Gas chromatography in air pollution analysis

JOURNAL OF CHROMATOGRAPHY LIBRARY - volume 49

gas chromatography in air pollution analysis

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JOURNAL OF CHROMATOGRAPHY LIBRARY- volume 49

gas chromatography in air pollution analysis Viktor G. Berezkin and Yuri S. Drugov Academy of Science of the U.S.S. R. A. V. Topchiev lnstitute of Petrochemical Synthesis, Moscow, U.S.S. R.

ELSEVlER Amsterdam

- Oxford - New York

- Tokyo 1991

This book is exclusively distributed in all non-socialist countries by ELSEVIER SCIENCE PUBLISHERS B. V. Sara Burgerhartstraat 25 P. 0. Box 211, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655 Avenue of the Americas New York, NY 10010, U.S.A.

Library of Congress Cataloging-in-Publication Data Berezkin, V. G. (Viktor Grigorkvich), 1931Gas chromatography in air pollution analysis / Viktor G. Berezkin and luri S. Drugov. 216 p. 16,5 x 24.0 cm. - (Journal of chromatography library ; v. 49) Includes bibliographical references and indexes. