrogen temperature and selective volatilization of the arsines. This approach has the advantage of simplicity and cheapness: an expensive gas chromatograph is unnecessary as only a few species with widely different boiling points have to be separated and therefore a simple separation device is satisfactory. Some of the availablespeciation data for arsenic in natural waters are summarized in Table 12-23. Arsenate is the thermodynamically stable form of arsenic under aerobic conditions: the non-equilibrium reflects the biological activity in natural waters. The biogeochemical cycle of arsenic in the surface ocean involves the uptake of arsenateby plankton in the biologicallyactive surfacelayer, the conversion of arsenate to a number of as yet unidentified organic compounds and the release of arsenite and methylated species into the seawater. Biological demethylation of the methylarsenicals and the oxidation of arsenite serve to generate arsenate. The concentrations of the arsenic species are then controlled primarily by the relative rates of biologically mediated reactions, superimposed on processes of physical transport and mixing [387]. Complete speciation of arsenic in the aquatic environment is not achievable as even simple operations may disturb existing equilibria. Sb (111) and Sb (V) are known to form complexes with high-molecularweight, multidentate ligands such as humic or fulvic acids which are dissociated below pH 4 [430].From ultrafiltration studies [388]it is obvious that a substantialfraction of the dissolved species consists of high-molecular weight arsenic species, a related behaviour can be assumed and so acidification has to be omitted at determinationof these species. A derivatization technique using NaBH, will only enhance speciation alterations, e.g. the species determined as As(V) could be present as a labile organic complex of arsenate as well as free hydrogen-arsenate ion. Large sample volumes might be beneficial in the acquisitionof real evidencefor the existence of these complexes and also for the occurrence of volatile arsenic species as only negative results have been reported at measuring these species [387,415].A speciation scheme using these large sample volumes is proposed in the next section.

12.5.4.2 Speciation scheme The best possible speciation of arsenic in natural waters can be achieved by a combination of techniques. In the described speciation scheme (Fig. 12-23) a differentiation between several classes of arsenic species can be made; further separation of each class leads to the utmost speciation.

522

H.A. DAS

Table 12-23.Occurrence of arsenic in natural waters Locality and sample type

As(II1)

Conc. species* (in ng As ml-') h(1V) MMAA

DMAA

< 0.0.2 < 0.02

fresh water 0.25 < 0.02 0.16 0.06

W141 0.30

< 0.02

0.27

0.11

0.20

0.02

0.32

0.12

0.62

0.96 2.41 0.49

Beaulieu River, Hampshire Restronguet Creek, Cornwall

0.79 2.74 0.89 0.10 1.6

0.05 0.11 0.22 0.06 < 0.2

0.15 0.32 0.15 0.23

Sacramento River, Red Bluff, CA Owens River, Bishop, CA Colorado River. Parker. AZ Slough near Topock, CA

0.040 0.085 0.114 0.085

Bay, Causeway Tidal flat McKay Bay

0.12 0.62 0.06

HillsboroughRiver Withlacoochee River Well water near Withlacoochee Rivr Remote Pond, Withlacoochee Forest University Research Pont, USF Lake Echols. Tampa Lake Magdalene. Tampa

Seawater, San Diego Trough Surface 25 m below surface 50 m below surface 75 M below surface 100m below surface Seawater, Snipps Pier, La Jolla, CA 5 Nov. 1976

11 Nov. 1976

1.02

19.4

1.08 0.021 42.5 0.062 1.95 0.063 2.25 0.13 saline water 1.45 < 0.02 1.29 0.08 0.07 0.35

Ref.

< 0.004 0.22 0.051 0.31 0.20 0.29 1.oo

0.017 0.016 0.016 0.021 0.060

1.49 1.32 1.67 1.52 1.59

0.005 0.003 0.003 0.003

0.002 0.002

0.019 0.034

1.75 1.70

0.017 0.019

0.12 0.12

* MMAA = monomethylarsonicacid (CH3)AsO(OH)z DMAA = dimethylminic acid (CH3)2AsO(OH)

0.004

0.21 0.14 0.004


523

Figure 12-23. Speciation scheme

In the following part the individual sections of the speciation scheme are discussed in &tail. Centrifugation

Removal of suspended matter from a large sample volume (100 1) is preferably performed by means of a flow-through centrifuge [416] with the equivalent action of a 0.45 pm membrane filter). The total amount of arsenic in suspended matter can be determined by INAA. Speciation of particulate arsenic has not been performed yet. However, a modification of the technique as described by Mukai and Ambe [417] for the determination of methylarsenic compounds in airborne particulate matter might be suited to this purpose. Gas stripping

Stripping of naturally occumng arsines from a large sample volume (100 1) means practically that a streaming sample has to be purged continuously; this can be achieved with a purge unit in which sample streams in and out are in the mean time constantly being purged. Cold trapping of the volatile species is not recommended, as much water vapour is generated during purging of a large sample, resulting in clogging of the trap. A water-removing pretrap or a drying tube will probably not

524

H.A. DAS

be sufficient[418]. Therefore preconcentration at ambient temperature is preferred. An example is reported [419] for absorption of trimethylarsine, dimethylarsine, methylarsine and arsine onto clean silver-coated glass beads. After desorption by a warm NaOH solution, speciation of the oxidized forms can take place. Another example is the preconcentration of methylarsine and dimethylarsine on Tenax as described by Paudyn and Van Loon [421]; they determined these compounds in air by heating the Tenax column to 100'C followed by the collection of these compounds in benzene and measurement by GCAAS. Solvent extraction Determination of organically-associated trace metals in estuarine sea-water by solvent extraction with chloroform and AAS has been performed by Hayase et al. [429]. Solvent extraction of organic arsenic complexes can be performed by means of a continuous extraction. Proper operation of the continuous extraction system (Fig. 12-24)as designed by Hillebrand and Nolting [420]is achieved by using the counter-current principle and the differencein specificweight of two immiscible solvents. In a continuous cycle freshly distilled solvent is added to the system. So with a limited amount of solvent (500 ml) an almost infinite amount of another solvent can be extracted with an extraction rate of about 300 ml min-'. Extraction of organically-associated arsenic with different solvents will yield information about the extractability of naturally occurring organic arsenic complexes. The prevalent arsenic species As(III),As(V), MMAA and DMAA are not extracted [421] but need speciation further on in the speciation scheme. An estimation of the total amount of extracted organic arsenic complexes can be found by complete evaporation of the solvent followed by INAA. If further speciation is desired, separation by HPLC can be considered. Mackey [422] fractionated organic complexes of magnesium, iron, zinc and copper by reverse phase HPLC with gradient elution and on-line atomic fluorescence detection. A related method might be used for the separation of extracted organic arsenic complexes. However, it must be emphasized that even successful separationwill only yield fingerprintsof samples;further characterization is impossible so far. Selective volatilization [431] Speciation of arsenite, arsenate, MMAA and DMAA is usually performed by pH-selective hydride generation followed by cold trapping and selective volatilization. A fraction (100 ml) of the large sample volume is sufficient to reliably determine these species. Many authors have used this technique or a modification of it. In spite of its popularity we have to face the limitations. Quantitative or reproducible hydride generation is necessary whereas complete atomization of the different arsines prior to AAS will result in the same calibration curves. The most

525


TECHNICAL S P E C I F SCAT IONS

"

ISABELLE"

(Lenr-Labor-Instruments)

A

Mixing and senling chamber: 0 60 mm length 1500 mm. 0 ring sealed flens connections

B

Solvent overflow, glass

C

Roundbonemed fbsk. Mo ml. glass

D

Electric heater, 220 volt. 280 watt, manual control

E

Level tube. glass, coune re. gulation of water-solvent interface

F

Waste drain.

G

Magnetic stirrer. adjustable soced control. 220 d t . 18 watt

H

c

PTFE tubing

RVS electrodes. fine regulation of solventwater interface. bared on differenceof

conducuviw. 220 volt.

I

Solvent pum. 220 volt All wetted Parts made of PTFE Adjustable sDeed and pumvnte

J

and :.J

Electric solenold Val. volt, all wetted pans made of PTFE. caoaclty 8.l.h' I t 23 PSI

yes.

a

K

Flow meter. flow contmf &

L

Condensor. glass

needle valve. glass float

M N

Solvent inlet

0

Mixer axe

P

Stirrer

RVS mixer axe auidance

C

Figure 12-24. System for continuous solvent extraction; configuration in which the solvent has a lower specific weight than the sample

H.A. DAS

526

HYDRIDE GENERATION

GAS STRIPPING

DRYING

COLD TRAPPING AND DETECTION SELECTIVE VOIATILIZATION

PERM4 PURE DRYER

HCI

DRY Nq

NaBH, SOLUTION

SAMPLE SOLUTION

Figure 12-25. System for speciation of methyiatedarsenic species with AAS detection [433].

I

As (v)

-

Time (min)

Figure 12-26. Differentiation of a mixture of AS (V), MMAA and DMAA using cold trapping and selective rotation of the 40 ng level [433]


527

severe problems are encountered in the hydride generation step: (a) Incomplete conversion of arsenic species to corresponding arsines caused by the presence of trace elements which interfere in the reduction step is one of the major drawbacks. Problems can be overcome by removing interferences or by masking interferences. Howard [423] described their removal by using either a Chelex-100 column or a dithizone-extraction;masking of interferences was performed by addition of EDTA prior to hydride generation. (b) Insufficient selectivity during the the reduction step is another problem; at pH 5 not only As(II1) is reduced but MMAA and DMAA to a certain degree - dependent on kinetic factors and complex formation, as well [424]. Calibration problems due to this insufficient selectivity at reduction were reported by Hinners [425]. (c) Talmi and Bostick [426] were the first to observe molecular rearrangements during hydride generation; originating from one methyl arsenic compound several arsines were generated. (d) Insufficient degassing of generated arsines from solution is not a problem in the normally used batch hydride generation vessels as purging of the solution with an inert gas is one of the standard procedures here. As the final research goal is to design a computer controlled automatedsystem, a continuous flow hydride generation system as, e.g. described by Arbab-Zavar and Howard [427] is desirable. However, additional purging and therefore redesigning the conventional G-L separators 14281 for continuous purging might be necessary as hydrogen as the only means of degassing arsines from solution is probably not feasible at pHs approaching neutrality. Computer controlled manipulation means that the U-tube normally used for preconcentration and separation also has to be adapted to automatic operation. The apparatus, used at ECN,is schematized in Fig. 12-25. A speciation chromatogram is given in Fig. 12-26.

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400 Ficklin. W.H.,Talanta, 30 (1983) 371. 401 Aggett, J. and Kadwani, R.. Analyst, 108 (1983) 1495.

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CHAPTER 13

Trace elements in environmental and health sciences

G.V. IYENGAR’ and V.IYENGAR2 Iiiteriiational Huniaii Nutritioii Project NIST. Gaithersburg.MD 20877. USA 2Georgerowir University Medical Center. Department of Pathology. Washirigton DC 20007 USA

.

.

Contents 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544 13.2 Need for trace element analysis of biomaterials . . . . . . . . . . . . . . . . . . . . . 545 13.2.1 BTER a multi-disciplinaryscience . . . . . . . . . . . . . . . . . . . . . . 546 Trace element speciationand bioavailability . . . . . . . . . . . . . . . . . 548 13.2.2 549 13.3 Biological standardization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549 13.3.1 The “Bio” sources of analytical errors . . . . . . . . . . . . . . . . . . . . 13.3.2 Presamplingfactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 550 13.4 Analytical standardization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 556 13.4.1 Analytical quality assurance . . . . . . . . . . . . . . . . . . . . . . . . . 557 559 13.4.2 Harmonization of measurements . . . . . . . . . . . . . . . . . . . . . . . 13.4.3 Trace element determinations . . . . . . . . . . . . . . . . . . . . . . . . . 559 13.4.4 Multianalytedeterminations . . . . . . . . . . . . . . . . . . . . . . . . . 559 Matrix related problems in sample treatment . . . . . . . . . . . . . . . . . 560 13.4.5 Sample preservation and storage . . . . . . . . . . . . . . . . . . . . . . . 560 13.4.6 Contaminationby trace elements . . . . . . . . . . . . . . . . . . . . . . . 562 13.4.7 13.4.8 Losses of trace elements . . . . . . . . . . . . . . . . . . . . . . . . . . . 562 563 13.5 Clinical specimens from human subjects . . . . . . . . . . . . . . . . . . . . . . . . 13.5.1 Special features of biofluids . . . . . . . . . . . . . . . . . . . . . . . . . . 563 13.5.2 Medico-legal implications . . . . . . . . . . . . . . . . . . . . . . . . . . 564 13.5.3 Sampling and preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 565 567 13.6 Environmental biomonitoring for toxicants . . . . . . . . . . . . . . . . . . . . . . . 13.6.1 Chemicals in the environment . . . . . . . . . . . . . . . . . . . . . . . . . 567 13.6.2 Bioenvironmental surveillance . . . . . . . . . . . . . . . . . . . . . . . . 570 571 13.6.3 Real time and long-term biomonitoring . . . . . . . . . . . . . . . . . . . . Human specimens for biomonitoring . . . . . . . . . . . . . . . . . . . . . 571 13.6.4

.

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13.6.5 Environmental Specimen Bank (ESB) . . . . . . . . . . . . . . . . . . . . 572 574 13.6.6 Proven applications of ESB . . . . . . . . . . . . . . . . . . . . . . . . . . 13.7 Biomineral imbalances and health effects . . . . . . . . . . . . . . . . . . . . . . . . 575 575 13.7.1 Nutritionaland metabolic factors . . . . . . . . . . . . . . . . . . . . . . . 13.7.2 Nutritional surveillance of trace elements . . . . . . . . . . . . . . . . . . . 576 13.7.3 Recommended dietary allowances (RDA) . . . . . . . . . . . . . . . . . . 576 577 13.8 Trace elements and high altitudepopulations . . . . . . . . . . . . . . . . . . . . . . 13.8.1 Iodine and selenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579 13.9 Reference values for trace elements in human specimens . . . . . . . . . . . . . . . . 580 13.9.1 Reference values vs normal values . . . . . . . . . . . . . . . . . . . . . . 580 13.9.2 Reference concentrationsin clinical specimens . . . . . . . . . . . . . . . . 580 13.9.3 Trace element content in Reference Man . . . . . . . . . . . . . . . . . . . 581 586 13.10 Reference parameters for data interpretation . . . . . . . . . . . . . . . . . . . . . . 13.1 1 The future. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586

13.1 Introduction

With the exception of iodine (since 1850) and iron (since the 17th century), most of our knowledge concerning the essentiality of trace elements in human and animal health, however, belongs to this century, particularly to the last 30 years. The availability of refined analytical methods over time has provided evidence of essentiality of 16 such trace elements: arsenic, boron, cobalt, chromium, copper, fluorine, iodine, iron, manganese, molybdenum, nickel, selenium, silicon, tin, vanadium and zinc. Although all these are thought to be essential for humans, for some of them, (e.g. arsenic, vanadium and tin) the evidence is not direct but comes from animal world. The present day analytical techniques are capable of detecting extremely small quantities of chemical elements in the biosphere and have reached the potential to serve as routine tools for ultra-trace measurements. Undoubtedly, the technological progress has greatly contributed to the advances in metrology (the science of measurements) of trace element determinations in biological systems. Similarly, analytical quality assurance brought about by the development and use of a variety of biological certified reference materials (CRM) have further enhanced the metrological excellence. However, there are still some lingering inconsistencies in the quantitative data, limiting progress in Biological Trace Element Research (BTER) areas [ 13. This is an indication that high detection capability and sensitivity of analytical techniques alone is not the solution to the problem of reliable data generation in the BTER area. This is substantiated by the concern in the minds of many life sciences researchers [11 who are apprehensive about the usefulness of these measurements since the improved capability for quantification is not harmonized with appropriate bioanalytical perceptions. The purpose of this communication is to focus on the emerging trends in the study of trace elements in the life sciences; the goals, challenges and accomplishments will be briefly reviewed.

TRACE ELEMENTS IN ENVIRONMENTAL AND HEALTH SCIENCES

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13.2 Need for trace element analysis of biomaterials “Essential chemical elements whether required in major, minor or minute quantities, have the same mission - to sustain life [ 11.” Trace element analysis in biomedical and environmental media is required for a variety of purposes: In the biological sciences, we know very little about a number of trace elements with regard to their participation in the innumerable biochemical processes. The first step in this direction calls for their unquestionable identification and precise quantification to provide firm basis for metabolic investigations. For example, natural concentrations of lithium (an element familiar to clinicians as a therapeutic agent), is becoming available only recently [2]. Similarly, reliable baseline concentrations for elements such as arsenic, barium, boron, bromine, cesium, fluorine, gallium, germanium, rubidium, silicon, strontium, tin, titanium and vanadium need to be established. In the medical sciences, establishment of reliable reference values with a narrow spread of ranges for several trace elements, is crucial for initiating therapeutic measures in deficiency or excess situations. Similarly, identification of a role for an increasing number of trace elements in human nutrition, accurate assessment of human daily requirements of trace elements to evaluate the existing recommendations for dietary allowances, investigation of the deficiency states of essential trace minerals, development of strategies related to fortification of foods with nutrients need for the preparation of chemically defined elemental diets and meeting the requirements of total parenteral nutrition, surveillance of medications containing trace elements to scrutinize dosage claims, and clinical surveillance of patients undergoing kidney dialysis kind of treatment, and restorative or replacement surgery, call for elemental assays in clinical laboratories. For example, a wide range of materials is used as prosthetic implants, and it is essential to ensure the biocompatibility of such medical aids [3]. In the environmental sciences, ever increasing threat of pollution, and the demand for continuous surveillance and monitoring activities (e.g. toxic trace elements in foods), has resulted in extensive investigations, some extending beyond national boundaries. The relationships between health and exposure to elements through food, air and water is a major area of analytical endeavor that is bound to attract ever increasing attention. Establishment of specimen banks for biological, dietary, marine and medical samples and efforts to produce biological reference materials for analytical quality control is assuming significant proportions in many countries, and require enhanced measurement activities. Similarly, water, soils and plants are being analyzed in large numbers for geochemical purposes. In the veterinary sciences, trace element analysis has particular significance in commercial management systems where domestic animals are wholly dependent on what is supplied through water and diet. Continuous elemental status monitoring

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activities are not only beneficial in identifying and correctingdeficiencies, if any, but also useful in revealing incidences of over dosages, which can also be economically devastating. The fact that data obtained on one species cannot be automatically transferred to another, illustrates the bulk of analytical requirement when one considers that there are very many animals of economic importance such as buffalo, cow, goat, hen, horse, pig, sheep, etc. Therefore, for the above mentioned and many other reasons, the trace element field is still wide open, practically inexhaustible, and is limited only by the imagination of the investigators. “A prudent combination of analytical awareness and biological insight is crucial for success in biomedical trace element research studies [ 11.” Over one hundred years ago, macro-scale analytical techniques were used to discover the roles of special compounds (especially of metallic elements) in living organisms, and investigations were focused on selected proteins and pigments suspected of containing percentage quantities of metals. In contrast, present day analytical techniques are capable of detecting extremely small quantities and have become routine intra-trace measurement tools to probe elemental interactions at cellular levels. The scientific achievements connecting these two boundaries are punctuated with an array of analytical developments; some highlighting the phenomenal advances in the measurement technology and others reflecting the exceptional bioanalytical perception and the multi-disciplinary outlook of trace element investigators. Earlier investigationswith biological materials merely demonstrated the powerful capabilities of the newly emerging methodologies (e.g. multi-element techniques) since little or no consideration was given to incorporate the biological basis for the investigations. As a result, the earlier trace element studies indiscriminately attributed the observed changes to “biological variations” arising from age, sex, environmental and dietary factors. This is of course true for some elements that did not pose insuperable analytical difficulties and the results were reliable. Similarly,in some cases, the conclusionsdrawn were based on relative measurements to estimate the differences, and there were grounds to accept these findings as valid. In addition to these, there were instances where an investigation was carried out with meticulous care to understand the problems confronted. Several milestones have been established in the BTER arena [4] characterizing the growth of this dynamic branch of science as shown in Table 13-1. 13.2.1 BTER, a multi-disciylinaryscience

“The lack of a multi-disciplinaryapproach has been the Achilles heel of biological trace element research [ 11.”


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Table 13-1. Milestones in biological trace element research Recognition of the essentiality of trace elements and related developments Cart before the horse period! 0 The “cure-all” situation 0 the “artifact” variations Developments in metrology 0 Historical perspective 0 Trace element detemiinations 0 Multi-element methods 0 Analytical quality assurance Developments in bioanalytical concepts 0 “Metal-free Environment”concept for metabolic studies 0 Recognition of biological trce element research as a multi-disciplinary science 0 Speciation and bioavaiiability concept 0 Preparation and certification of reference materials for analy tical quality assurance Selected applications 0 Reference elemental concentrations for trace elements in tissues and body fluids 0 Elemental compositionof reference man 0 Recommended dietary allowances 0 Nutritional surveillance of essential trace elements (dietary intake) 0 Fortification of foods and feeds with selected nutritional trace elements 0 Environmental biomonitoring for toxic trace elements

BTER is an emerging multi-disciplinary science [5] which explores the metabolic roles of biominerals to seek solutions to a variety of biomedical problems, using several analytical techniques. Hence BTER researchers should have physiological, biochemical and analytical insight into the problems, or form a multi-disciplinary team to deal with the task appropriately. The multi-disciplinary requirement of BTER is clearly seen in some cases where collective efforts by physicians, biochemists and analysts will be required to safeguard the biologic validity of a specimen. This refers to the preservation of certain biological properties such as viability (e.g. cells) motility (e.g. sperm and bacteria) as well as electrolyte and osmotic equilibria in tissues and body fluids. This is essential to meet the requirements of specific biochemical steps, e.g. cell separations, fractionations, enzyme estimations, etc. In practice, it is not a simple task since it involves a systematic approach to the entire problem including the use of several techniques from different scientific disciplines. This is effectively illustrated in sampling platelets (thrombocytes) for trace element analysis [6]. The process iiivolves complicated steps such as obtaining sufficient quantity of the sample, preserving the viability of

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G.V.IYENGAR AND V. IYENGAR

the isolated platelet fraction, achieving the required cell purity, accounting for the trapped plasma fraction and preventing the problem of sample contamination in the inorganic sense. BTER demands both biological and analytical standardization while planning an investigation. This is required in order to (1) ensure that the information obtained is compatible with the purpose of the investigation, (2) to relate the elemental concentration data to desired biomedical needs and parameters, e.g. to specific cells, cellular components,enzymes or proteins to facilitatemeaningfuldata interpretation, and (3) to c a ~ out y race element speciation in the same specimens, if required. Therefore, the specimen collection and preparatory techniques must be capable of preserving the analytical quality, biological property and biochemical integrity of the sample. To give a simple example example, clotted blood (useful for obtaining serum), heparinized blood collected in the conventional manner, and the same collected under controlled contamination conditions are samples of different biochemical validity with respect to their usefulness for BTER studies. A range of analytical problems particularly relevant to clinical [7] and bioenvironmental[8] specimens have been outlined. 13.2.2 Trace element speciation and bioavailability “Human milk is for the human infant; cow’s milk is for the calf” [9]. The late Paul Georgy stated this point almost derisively 60 years ago and persisted in his opinion when others chuckled! In our opinion, this sentence powerfully symbolizes the concept of relevance of an elemental species and its bioavailability. As for understanding the biochemical properties (i.e. speciation chemistry), even an analyst with a proven ability to determine extremely small quantities of “total” element concentrations in biological systems, must be ready to recognize an entirely new dimension, namely complex problems of speciation chemistry of biological matrices. In studies of biological effects of trace elements, besides the determination of their total contents in an organ or its components, it is often necessary to obtain information on the chemical forms in which trace elements occur. Many trace metals are bound to proteins and therefore, biochemical procedures would be necessay to estimate the fraction bound to organic molecules. Most biochemical procedures can introduce serious extraneous contamination into the processed specimen since the concentrations of several elements of biochemical interest are very low in some clinical specimens such as whole blood, blood serum and urine. Therefore, frequently used materials e.g. gels and reagents such as buffer solutions need to be of very high purity. In dietary intake studies it is being recognized now that the biologically available fraction of a trace element differs among different foods and therefore, bioavailability of various elements from single or mixed foods should be determined for


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accurate estimation of the intake from diets. Many biochemical methods for isolation of metallocomplexes such as ion-exchange chromatography, affinity chromatography, immunoprecipitation, electrophoresis and isoelectric focusing are in use, but are faced with problems of loss and contamination of the metal and its species. Radioactive labeling of organo-arsenic compounds has been described for animal studies [lo]. A two-stage in vim system to simulate gastric and intestinal spectrometry has been successfully used in recent investigations. Several techniques based on gas chromatography and high pressure liquid chromatography and anodic stripping voltammeter have been applied for speciation chemistry. Many analytical methods are useful for speciation chemistry work provided that their limitations are recognized [ 111. The use of stable isotopes in mineral nutrition research is well established. Stable isotopes as isotopic tracers have no radiation risk and therefore, are widely accepted. Use of various versions of Mass Spectrometry and Neutron activation analysis for in vitro specimens following stable isotope application have resulted in considerable progress in mineral metabolism research.

13.3 Biological standardization 13.3.1 The “Bio” sources. of analytical errors

The need for biological standardization in BTER studies is amplified by the fact that biological systems are dynamic entities unlike the static inorganic situations. Therefore, the mechanisms for representative sampling of biomaterials are generally more difficult than sampling inorganic pools. It should be recognized that any sampling plan developed for obtaininga biological specimen must take into account a host of situations recognized as presampling factors [12]. Failure to recognize these parameters at the right time can lead to erroneous results, inappropriate data interpretation including the development of inaccurate baseline values for reference purposes. Although there is ample evidence in the literature that the above mentioned factors seriously affect the biological integrity of the sample, the severity of presampling factors continues to be neglected by the bioanalytical community [121. This situation has to be corrected since there is a critical need for incoiporatingmore biological considerations into the sampling protocol. Two major sources of errors need to be considered in this context: conceptual (arising from limited understanding of the “bio” dimension of the specimens such as biological variations that are identifiable but not always quantifiable, wrong statistical approach to data processing, wrong basis for expressing the data, and incompatible data interpretation); and measurable (stemming from sampling and preparation, and measurement related ones such as due to calibration, matrix effects, proceduralhnstrumentetc.). Of particular concern is the fact that a significant portion of the existing analytical information is derived from analyses of uncontrolled and often “spot” samples (e.g. hair, urine, soil and foods) with deficient

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sampling plans and inadequate AQA procedure. Yet, major decisions of public health concern are being made that are dependent on the analytical information obtained by monitoring and other biomedical studies. Therefore, for obtaining conceptually relevant information from BTER studies, measures must be introduced to ensure both biological and analytical standardization. Unfortunately, the “bio” dimension of the analytical problem in the BTER area, has been rather slow in unfolding. This lack of appreciation has been a major setback since the early days of BTER, delaying the onset of multi-disciplinary approaches that are vital for conceiving meaningful investigations. The presampling factors e.g. biological variations, environmental influences, post-mortem changes and intrinsic (e.g. differences in residence times of various elements in the blood stream) and inadvertent situations(e.g. medico-legal restrictions), are potential sourcesof serious errors, and require careful evaluation and data interpretation [12]. Yet, these are poorly identified and currently, very little effort is expended to evaluate them.

13.3.2 Presampling factors Presampling factors represent a stream of biochemical processes that affect the analytical integrity of biological specimens beginning at the in situ state and prevailing through the sampling phase and beyond as the specimens are delivered to the laboratory for analysis. These processes can be classified under biological variations, post mortem changes, internal contamination (caused by intrinsic and inadvertent sources for specific constituents) and situations such as preferential accumulation of chemicals in selected organs or even within different compartments of an organ, etc. The impact of these factors is clearly reflected in sampling normal human tissues and body fluids, a process complicated by a host of inherent difficulties. Biological variations: These include physiological aspects of age, sex, habit, geographical and environmental factors, diet, pregnancy and lactation. Further, circadian rhythm, recent food, fasting, posture and stress during or jsut before sampling should also be considered. The impact of biological variations is clearly reflected in sampling body fluids, a process complicated by a host of inherent difficulties. In this context, the “status” of subjects at the time of sampling plays an important role by introducing random changes to the sampled material. For example, Zn in serum changes with age [131, and pre- and post-prandial situations have profound effects on the elemental concentrationprofiles in blood serum due to thechelating effects of amino acids [ 143. Similarly, certain foods are known to alter the trace element concentration of body fluids: consumption of tea has been shown to induce abrupt changes over a short time in F levels in serum and urine [ 151; ingestion of fish has been demonstrated to elevate as levels in blood serum over intermediate time intervals [16]; and use of foods such as dietary algae naturally rich in iodine has been documented to increase


55 1

levels of this element in breast milk over longer periods of time [ 171. The changes

are due to the differences in gastro-intestinal absorption of various dietary items as well as due to the differences in residence times (Table 13-2) of the absorbed fractions. In younger animals, the physiological status of the development of the gut is incomplete and passive diffusion accounts for the generally high absorption reported (Table 13-3). Minimizing the effect of variations arising from biological variations in clinical specimens is important but not easy. In Table 13-4, a few examples are listed which emphasize the impact of physiological status of subjects at the time of sampling. As seen from Table 13-4, a sampling condition suitable for one element may not necessarily be valid by some other element. Importantly, Table 13-4 illustrates two situations: the need for an understanding of the metabolic behavior of different Table 13-2. Normal dietary intake. gut absorption and urinary excretion in human subjects (median values from the literature*) Element Boron Bromine Fluorine Iodine Molybdenum Thallium Arsenic Cobalt Copper Iron Mercury Lead Selenium Strontium Zinc Aluminum Cadmium Chromium Manganese Nickel Tin Vanadium

Average absorption (% ingested dose)

High (50-100)

Medium (10-50)

Dietary intake

Urinary

( Pdd)

( Pg/d)

1300 4000 2000 200 200 15 100 20 2500 16000 10 300 150 1500 13000

loo00 40 50 4000 170-400

lo00 10

1000 4000 1700 170 100? 8 50 1 50 200 4 50 50 240 440 100 1 1 4?

10 20? I?

* Pooled infomiation; literature survey undertaken in connection with revision of Ref. [ 1 181. ? value uncertain


552

Table 13-3.Gastrointestinalabsorption of selected trace elements by human adult and infant subjects (% of ingested dose)' -

Element

Adult

Reiiiarks

A1

2?

AS

15-25

Increased absorption in patients with renal failure Absorptionmay vary even among similar types of food due to chemical fomis of As [ 161

cd Ce co

Infant

25 5-60

60

Cr

cs

6 < 1 30-100 3-15 40 25-50

cu

68 60

F Fe

16-50

10

10-60

15 < 5 50-60 < 2 < 5 10 5 20 5 50-80

Hg Mn Mo Nb Ni Pb Ru Sb

100

(High)

540 40-60

7

sc Se

Sr

100

Ti W

> 90

35-58 5 20 (3-38) 100

2I-I

50

20-40

Zr

5

Si Sn

3

Even higher absorption from par boiled rice Varies with Zn status of food Net retention approx. 3040% Enhanced absorption from human milk than from cow's milk

Typical values 1-296

In man, probably 5% Both organic and inorganic forms are well absorbed

m20 Both organic and inorganic forms (with few exceptions) are well absorbed

< 1

*Values are total absorption; net absorption is lower, depends on the element and its excretory patterns; literature survey carried out in connection with revision of Ref. [ 1181).

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Table 13-4. Parameters influencing reference values: some examples* ~~

Sample

Element

Condition

Influence

Blood

Cd

Tobacco smoking

Elevated

Remains chronically high in smokers

Pb

Alcohol consumption

Elevated

Especially wine drinkers

Zn

Fasting After nornial food After high Zn food Low Zn intake

Elevated Lowered

Even Overnight fasting During first few hours

Elevated

e.g. oysters, liver

Pregnancy

Lowered Elevated Lowered

Rather sudden effect E.g., standing posture Progressive decline

cu

Pregnancy

Elevated

Progressive increase

F

Fasting Normal food Tea consumption Low F intake

Normal Variable Elevated Lowered

Overnight fasting If sampled immediately If sampled immediately Over a few days

I

High intake e.g. sea foods, iodinated salts Low I intake

Elevated

Rapid uptake but slow clearance; biological half-life is 2.5 weeks Over a few days

As

Fish intake

Elevated

Well absorbed and slowly excreted

Mn Fe I

High input Fore-milk Hind-milk Low I intake High I intake e.g. dietary algae

Elevated Elevated Elevated Lowered Elevated

Mn rich cereals Variation due to high fat content Over a few days Extremely high levels of I excreted within a short period

Several elements

Low or high intakes

Variations

Sensitive to fluid intakes, requiring 24 hr collections.

Serum

stress

Milk

Urine

*Information summarized from Ref. [ 121

Lowered

Remarks

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elements in the specimen to be sampled and the necessity for adopting compromised but well documented conditions for specimen collection. Indeed, it raises questions on the conclusions drawn from multi-element analyses, e.g. serum, for establishing baseline data. The influence of geographical and environmental factors has been established in several cases. Elevated blood Pb levels in several urban areas of the world linked to the use of leaded gasoline is a good example [ 11. Other examples include a dramatic accumulation of Sc in lung [ 181 and elevated levels of Cu and As in urine [19]. In a recent investigation dealing with human milk, distinct geographical differences were shown for As, Mn, Se and Zn, among others, between Guatemala, Hungary, Nigeria, Philippines, Sweden and Zaire [20]. Seasonal changes can be of physiologic or climatic origin. The fluctuations observed for I concentrationsof dairy milk samples collected in different seasons is a good example [21]. Smokingtobacco (elevated Cd levels in blood) and consumption of wine (elevated Pb levels in blood) are good examples of the impact of habit on trace element metabolism as seen from a number of studies documented in the literature [ 1,221. Postmortem changes: Post-mortemchanges are also of significance to the analyst since they influence the “sample status” as long as the specimens to be sampled remain inside the body. The processes responsible for this are cell swelling, tissue dehydration, imbibition, putrefaction and autolysis [23]. A survey of the literature has revealed that elemental concentrations based on autopsy samples generally show great variations in contrast to blood or serum which are obtained from living subjects [24]. These changes stem from the fact that human autopsy process entails a certain time lapse between death and sample collection. Immediately after the death of an organism, several post mortem changes set in with varying rapidity depending upon environmental temperature, humidity, body temperature at the time of death, insulating effect provided by the fat layers in the body, time elapsed before the body was put under cooling and the storage time. Of the many changes that occur, cell swelling, tissue dehydration, imbibition, putrefaction and autolysis are of particular significance to the analyst since they influence the “tissue status” as long as the organs remain inside the body. Rapidly metabolizing organs such as liver, spleen, kidney and heart are severely affected by cell swelling, imbibition and autolysis. The former two events produce changes in organ volume due to fluid influx and expulsion. Great changes in the elemental concentrations in liver have been demonstrated in an animal model study that evaluated the effects of post mortem changes [23,24].

13.3.2.1 Intrinsic errors In routine clinical samples one has to deal with numerous intrinsic aspects that introduce changes by way of physiologic shifts, accentuating the difficulties in


555

establishing reference values. The following example clarifies the situation. Stress conditions induce water shifts in the circulating blood. Since the red blood cells (RBC) and protein molecules are too large to pass through the water permeable membranes, they undergo changes resulting in higher protein concentrations and elevated hematocrit values. The impact of changes in hematocrit value is best illustrated by considering concentration changes of an element such as Pb which is predominantly bound to RBCs, and shifts in protein content which affect the results for protein bound elements such as Zn. Since both hematocrit and serum protein concentration can be estimated with high precision, these two parameters are useful in tracing water shifts. Conversely, non-stress conditions can create the opposite effect. The RBCs are also very sensitive to osmolality (total dissolved solids per kg of water) and react by changing shapes. Addition of water increases cell size and vice versa. These size changes are also easy to measure. Since the factors that are responsible for shape changes also initiate concentration changes, e.g. trace metallic elements, one can link them to the wide scatter often observed in the analytical findings in specimens associated with RBCs. Seasly [25] has reported the dynamic variations in hematocrit and serum protein concentrations in 200 test subjects. The average daily spread of hematocrit was about 7%; the greatest individual variation in one day was 12.5% and the average 4-day spread between the highest and the lowest values was about 11%. Peaks of hemo-concentrations were observed at meal time. For serum protein (reference value = 70 g per liter), an average drop of 5 g per liter was observed in changing position from standing (which is a form of physical stress) to recumbent (non-stress condition). This difference varied from day to day between 3.5 and 6.5 g per liter (95% confidence limit). 13.3.2.2 Internal contamination Internal contamination of tissues and body fluids from elements may arise due to a number of reasons governing many aspects of presampling factors and lead to intrinsic errors. These are sources that are inherently present in the sample and may falsify the results. These errors, as the definition itself suggests, are difficult to detect and the analyst has little or no control over them. Good examples in this context are medication, hemolysis, prevalence of subclinical conditionsand certain inescapable medical restrictions. There are certain baffling situations such as prior exposure to I containing drugs or X-ray contrast media which generally elevate the tissue I levels with varying retention times in body compartments [26,27]. Widespread use of I containing drugs (some as prophylaxis) and X-ray contrastmedia signal a formidable source of internal contamination for this element. Careful evaluation of case history is necessary to minimize such errors. Hemolysis is another source of intrinsic ei-rors [28]. Normal plasma contains much less hemoglobin in relation to serum which may contain 10 to 20 mg in 100 ml, an equivalent of 350 to 700 ng of Fe per

556


ml. Since naked eye cannot distinguish hemolysis in serum below a certain degree, errors of this kind virtually go unnoticed at the sampling stage. Careful evaluation of case history is necessary to minimize the impact of these errors in terms of guarded data inteipretations. Even habits such as smoking tobacco (elevated Cd levels in blood) and consumption of wine (elevated Pb levels in blood) can have an impact on trace element metabolism as seen from a number of studies documented in the literature [ 13.

13.4 Analytical standardization “The analyst is the most important component of any analytical system [l].” Concerning the analytical standardization, progress in technical aspects of sampling and preparation, instrumentation, calibration procedures, increased awareness of matrix interferencesand preparation of primary standards have collectively contributed to several improvements. However, information on long-term storage (several years) and its impact on the validity of the stored biomaterial, and reliable experimental findings on the quantitative recovery of biominerals during acid decomposition of different types of sample matrices are still scarce. The analytical standardization problems are encountered at all stages of an analysis. Currently, many of the problems faced at the measurement stage are being addressed by superior instrumentation, and development and use of appropriate analytical quality assurance materials. To resolve the concerns related to analytical quality of a sampled material, guidance to procedural steps (e.g. good laboratory practices) and emergence of concepts such as controlled contamination (i.e. use of specific materials as sampling tools that do not interfere with the purpose of the investigation), have provided a reasonably good understanding of the problems of extraneous contamination. The sampling and sample handling strategies practiced by the National Biomonitoring Specimen Bank (NBSB) at the National Instituteof Standardsand Technology (NIST) is a good example of the sustained efforts over a period of ten years dedicated to the improvement of specimen collection problems [29]. Due to extremely low level of trace element and trace organic pollutants found in most environmental samples, extreme caution must be exercised during sample collection and processing to avoid contamination. For this NIST effort, a detailed sampling protocol (for sampling human liver at autopsy) was developed in conjunction with individuals performing the autopsies, and implementation of the protocol required periods of education and close cooperation to arrive at suitable conditions within the bounds of practicality. Teflon materials (e.g. sheets, bags and storagejars) were selected as the most suitable material for non-contaminationof sample with respect to both organic and inorganic constituents and for low diffusion rates of water. The protocol specified such non-contaminating items as non-talced vinyl gloves; precleaned dust-free


557

Teflon sheets and bags; high-purity water; and a titaniumneflon knife. These items were provided to each collection site by NBSB to ensure uniformity in sampling and container materials. The titanium-bladed knife with a Teflon handle was designed and constructed at NIST to dissect the liver specimen after removal from the donor. These special knives were used to prevent trace element contamination by various constituents associated with a regular stainless-steel scalpel or knife (e.g. Ni and Cr) and to limit the contamination to an element of little environmental interest, namely titanium.

13.4.1 Analytical quality assurance “Frequent analysis of appropriatecertified reference materials is a key component of any well planned quality control program [ 11.” Frequent analysis of reference standards is the key for overcoming procedural inconsistencies in establishing quality control in any analytical laboratory. In this context, it should be recognized that the standards set for the quality of an analytical result is an important factor and is of course dependent upon the end use of the results. Thus, for example, if the aim is restricted merely to scan different biological matrices as a provisional step to establish the relative levels of elemental concentration profiles in them, it is obvious that a reasonable degree of quality standard is sufficient; whereas, for meeting the requirements of a typical biomedical trace element research laboratory aiming to use the results for medical diagnostic purposes and legal regulatory processes, depending upon the problem results with 5 to 10%total uncertainty may be acceptable. On the other hand, an exceptionally high quality standard (< 1% total uncertainty) is mandatory in some cases, e.g. certification of reference materials. If the tolerance limits are set narrower than the investigation really requires or not feasible under practical laboratory conditions, it can cause unnecessary expense and loss of time. In initiatingan AQA program use of two or more independent analytical methods to verify the accuracy of an analytical finding is a crucial requirement. This is reflected through the development and certification of a wide variety of reference materials for many inorganic constituents [30]. Therefore, reasonably well-founded baseline data for trace elements in biomaterials have been generated by few selected laboratories around the world.

13.4.1.1 Reference materials Although there are quire a few biological RMs that are presently available, they do not fulfill all the current analytical requirements. For example, for a group of elements (Al, F, Sb, Si, Sn and V,among others) not many RMs are available, and in some cases the number is very small. A major problem is that, trace element concentrations are generally subject to large errors, and it is imperative to match the

558


sample matrix with an appropriate RM, so that the level of a particular analyte retains its proportion in relation to the overall matrix composition of the sample. Only then can the sources of systematic errors arising from matrix effects be identified. To quote one example, using instrumental neutron activation analysis Hg and Se could be determined in human milk even in small size samples, but not in cow milk, although the levels of Hg and Se were comparable in these two matrices; this was due to matrix effects from predominant presence of P (ratio 1:10) in cow milk [31]. The following are selected examples of reference materials that are available for quality assurance of inorganic analysis as body fluids or solids: animal blood (IAEA-A-13); bovine blood (BCR-CRM-194 to 196); bovine serum (NIST-SRM1598); Pb in blood (NIST-SRM-955 a,b,c,d); human whole blood (NYCO 105,112); urine (KL-110-H to KL-142-11, NYCO 108, NIST-SRM-2670); milk power (IAEAA-1 1, NIST-SRM-1549, BCR-CRM-150, 151 and 063). Information on matrix composition of these standards can be found in the following sources [1,30,32]. CRMs for speciated forms of trace elements in biological matrices are extremely scarce. However, for elements such as Hg and Sn, information for both organic and inorganic constituents is required for making public health decisions. For example, many types of biological and dietary matrices are available for total concentrations of Hg (e.g. fish flesh, IAEA-MA-A-2m); bovine liver, NIST-SRM-1577a; rice flour, NIES-CRM- 10a; bovine muscle, BCR-CRM-184; and wheat flour NISTSRM-1567a). For Sn, a limited number of reference materials are just beginning to appear, although only with provisional values needing further work for certification. Total Hg and Sn in different matrices are determinable at pg/g to ng/g levels using either neutron activation analysis (instrumental or radiochemical mode), or by graphite furnace atomic absorption spectrophotometry following a wet ashing step. As witnessed by the example of Minamata disease and the bioavailability requirements in nutritional and environmental fields, speciation of elements is an important requirement in future SRMs. Chromatographictechniques are applicable for the determination of methyl mercury and organotin compounds. However, there are hardly any available that are certified for the organic forms of these elements; a few exceptions are dog fish (NRCC-DORM-I), dog liver (NRCC-DOLT-l), lobster hepatopancreas (NRCC-TORT-1) for methyl mercury, and fish (NIES-fish tissue) for uibutyl and triphenyl tin. Very recently, frozen whale blubber (NIST-SRM1945) has been developed at the NIST for use as control material [33] for organic and inorganic contaminants including methyl mercury. Information on additional matrices can be found elsewhere [34]. The future efforts towards developing analytical methodologies and reference materials should also take into account the multi-disciplinary needs of BTER, which include development of a wide variety of speciated-CRMs, required for risk assessment studies related to environmental chemicals, and selected nutritional constituents with widely varying bioavailability characteristics.


559

I3.4.2 Harmonization of nieasurenients “The ultimate purpose of an analyticalresult is to address the problem, not merely generating numbers for inter comparisons [ 11.’’ The present day analytical techniques are capable of detecting extremely small quantities of chemical substances in the biosphere and have taught the potential to serve as routine tools for ultra-trace measurements. Undoubtedly, the technological progress has greatly contributed to the advances in metrology (the science of measurements) of trace analysis of bioenvironmental systems. Similarly, analytical quality assurance brought about by the development and use of a variety of Standard Reference Materials (SRM) have further enhanced the metrological excellence. However, this capability is somewhat neutralized because of failure to observe proper procedures, analyzing inadequately prepared samples and generalizing the findings. Hence, in spite of the technological supremacy, inconsistencies in the quantitative data of environmental measurements are still prevailing. This is an indication that high detection capability and sensitivityof analytical techniquesalone is not the solution to the problem of reliable data generationin bioenvironmentalsystems. Thus, there is much concern in the minds of the life sciences researchers that improved capability for quantification is not harmonized with appropriate analytical and biological perceptions.

13.4.3 Trace elenletit detemzinations Several analytical techniques namely atomic absorption (flame and flameless), atomic emission (direct current and inductively coupled plasma), chemical and electroanalytical methods, gas and liquid chromatography, mass spectrometry in different modes, nuclear activation techniques and X-ray fluorescence, which offer sufficiently low detection limits to a variety of biomatrices are now available. Yet, very few laboratories in the world carry out reliable trace element determinations, while a large proportion of the laboratories working with biomaterials find it difficult to achieve a consistent capability to maintain even a 10-20% accuracy and precision. This is an indication that high sensitivity alone is not the solution to the problem of accuracy and precision, and that unawareness of various interferences (e.g. matrix related problems), flaws in sample and standard preparation and inadequate calibration procedures are evident.

13.4.4 Multianalyte determinations Many interesting findings have surfaced unexpectedly as a result of multi-analyte studies. From an analytical point of view, the non-destructive modes offer possibilities for generating simultaneous data for several elements for comparison, thus acting as internal quality control agents so that unusual situations involving any

560


specific element can be evaluated. Moreover, in a carefully designed study, multielement assays can provide very useful information on a large number of elements at relatively low costs [ 151. On the other hand, single element assays have their own role to play. In some cases, some elements have to be determined alone due to severe analytical problems. In clinical, environmentaland nutritional laboratories involved with specific elements frequently need single eleineiit assays. Therefore, in a comprehensive laboratory, a combination of both single and multi-element capability is essential for effective functioning. A recent report summarizes the capabilities of various analytical techniques as applied to a variety of biomatrices [35]. 13-45 Matrix reluted problems in sample treatment

Solid samples have to be brought to solution either by an acid decomposition step or by some kind of fusion treatment. This is an essential requirement for many analytical techniques before the analytical signal can be obtained. There have been many studies to assess the possible loss during dissolution due to escape of gaseous products, precipitation and sorption, and undissolved fractions. It is obvious that unrecovered fractionseither due to loss or due to retention on various surfaces would introduce a systematic error. Plant materials are good examples since they contain considerable amounts of silica in them. There have been very few systematic studies to assess the undissolved material under common methods of matrix dissolution. A method such as the instrumental neutron activation analysis is ideally suited to study this problem. The neutron irradiated material can be first assayed instrumentally (without any treatment), then subjected to various dissolution procedures, filtered and residual radioactivity on the filters measured. This is an elegant way of assessing the undissolved fraction. Using this approach, in a recent study [36], significant quantities of residual Cr,Sc and Fe from apple and peach leaves were recovered from the filter papers, as undissolved fractions. Similarly,the physical and chemical characteristics of siliceous or calcareous matter in a number of botanical samples have been investigated [37]. These kinds of studies are needed for different kinds of matrices to establish standard conditions for mineralization.

13.4.6 Sample preservation and storage Preservation is the process of retaining morphological and cellular integrity of tissues or organs, enabling restoration of all the “functional” properties when required. This is the kind of preservation (with “total” functional properties) that is of great interest to the developmental biology discipline where cells are preserved (sometimes using a cryoprotectant such as dimethyl sulfoxide) and recovered for various applications [38]. This form of preservation would also be useful for some of the chemical speciation studies. On the other hand, in the case of specimen banking (systematic collection and preservation of environmental and other biological


56 1

specimens for deferred analysis and evaluation), a slightly different kind of example, preservation primarily relates to the retention of structural integrity of biochemical moieties (e.g. enzymes, antigens). Therefore, rupture of the cells, if any, during specimen banking may not necessarily disquality the specimen. The standard technique used for preservation is at cryogenic temperature which is considered to be below -80" C. Because of the simplicity, convenience and the low temperature attainable associated with liquid nitrogen, use of this medium for cryopreservation is quite common. However, use of mechanical freezers permitting cooling up to -80" C is adequate for most BTER related preservations. Storage, on the other hand, especially from trace element studies point of view, basically implies an overall safe, clean, uncontaminated and non-degraded containment. Investigations have been carried out on long-term storage by retaining human liver samples under four different conditions: freeze dried tissue at room temperature, fresh tissue frozen at -15 and -8O'C, and fresh tissue frozen at -195°C (liquid nitrogen). These aliquots have been reanalyzed over a period of 7 years of storage and compared to data from real-time analysis, (i.e. analysis performed soon after homogenization)to determine if changes in the concentration of trace elements have occurred. The results show no significant changes for Se and Zn. Contrary to this, losses of As from bovine liver marix have been suspected with prolonged storage [6]. Stability information over a five and a half years of storage at -30 to -4O'C has also been carried out in a Canadian study demonstrating that several chemical residues have been shown to be stable in herring gull egg homogenate by systematic reanalysis [39]. Similarly, in a German study, human specimens of blood and liver have been shown to be stable for Cd and Pb over several years [40]. Lyophilized human body fluid reference materials have been shown to be stable for a period of 5 years for Hg, Pb and A1 [41]. However, the behavior of mercury was unpredictable among different reconstituted materials if the reconstituted material was not assayed within a few hours of reconstitution. Use of polyethylene (PE) vials and bottles are acceptable for storing dry powders for several years as very low temperatures are not required. Polycarbonate tubes are adequate for storage of serum and plasma for up to two weeks under refrigeration temperature. These tubes tend to develop cracks under sub-zero conditions, and for storage up to about -20' C, polyethylene tubes are recommended. If deep freezing conditionsare desired,PE is not suitable and Teflon bags or bottles arerecommended. The relative merits of container materials with respect to their suitability for trace analysis work has been reviewed [42]. Extensive investigationshave been carried out by a Swedish group to understand the storage problems of whole blood for Cd and Pb over a wide range of concentrations in several species [43,44]. Their results indicate that after 6 years of storage at -20" C in 5 ml polyethylene tubes, the concentration of Pb in blood had decreased

562


by 15%. However, up to 2 years of storage at -20” C no losses were encountered. Similar observations have been recorded for Hg in whole blood 1451 and for Se in blood serum [46] especially for the dimethylselenide form. Dahl et al. [41] have demonstrated that freeze dried blood serum can be stored for over 4 years (198286) at -20’ C without any problem. After reconstitution, the duration of storage is reduced drastically: one month at -20’ C, 4 days at 2-8” C and only 8 hr at room temperature. In some cases, conventional storage is inadequate for preserving the natural state of organic compounds (e.g. metallo proteins, enzymes). In this context it should be recognized that preservation relates to the process of retaining morphological and cellular integrity of an organ or tissue. Cryogenic preservation is one such approach which allows the specimens to retain the biochemical identity of the constituents. 13.4.7 Coiitantiriatioriby trace elenreiits

‘Contamination” is a double-edged concept in sampling body specimens from human subjects. It calls for sterile as well as free of chemical (trace element) contamination. Contamination is a relative concept, the danger of which is related to the concentration of the analyte. For example, aluminum, chromium, manganese and lead are more susceptible to air-borne contamination than are certain other elements. The case of chromium in blood plasma provides a good illustration in this context. The amount of ethylene diamine tetraacetic acid (EDTA) needed (approximately 4 mg) as an anticoagulant for 1 ml of blood, would introduce an equivalent of 1.2 ng of Zn and 0.2 ng of Cr to the resulting plasma, due to the Zn and Cr present as contamination in EDTA.This increase although would not affect the determination of Zn, for Cr it is unacceptable since the contaminant Cr fraction exceeds 100%of the actual Cr level of the serum samples [47]. Another example is that of Cr in urine; it took several years to understand the problems of external contamination by Cr before reference values could be established [ 1,221. Other crucial factors related to contamination control are, the length of storage time before the samples are analyzed, and whether the available technical facilities for storage (e.g. container materials and dust and humidity controlled environment) are adequate for safe storage of the sampled material. A practical solution here is to use tools and containers of highest possible purity, made of materials that do not interfere with the analytical parameters under investigation, e.g. accessories made of titanium, quartz and Teflon. 13.4.8 Losses of truce elenrents

There are several possibilities of loss of trace elements during sampling as well as sample preparation stages. Adsorption on container walls and tools is a distinct possibility during sampling and storage. Losses due to volatilization, sputtering


563

and chemical conditions during drying and ashing should be prevented by suitably modifying the working procedures. Retention of protein bound trace elements under moderate drying coiiditions is in most cases quantitative [48,49]with the exception of methyl-Hg which has been reported to be lost even during freeze drying [50]. Matrix specific problems encountered with various biomaterials during dehydration treatments have been documented [51].

13.5 Clinical specimens from human subjects Solid (organs) sampling from living human subjects for BTER studies is usually restricted to feces, hair and occasionally nail specimens. For sampling strategies of these and other soft tissues, the reader may consult the following sources [5 1,52, 54,551. Since body fluids are commonly sought for BTER studies, discussion will be extended to these types of samples. 13.5.1 Special features of biofiuids

Body fluids may be grouped under three categories: commonly sought (whole blood, milk, urine), easily accessible but not commonly sought (semen, sweat, tears, sputum, and saliva) and fluids requiring special procedures for collection (amniotic, bile, pancreatic, gastric and cerebrospinalfluid). Biological fluids are susceptible to bacterial growth in unfrozen state and sampling and preparation should be designed to overcome these difficulties [53]. Since most biological fluids are suspensions (e.g. fluids containing cells) or emulsions (e.g. milk and serum containing excess fat), care should be taken to select the phase of interest, or to ensure that the phases are properly mixed if sampled as a whole. The complexities involved in sampling biological fluids for trace element research studies requires both analytical and biomedical perceptions. Every effort should be directed at procuring samples that satisfy both biological and analytical validity. Some clinical samples are very valuable since repetitive sampling may not always be possible (e.g. samples from infants). Therefore, to cope with the ethical restrictions involved with sampling clinical materials, it is essential that medical sciences personnel appreciate the dangers of sample contamination during collection. This can be achieved only when both the analyst and the physician work in close cooperation. From a trace element composition point of view, biological fluids are generally heterogeneous media containing suspended or fragmented cells and other proteins or crystalline particles. Fluids such as milk are emulsions. The body produces a variety of fluids required for numerous functions under normal health conditions. These include: aqueous humor, whole blood (includes erythrocytes, leukocytes, plasma, platelets and serum), cerebrospinal fluid, fetal fluids, (allantoic and amniotic), intestinal fluids (cecal, duodenal, ileal and jejunal secretions), gastric juice,


564

bile (gallbladder and hepatic), milk (colostrum, transitional and mature), pancreatic juice, pericardial fluids, phlegm, pleural fluids, prelactation secretions of the mammary sperm), sputum, sweat, synovial fluid, tears, urine and vaginal secretions. Besides these, transudates and exudates are produced under pathological conditions. Transudate is a non-inflammatory fluid, characterized by a few cells, and contains less than 30 g/l of protein. Exudate consists of the fluid and cells of acute inflainmation, containing protein in excess of 30 The essential features and physiological significance of these biofluids has been recently summarized [ 13. Trace element determinations in body fluids is characterized by the extremely small amounts of elements. The predominant presence of organic components along with high concentrations of certain inorganic elements complicates the analytical procedures requiring the removal of these interfering components before analytical signals are registered by the measuring instruments. Such analytical problems associated with each trace element and applicability of various analytical techniques fortrace element determinations have been discussed on an element by element basis in the following monographs [54,55].

a.

135.2 Medico-legal imylicatiom Medico-legal implications are of very special concern in dealing with samples from human subjects. Although a physician is justified to obtain tissue and body fluid samples froin human subjects (e.g. diagnostic purposes), the end use of the samples is the deciding factor as to what extent the medico-legal implications come into picture. Hence, there are well defined medical restrictions governing human specimen collection. These regulations can be severely restrictive if samples are obtained from living subjects, and greatly differ among different countries. First of all, “informed consent” should be sought from donors to obtain samples such as blood, and for collecting biopsies during surgery. The following information should be provided to the donors before obtaining their consent: procedure to be followed for taking the specimen; description of any attendant discomforts and risks which might reasonably be expected for the individual and the community; an assurance that the data and results will be kept confidential but accessible to the donor; and the donor is at liberty to withdraw from the investigation at any stage if the process involves repeated sampling. Similarly, in case of autopsy sampling, medico-legal restrictions are basically dependent on the country in question, and the existing specifications have to be considered in preparing the protocols. Transboundary situations if prevalent, would probably complicate the situation. However, acceptable norms that have been established for shipping biological reference materials from one country to the other provide useful guidelines. Medical personnel engaged in specimen collection (e.g. blood), will be required to follow both sterile (medical stipulations) and non-contaminatingconditions (trace


565

analysis requirements) if the samples have to be valid for analysis. This involves not only training the medical personnel in trace analysis problems, but also educating the analytical community to observe safety precautions in handling specimens that have potential for infection. For example, the medical personnel should get acquainted with dust-free requirements and use of titanium, Teflon or other acceptable nonmetallic accessories, while the analysts should learn to work in biohazard hoods (special work benches).

135.3 Sampling and preparation 13.5.3.1 Blood The blood is obtained conveniently by venipuncture. Since various considerations limit the widespread use of optimal needles made of alloys such as platinum-rhodium for draining of blood, disposable type needles and syringes are generally used. Collection of blood in 10 or 20 ml fractions successively using the same needle and discarding the first two or three as rinse fractions is recommended. However, this method is not entirely suitable for elements such as Cr and Mn. Use of a plastic catheter reduces the contamination problem for difficult elements such as Mn and the discomfort to subjects since a single rinse provides a valid sample. Use of Teflon catheters is becoming a recognized mode of blood collection for the determination of trace elements. Preparing neonatal blood samples presents additional problems since only small volumes of sample can be obtained. A technique of handling these samples using a flexible Teflon tubing to sample capillary blood from the heel has been described

1561. Hemolysis must be avoided since elements such as K and Fe which are at higher concentrations in the erythrocytes may affect the serum value depending upon the degree of hemolysis. Visible hemolysis begins at about 50 mg of erythrocytes per 100 ml serum. Use of a dry syringe, slow transfer to a dry test tube and allowing sufficient time for clotting are helpful in avoiding hemolysis [28]. 13.5.3.2 Serum and plasma The serum should be separated from the clot within one hour. The clotting of blood takes about 15 min at room temperature and will be delayed when siliconized glassware, Teflon or polyethylene containers are used. Blood is fractionated by centrifugation at about 3000 rpm for 10-15 min. The contents of the tube should be kept closed until separation to prevent contamination and loss by evaporation. Recentrifugation of the separated serum is sometimes necessary to spin down the residual erythrocytes. One disadvantage with serum samples is that hemolysis is greater than with plasma. In addition, clotting releases K from platelets. For this

566


reason K values are lower in plasma than in serum. Plasma can be obtained by immediate centrifugation of heparinized blood, but the presence of anticoagulants is undesirable for trace element analysis. However, it is possible to obtain plasma without anticoagulant if fresh blood is immediately centrifuged.

13.5.3.3 Red blood cells Careful removal of all the serum is necessary to separate the red blood cells (RBCs). However, a certain amount of trapped serum (usually 5 to 8%) is unavoidable and needs correction. Use of a syringe and needle to transfer centrifuged RBCs minimizes the presence of trapped plasma. A check on hematocrit is recommended if RBC values are used to compute whole blood values and vice versa.

13.5.3.4 Human milk Samples of breast milk are collected either by manual expression or by using plastic breast pumps after cleaning the nipples with distilled water and letting them air dry for a couple of minutes. It is necessary to empty at least one breast completely and mix before collecting the sample, as the mean composition changes during the feed itself, e.g. fat content. Status of the sample, i.e. colostrum, transitional or mature should be clearly defined since the stage of lactation influences the protein content and therefore the metal concentration [57].

13.5.3.5 Sweat Sweat is usually collected either by the so-called arm bag method using a polyethylene bag around the arm or from the whole body by collecting the free flowing drops from various points of the body. This is done in a specially created sweating environment after the subjects have showered. However, collection of a valid sweat sample is difficult because of the numerous contamination hazards. 'The sweat collected should be homogenized by rigorous shaking and prepared immediately for the analysis. It is also necessary to centrifuge in order to obtain cell-free sweat. Unpredictable dilutions of the sample may occur since the profuseness of the sweating varies greatly in different parts of the body [59].

13.5.3.6 Urine Random collections of urine have limited use, if any, and therefore, 24 hr collections should be made. This is done after omitting the first void at the start and including the last one at the end of the 24 hours using high pressure polyethylene bottles, precleaned by leaching separately with nitric acid and hydrogen peroxide for a few hours followed by liberal rinsing with double distilled water. Following the sample collection, aliquots are taken as soon as possible after vigorous shaking of the contents using pipettes which are rinsed several times with the urine to be transferred. It is preferable to freeze and lyophilize these subsamples (10-20 ml) to avoid the interaction of the urine with the container.


567

13.6 Environmental biomonitoring for toxicants The concern about toxic substances in the biosphere and the need for developing strategies to eliminate or minimize the health hazards caused by these pollutants is well recognized. Contaminants such as pesticides, polycyclic aromatic hydrocarbons, halogenated hydrocarbons, heavy metals and other trace elements, enter the food chain through various sources (e.g. anthropogenic) in the biosphere. The organic pesticides are generally highly stable, lipophilic substances that tend to accumulate in adipose tissues over time, culminating in environmental toxicity and mutagenic changes. These events call for stringent measures for environmental surveillance, but the scenario is not easy to comprehend; there are too many chemicals and practically endless routes of exposure requiring comprehensive schemes for monitoring. Moreover, the state-of-the-artof analytical problems warrant skillfully conceived multi-disciplinary approaches including identification of appropriate specimens to assess human exposures to xenobiotic toxicants in the environment. However, any monitoringprogram should be designed to meet both real time pursuit (tracking short-term trends) and provisions for retrospective research (identifying long-term trends) as analytical capabilities improve. The ability to reevaluate data retrospectively extends the scope of data accumulation, and provides a sound basis for evaluation of the biological effects of various pollutants. Therefore, dedicated efforts are needed for (1) consolidating the biologic basis for selection of appropriatespecimens for environmental surveillance, (2) strategies for long-term preservation or sampled materials, (3) harmonization of the analytical measurements, and (4) multi-disciplinary expertise [ 13 to evaluate the pollution trends. The benefits are: improvements in problem oriented analytical approaches, establishment of reliable baseline values for numerous chemical constituents in selected environmental media, and a proven experimental tool for assessment of the environmental health criteria. These aspects will be addressed in the following sections. 13.6.1 Chemicals in the environment

From the environmental pollution point of view, various anthropogenic activities, especially the burning of fossil fuels required in various industrial operations, are a major source of several toxic trace elements, including selenium. Among these arsenic, cadmium, lead and mercury deserve special mention since they have profound effects on both domestic animals and human beings. Froslie and coworkers [59] have shown that heavy-metal contamination of natural surface soils from atmosphericdeposition occurs even at very long distancesfrom the major point source. Several examples of human exposure to arsenic, cadmium, lead, mercury and selenium have been recorded in the scientific literature [ 13. In some cases, in-depth

568


investigations with multiple population groups, e.g. mercury in hair from subjects in 13 countries [60] has yielded very valuable information. 13.6.1.1 Arsenic It is known, for instance, that sections of human populations from the south and far east Asia such as Japan, Taiwan and the Philippines have high levels of arsenic in their blood, milk or hair [61]. The source of arsenic is linked to the consumption of fish which is a significant component of an average diet. The arsenic content of the soil also plays a role. Similarly, it has been reported that subjects residing in the vicinity of copper smelters near Cluj-Napooa in Rumania excreted higher levels of arsenic in their urine and hair than did controls. A five-fold increase in urinary arsenic (6.4 vs. 31 pgA) and an increase of up to 32 times in hair content (0.25 vs. 8 &l) was demonstrated [ 191. Analysis of the body burden of arsenic clearly indicated excessive exposure to this element resulting from copper-ore smelting activities. 13.6.1.2 Cadmium Cadmium is a toxic industrial environmental pollutant. Tobacco smoking is a significantcontributer to the high levels of cadmium found in smokers. Blood levels of cadmium in smokers are higher than in those in nonsmokers by a factor of 3 to 12 [13. It is now well established that excess exposure to cadmium can cause renal tubular damage and obstructive lung disease. The amount of cadmium transferred from soil to plant to animal or human can be of concern, especially where sewage sludge is used to fertilize the food affected (e.g. commonly consumed food and foods consumed in large quantities) by the application of the sludge, soil pH and the amount of cadmium entering the soil. 13.6.1.3 Mercury Food is a major source of mercury intake, especially in areas where fish and other sea-foods are the main components of the diet. For example, it has been shown that Alaskan Eskimo mothers have rather high concentrations of this element in placenta, hair, blood and milk [64]. It is also known that these concentrations of mercury are directly proportional to the quantity of seal meat consumed. Subjects who consumed seal meat daily had the highest levels of mercury [64]. This brings into focus considerations of infant nutrition since breast-fed babies of these mothers are subject to toxic doses of mercury. 13.6.1.4 Lead The ever-wideningperspectives of lead toxicity indicate that lead intake by human populations has increased about one hundred-fold above the natural level based on


569

analysis of samples of prehistoric human skeletons [65]. Among the innumerable sources of industrial pollution, gasoline is the major source of environmental Pb in those countries where unleaded petrol is not mandatory. In many urban populations blood-Pb levels have been shown to be well over 200 ng/mL [ 11. This figure may be compared with the moderate or low levels of 90 ng/mL in Japan and Sweden, or even lower levels of 30 ng/mL observed in remote parts of Nepal and the interior regions of Venezuela [l]. Similarly, urban mothers have been shown to secrete more Pb into their milk than their corresponding rural controls [66]. A common problem is faced by young children who ingest several mg of soil (along with paint scrapings and house dust) per day resulting in significant extra-dietary exposure to Pb in particular, and few others such as Al, Pb, Si, 'Iiand V. This is a serious bioenvironmental problem problem since it is not easy to precisely estimate the child's daily intake of soil and other non-food sources [67]. The ever widening understanding of Pb toxicity indicates that Pb intake by human populations has increased about one hundred-fold above the natural level based on analysis of samples of prehistoric human skeletons [65,68]. 13.6.1.5 Selenium Mineral oil contains about 0.2 pg/g of selenium whereas coal contains as much as 3 ,ug/g, and in some exceptional cases levels can be much higher [69-711. Coal ash (especially the fine fraction of fly ash), which contains up to several hundred pgJg of selenium is carried over long distances and distributed over soil surfaces. The availability of this selenium to plants depends upon soil characteristics such as degree of alkalinity. The unique feature of selenium is that it can be present at toxic and deficiency levels within a relatively small geographical area. Added to this is the fact that the concentration window for the biological action of selenium is much narrower than that of other elements such as zinc. Thus, while a few parts per billion (ng/g) of selenium in food or tissues are essential, even a marginal increase over an extended period of time can have adverse effects because of background factors. In this context China is again an interesting example [72]. As a result of a combination of both soil and environmental pollution factors, neighboring countries of selenium deficient areas in Northeastern China reported cases of selenium intoxication [72]. Loss of hair and nails was seen in affected individuals. IN areas where incidence was high, there were symptoms such as skin lesions, and central nervous system and teeth disorders. A mortality rate of about 50 percent was recorded in some villages. The source of this environmental selenium was found to be a stony coal that contained high levels of selenium (average 300 &g). The selenium entered the soil and was readily taken up by plants because of the traditional use of lime as fertilizer in that region. The outbreak of human selenosis was due to a drought that caused the failure of rice crops thus forcing the villagers to eat more selenium-rich vegetables and maize and fewer protein-rich products. The margin therefore, between safe

570

G.V. lYENGAR AND V. IYENGAR

and potentially harmful levels of selenium is narrow. Several examples of human exposure to arsenic, cadmium, lead, mercury and selenium have been recorded in the scientific literature.

13.62 Bioenvir-onmerttulsun~eillunce The role of bio-environmental monitoring (BEM) or surveillance is that of an early warning system to identify factors responsible for adverse health effects and measures to prevent them. However, BEM per se has different meanings and these should be understood properly. In the context of human health, the expression biological monitoring (BM) of health is commonly used and is aimed at the detection of biological effects arising from exposure to chemicals in the environment (e.g. exposure to metals resulting in protein urea or perturbation of enzyme levels or other metabolites). BM is performed by measuring the concentrations of toxic agent and its metabolites on representative biological samples from the exposed organism. If appropriate indicator specimens, e.g. blood, urine, expired air and hair are used, the body burden of certain pollutants absorbed or retained in the organism during a specific time interval can be assessed, dose-response relationships can be established, and the data can be used for risk-assessment purposes. Bioenvironmental surveillance of pollutants is a complex task which requires careful attention to several aspects: an understanding of the multi-disciplinary perspectives involved; an objective evaluation of the suitability of bioenvironmental specimens; proven methods for collection of valid samples and their processing; availability of appropriate analytical methodologies to meet the desired accuracy and precision criteria; expertise required for data processing and meaningful interpretation of results; effective mechanism for dissemination of the acquired information; and provisions for a technical facility for long-term storage of samples for retrospective analysis (see section on Specimen Banking). Sample selection and collection are critical components of a biomonitoring system and any compromise at this stage would vitiate the purpose. Special attention should be paid to minimize the impact of presampling factors to safeguard the biological validity of the sample [121. For a comprehensivebioenvironmentalmonitoring program, the basic planning should take into consideration the requirements of both inorganic and organic pollutants. In developing analytical schemes it is prudent to plan for a broad range of analyte coverage since the aim is environmental surveillance and the need is to establish baseline values for as many parameters as possible. If the focal point is organic pollutants, then retention of the biochemical integrity of the specimen (e.g. by cryogenic preservation) is of highest consideration during pre- and post-sampling stages. On the other hand, sampling tools (e.g. tools made of Ti)and ambient conditions play a crucial role if inorganic pollutants are of primary concern [20].


57 1

13.6.3 Real tinie arid long-term bionionitoring Real Time Monitoring (RTM) is a means of frequently checking short-term changes of pollutant profiles using samples such as blood, milk, saliva and urine. The samples collected for RTM are analyzed as soon as laboratory facilities permit. RTM differs from ESB in terms of time interval between sample collection and analysis. On the other hand, samples for Long-Term Monitoring (LTM) should be reliable indicators of long-term body burden of the chemicals identified in them. Examples of specimens suitable for LTM are hair (with some limitations), adipose tissue (for organic pollutants) and liver (for both organic and inorganic pollutants).

13.6.4 Himtan specintensfor bionionitoring Several clinical specimens such as whole blood (for RTM of Cd and Pb, and their metabolites), hair (for arsenic and methyl mercury), and urine (for both organic and inorganic pollutants) are good specimensfor RTM [ 1,731. Determination of urinary arsenic is a good example of RTM for environmental exposure to arsenic [ 191. Breast milk has also been shown to be useful for monitoring the concentration levels of arsenic, cadmium, fluorine, mercury and manganese [20], and lead [66]. Placenta has been used to identify elevated levels of cadmium [74] and mercury [75] under different exposure conditions. Under normal environmental situations, the information obtained from placental analysis is an indicator of a time aggregate. However, the data interpretation should include the effect of placental age on its weight. Liver (many elements) is a good specimen for LTM. Choosing the right kind of samplesfrom human subjects for biomedical investigations in general [73] and biomonitoring of toxic trace elements in particular [76-781 is a much debated subject and poses many difficult problems. However, there is general agreement that no single specimen can answer all the commonly sought xenobiotic chemicals. As a first step in evaluating the usefulness of a given specimen for biomonitoring, it is necessary to understand the metabolic roles involved. The example of blood and hair clarifies the situation. These two compartments reflect the body status on two different scales of time. The blood generally represents short durations, in some cases a few hours or even less (e.g. response to changes in fluoride intakes), whereas the hair records events over longer periods of time and therefore, offers other advantages. Selected examples of suitable specimens for pollutant monitoring (Table 13-5),and risk assessment (Table 13-6) are presented. Several basic considerations have to be fulfilled in collecting human tissues and body fluids to ensure their usefulness as biomonitors of xenobiotic chemicals. These include (1) the biological relevance (i.e. suitability) of the chosen specimen, (2) quantity of the biomaterial obtainable and the associated statistical considerations and, (3) the analytical quality of the sample including the technical aspects of storage and preservation. These requirements have to be considered keeping in view that


512

Table 13-5. Human biological specimens useful for biological monitoring of selected toxic elementsa Tissues Blood

Arsenic

Cadniium

Lead

Inorganic mercury

Methyl mercury

X

X

X

X

X

X

X

Bone

X

Brain Feces Hair

X

X

Xb

X

Kidney

X

X

Liver

X

X

Placenta

X

X

Teeth

Urine

X

X X

X X

X

X

X

X

a Pooled

information. See Ref. [I] for original sources. bSaniplingand preparatory steps critical only a few samples such as blood, feces, hair, milk, saliva and urine and occasional biopsy samples (limited to a few organs and adipose tissue) are obtainable from living subjects while most other specimens must be sought at autopsy. Besides, there are ethical, legal and medical barriers to be confronted before the sampling process can be initiated. Hard tissues namely bone (selected inorganic constituents, especially bone seekers such as lead) and hair (selected inorganic constituents such as arsenic and mercury and specific organic species such as methyl mercury) are good indicators of long-term exposure. Soft tissues (e.g. liver, kidney, etc.) accumulate high concentrations of both organic and inorganic constituents. Body fluids (whole blood, milk, urine, sweat, saliva, etc.) offer limited but various possibilities. Blood is useful for monitoring inorganics such as arsenic, cadmium, lead and selenium. 13.6.5 Eiivir.onmeiita1Specinieii Bank (ESB)

Environmental specimen banking is an emerging discipline that has made impressive progress during the past ten years. ESB enables a systematic collection and careful preservation and storage of an array of environmental samples for deferred chemical characterization and evaluation. Specimen banking enables tracing newly


573

Table 13-6.Suitability of human blood and urine for exposure and risk assessment in biological monitoring programa ~~~

Metal

Specimen ~

~~

~

Risk assessment

~~

Antimony Aluminum Aluminum Arsenic (inorganic) Arsenic (organic) Cadniium Chromium Cobalt

Lead (inorganic) Lead (organic) Manganese (inorganic) B, U Manganese (organic) Mercury (inorganic) Mercury (organic) Nickel Selenium Tin Vanadium a

Exposure assessment

Bb U' B U U B, U U U B U

-

U €3, U B, H' B, U B, U B, U B. U

+c

?d

++ +++ ++ ++ +++ ++

?

+

+++ +

+++

+ ?

+ ?

+++ +

?

++ ++

+++ + ++ ?

+

?

++ +++ ?

++ ? ?

Pooled information. Whole blood. plasma or serum, see Ref. [I] for original sources. + = weak, ++ = moderate. +++ = well established association. Not known Urine Hair

recognized pollutants (retrospective evaluation) and permits reevaluation of an environmental finding when new and improved analytical techniques become available. This is particularly relevant in the context of the estimated 60,000 or so chemicals of industrial consequence, of which only a handful are currently being examined for their environmental impact 1781. ESB also permits identification of environmental trends and development of baseline data for use in public health issues. An important aspect of specimen banking is that information on both organic and inorganic constituents can be obtained on the same specimen. This has particular relevance to human ecological problems when multiple environmental parameters are involved. Specimen banking is an intrinsic part of a comprehensive analytical process. Sampling whether for storage or for spot analysis requires the same stringent precautions during sample collection. For a viable specimen banking program, access

574


to more than one analytical technique (e.g. atomic absorption spectrophotometry and neutron activation analysis for inorganic assays) is a crucial requirement to support a stringent analytical quality assurance program. It is highly desirable that one of the chosen techniques is a multi-element determination method. Whether it is for the purpose of storage or for preservation in terms of specimen banking, sample collection and preparation procedures must be devised to introduce minimal contamination, and storage conditions must cause minimal changes in sample characteristics. In essence, specimen banking provides the logical link between real time analysis, storage and preservation, and future trends in monitoring activities, since it permits analytically valid comparisons of changes taking place in the environment over extended periods of time. Hence useful preservation and storage information has originated from the tissue banking programs [29,79]. Presently, specimen banking activities are taking place in several countries of which two facilities in Germany and one in the USA have made substantial progress in setting up guidelines for collection and long-term preservation of both human and non-human specimens. Further, systematic approaches for sample selection, collection, preservation, processing and measurement of the analytical signal through data handling have been developed for the characterization of organic and inorganic constituents. The U.S. program has successfully completed the pilot phase, has established the potential usefulness of specimen banking that has led to the establishment of a National Biomonitoring Specimen Bank [29,80], while in Germany a national facility is already in operation [81]. 13.6.6 Proven applicatioris of ESB The ESB has entered the application phase as witnessed by several examples. The Canadian investigations with herring gull eggs [82] through retrospective determination of biphenyls (PCBs) using improved methodology have dispelled earlier high values. The NIST has conducted several investigations in collaboration with other governmental agencies and representative specimens of several ecosystems have been preserved. For example, 550 human livers have been collected, and about 20% of these livers have been analyzed to establish baseline values. As a result of this effort, significant findings such as continued decline in liver lead concentrations have emerged [29]. The German investigations focusing on both ESB and real time monitoring (RTM), have observed a shift in the distribution of pentachlorophenol (PCP) towards the lower end of the scale in recent years. Another notable example in this context is the establishment of anthropogenic sources to account for the existence of chlorinated dioxins in human tissues thus disproving the trace chemistry theory of the origin of dioxins (naturally occurring in woods not treated with chemicals) [83]. Similarly, several other examples have resulted from monitoring activities related to grain and soil [84], seal oil and sediment [85] and fish [86] have been established to demonstrate the usefulness of specimen banking.

TRACE ELEMENTS IN ENVIRONMENTALAND HEALTH SCIENCES

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13.7 Biomineral imbalances and health effects “There is no proportionality between the low quantities of some elements in the body and their vital importance for our well being because of their indispensability for the effective and proper functioning of biological systems” [87]. The health effects of trace elements in man and animals are basic manifestations of their deficiency or excess syndromes, irrespective of the origin of their disequilibria; besides quantitative relationships of trace minerals, a host of circumstances collectively contribute tot he occurence of trace element imbalances and, therefore, the causative factors should be viewed in relation to each other. Primary deficiencies and toxicities mainly stem from the following sources: diet, excessive exposure to antliropogenic lactation and rapid growth; living at elevated altitudes can also be a factor. Smith has recently reviewed the historical aspects of health effects of trace elements [88]. There have been recent findings linking selenium deficiency to the endemic Keshan Disease in China [89] and the growth retardation syndromes linked to zinc deficiency in the Middle East [go]. The factors related to mineral imbalances in man and animals are interlinked, and need to be considered in context of geochemical influences including high altitude effects, anthropogenic pollutants and nutritional factors and related metabolic considerations. In general, mineral deficiencies are often dependent and influenced to a large extent by world geographical location. For example, young and alkaline geographical formationshave greater natural abundance of most trace elements than the older, more acid, coarse and sandy formations. In tropical regions there is a marked leaching and weathering of soils under conditions of heavy rainfall and high temperature thus making them deficient in plant minerals [91-951

13.7.1 Nutritional atid metabolicfuctors It is necessary to know not only the amount of a given trace element present in a diet, but also the major components constituting the diet, i.e. fiber, phytate, protein, fat and carbohydrate. These proximate constituents in the meal influence the way the body absorbs and utilizes a given mineral. Because of this, the bioavailability of nutrients from different diets can vary significantly. For example, diets predominantly based on vegetables, cereals and other pain products (e.g. brown rice, wheat bran, wheat germ and breads made of whole grains) all contain high amounts of fiber and phytate. It is necessary to consume large quantities of these foods to partially offset the inefficient absorption. The effect of phytate and fiber on the absorption of nutrient metals can be very great. They were identified as the primary cause of growth retardation in dwarfs who subsisted on unleavened bread [96]. Zinc supplementation promptly induced growth in these subjects. Similarly, ingestion of very high amounts of calciuin can depress magnesium, iron and zinc utilization 1971.

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Widespread geophagia (pica), which affects zinc and iron absorption, occurs in many regions of Turkey covering also hilly areas [98]. Growth retardation is the principal effect. Zinc deficiency is also suspected in some parts of the Middle East and sections of aboriginal populations in Australia [99]. For a growing number of elements, evidence of trace-element deficiencies in human populations is only now emerging, [ l , 100,101]. Recently it was demonstrated that cattle grazing grass in St. Augustine, grown on peaty muck soils in the Florida Everglades, developed anemia associated with the presence of Heinz bodies (excess oxidation resulting in denaturation of hemoglobin and precipitation within the erythrocytes) and suboptimal concentrationsof selenium in blood. Selenium supplementation promptly corrected the anemia, prevented Heinz body formation and increased the body weight of cows and calves. This provides yet another example of the role of selenium in health and disease [ 1021.

13.7.2 Nutritional surveillance of trace elentents The growing importance of trace elements in human and animal health warrant systematic monitoring of animal tissues and forage concentrations in a number of geographical areas. Environmental and biological monitoring is necessary in predicting mineral deficiencies and toxicities. In a comprehensive global project on minor and trace elements in human milk, distinct global variations were demonstrated for the levels of arsenic, manganese and selenium, which were all high in the Philippines; 19,40 and 3 ng/ml, respectively. In contrast, in Guatemala, Hungary, Sweden, Nigeria and Zaire the values were low [20]. Lowest values for manganese and selenium were found in Hungary and Sweden, with an average of 3.5 and 13ng/rnl,respectively. Some of these differences can be directly related to the dietary habits. Soils from parts of Scandinavia and New Zealand are known to be low in selenium and human milk from these areas also reveals a correspondingly low value of about 10 ng/ml. The importance of these figures lies in the fact that in several parts of the world selenium intake may not be optimal for adults and infants. For arsenic, in countries where levels are high, the reverse may be true, i.e. the daily intake may be on the high side, thus causing toxicological problems. Numerous examples reflecting the influence of pollutant elements can be found in the literature [57,103,104]. The dietary habits of populations play a crucial role in trace element intake and, therefore, food surveillance studies merit major attention in all countries. Farreaching benefits for public health problems may be attained at relatively low costs if the prophylactic features of essential trace elements are skillfully exploited.

13.7.3 Reconmended dietary allowances (RDA) RDAs are defined as the levels of intake of essential nutrients that on the basis of scientific knowledge, are judged by the Food and Nutrition Board (of the United


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States National Academy of Sciences) to be adequate to meet the known nutrient needs of practically all healthy persons [105]. A similar recommendation has also been published by the World Health Organization [lo61 for the nutrients and by the Food and Agricultural Organization for permissible exposure levels for a number of trace elements among a variety of chemicals [ 107,1081. These developments are spread over several decades and represent the improvements in and contributions of analytical chemistry in context of public health problems. The recommendations for selected constituents are presented as examples in Table 13-7.

13.8 Trace elements and high altitude populations Health effects of trace elements are being increasingly recognized. For a number of elements, incidences of trace element deficiencies and excesses in human populations are only just emerging. Evidence is accumulating that populations at high altitudes are prone to develop essential element deficiencies, e.g. iodine and selenium. These problems warrant skillfully conceived multi-disciplinary approaches if future investigations are to elucidate the diverse interrelationshipsbetween geochemicalenvironment,anthropogenic pollution and nutritional and metabolic factors (including effects of high altitude physiology) that are related to trace elements and their profound impact on human and animal health. Efforts to identify the environmental pathways and continuous monitoring of various geographical areas for several chemical elements are of direct relevance to public health problems [ 1091. The quality of soils at high altitudes is generally depleted by the leaching out of several nutrients by, e.g. podzolization [93,94]. Trace element uptake is influenced by the acidity or alkalinity. For example, acid rain reduces the pH of the soils, and acidity inhibits selenium mobilization, due to complexation with metal ions [93,94] thus causing deficiency in plants, and as a consequence also in the animal-human food chain. On the other hand, alkaline soils promote selenium uptake and lead to the reverse situation, i.e. excess uptake resulting in possible toxicity. Selenite, a soluble form of selenium, is important in alkaline soils. However, because it combines with iron ions, selenite is not soluble in acidic soils. Haas [ 1101has demonstrated significant altituderelated differences in the growth rate of infants under 30 months of age in various socioeconomic groups in Bolivia. Infants born and living at high altitudes invariably show lower growth rates when compared to low-altitude infants. It is known that mountainous populations are generally short in stature, have high active lung volumes and broader bodies. These factors reflect adaptational responses to high altitude living. It is conceivable that in cases where growth-(height) limiting factors are not genetic, supplementation with zinc might induce growth in these subjects. Such beneficial effects have been demonstrated in Middle Eastern dwarfs who responded dramatically to zinc supplementation therapy [89]. Nutritional anemias, due principally to iron deficiency are also common in Latin America, as in many other parts of the world.

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Table 13-7. Dietary intakes of essential and toxic elements by adult human subjects (data source [105-1081 Recommended dietary allowance nig/day

Ca Mg P Fe

= = = =

800 300 (females) 350 (males)

800 10-18(age/sex)

Se = 0.055 (females) Se = 0.070 (males) Zn = 15

PtddaY

I

= 150

Estimated safe and adequate intake mg/day

1700-5100 1875-5625 1100-3300 1.5-3 = 1.54 Mn = 2.0-5

CI K Na CU F

= = = =

CO = 0.12 (=3 pg Vit. B12) Cr = 50-200 MO = 75-250 Maximum acceptable load clog

body wt./day

AS = 2

cu

= 50-500 Fe = 800 Sn =2oooO Zn = 300-1000

Provisional tolerable intake cl@g

body wt./day

Cd = 1-1.2 Hg = 0.7 (total Hg) Hg = 0.5 (methyl Hg) Pb = 7.1


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For high altitude populations, it is imperative that environmental lead levels be kept low. If quantities of lead in high-altitude environments are appreciable, or if lead-levels in the food consumed by these populations are high, lead accumulation becomes a potential danger since more than 95% of the absorbed fraction of lead resides in blood, namely in erythrocytes [73]. The erythrocyte-turnover cycle is 120 days, which means that accumulated lead will be partially released and redeposited in bone in which lead has a long biological halflife. In view of the elevated hematocrit in high altitude populations, the potential dangers are obvious. The risk factor for lead absorption is enhanced in two population groups: the iron-deficientanemic group (which forms a large proportion of the world population in both lowland and highland regions) and infants. Absorption of lead from the diet increases significantly when the individual also has an iron deficiency [ 1111. Infants tend to absorb most of the lead ingested, due to passive diffusion across the gastrointestinal (GI) tract. Selective absorption via the GI-system is not developed until later in childhood. 13.8.1 lodiine and seleriiirni

It is interesting to note that there are some similarities between the geographicaldistribution patterns of iodine and selenium, although there are several fundamental differences in the soil chemistry of these two elements. Nutritional deficiencies of iodine El121 and selenium I1131 have been identified in populations residing at elevated global regions. Many interesting associations of selenium with environment, high altitude and human and animal health are beginning to unfold. A good example in this context is that of the Andean populations living at 3800 meters above sea level. Investigations revealed that these subjects show significantly low blood serum levels of selenium and glutathione peroxidase (a selenium dependent enzyme), compared to subjects from low altitudes [ 1141. Glutathioneperoxidase is believed to protect cells because of its role as an antioxidant. Selenium deficiency in these high altitude Andean subjects implies suboptimal protection of cells against oxidant stress. Important evidence for the role of selenium in human health in context of Keshan disease [1131 has been found in China. The disease was generally observed in hilly and mountainous districts at altitudes of about 1600 meters and in many low-salted soil regions. Several thousand individuals were afflicted. All the areas linked to Keshan disease are relatively deficient in selenium. The malady is selenium responsive and since sodium selenite has a beneficial effect on prevention of Keshan disease, it has become an established prophylactic procedure in all the areas where this disease is suspected. It is now established that selenium plays an important role in tlie etiology of Keshan disease.

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There are some examples of altitude-related differences in selenium levels from the animal world too. It has recently been shown in the USA that in rock squirrels captured at different elevations, ranging from grass-land to pinyon juniper ecosystems (elevations 1600 to 2400 meters), the selenium content of kidney and liver was highest in grass land animals and decreased with increasing elevation gradient [1151. As grass-land soil is alkaline, it would appear that the mobilization and uptake of selenium by plants, and thereby also in animals, was the key contributory factor.

13.9 Reference values for trace elements in human specimens Establishment of “normal values” for trace elements in human tissues and body fluids is a desirable task with an undesirable feature: what is normal may not be easy to answer! [ 11.

13.9.1 Reference values 17s iroimal values With increased understanding of the sources of variations in elemental concentrations arising from physiological changes, pathological influences and occupational and environmental exposure efforts to generate reliable reference data bases for elemental composition of human tissues and body fluids are showing signs of success. For essential trace elements, the “biologic” reasoning supports that under carefully controlled conditions, levels of essential elements may fluctuate within narrow limits for a given species, eventually justifying the usage of normal values. However, for toxic and nonessential elements the ranges can be broad, depending upon the level of exposure of the subjects in question. In relation to human subjects, dealing with “normalcy” is a difficult and complicated task. It requires consideration of and compensation for a number of possible concurrent phenomena, and correlations may be very complex. Thus, baseline values can be deceptive and the question as to what is normal is not always easy to answer. On the other hand, by definition, reference values are expected to reflect baseline data in a well defined group of individuals. Obviously, factors such as age, sex, living environment and diet, among others, influence the concentration levels of certain trace elements, but some of these parameters can be well defined. In some cases, even the habits such as smoking tobacco (e.g. elevation of blood Cd) and consuming alcohol (e.g. elevation of blood Pb) should be considered. Importantly, the process of evaluating reference concentrationscalls for adequate attention to and careful documentation of numerous bioenvironmental parameters.

13.92 Refereiice concentrations in clinical specimens The recommended reference ranges of concentrations (Tables 13-8 and 13-9) are a combination of an extensive literature survey conducted between 1982 and


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1988, the data collected for the Revision of ICRP Reference Man, and additional information collected during the preparation of this report. These are presented in Tables 13-11 and 13-12 for Al, As, Au, B, Ba, Br, Cd, Co, Cr, Cs, Cu, F, Fe, Hg, I, Li, Mn, Mo, Ni, Pb, Pt, Sc, Sb, Se, Si, Sn, TI, U, V and Zn for selected clinical specimens. Since these tables reflect an evaluation of the pooled data, not all the individual references are cited. Readers may consult the main sources for locating the original references [ 116,1171. Rather large collection of data was possible for Cu, Fe and Zn, and for Mn in specimens such as hair and liver. The concentrations reported for Cu in whole blood and serum were generally higher in women than in men in most of the data set. Breast milk and blood serum share similar concentration profiles for many trace elements. For Fe in liver, variations were seen for different sets of results, even within the same country. Also, livers from female subjects contained lower concentrations than those from males. These are discussed in detail elsewhere [20]. Se levels in tissues and body fluids are known to be extremely susceptible to changes in dietary intake and reflect even short term variations in input. Therefore, changes in the concentrationlevels of this element in biological samples should be interpreted carefully. The number of sources available for elements such as Co, Cr, F, I, Mo, Ni, V and Sn are not that many mainly due to still persisting methodological limitations. Al, As, Cd, Hg and Pb have been investigated fairly extensively at least in some of the specimens. Several studies have shown that tobacco smoking increases tissue and fluid Cd concentrations. Similarly, wine consumption has been linked to elevated Pb concentrations in whole blood. Further, comparison of the whole blood Pb levels between man and women reveals that men have about 20 to 30% more of this element in their blood than do women. This is confirmed in several studies and in many countries. Since almost all of the Pb in whole blood is known to be associated with erythrocytes (RBC), a substantial part of the sex difference in concentration is explained by the hematocrit. The remaining part of the difference can be related to the variability in the working environment and food consumption (quantity). Therefore, expressing the concentration of Pb on the basis of the RBC weight is more helpful in identifying other factors that influence the concentration of this element in blood.

13.9.3 Truce element content in Reference Mait The Elemental Composition Data of Reference Man, is a concept developed by the International Commission on Radiological Protection (ICRP) in 1975, using then available analytical data [ 1183. The Reference Man is defined as being between 20 to 30 years of age, weighing 70 kg, is 170 cm in height, and lives in a climate with an average temperature of from 10" to 20°C. He is a Caucasian and is a Western European or North American in habitat and custom. Besides quantitative

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Table 13-8. Reference cleniental concentration in adult human blood serum, urine and milk' Element

Blood serum Pg/1

Aluminum Antimony Arsenic Barium Boron Bromine Cadm iurn Cobalt Chromium Cesium Copper Fluorine Gold Iodine Iron Lead Lithium Manganese Mercury Molybdenum Nickel Platinum Rubidium Scandium Selenium Silicon Thallium Tin Uranium Vanadium Zinc

1-5

0.0 1-2? < 1-5 -

15-45

3000-6000 1.1-0.3 0.1-0.3 0.1-0.2 1-2 800- 1 1OOa 1 1W140Ob 20-50 0.002-0.08? 60-70 800-1200 < I 0.2-0.8 0.5-1

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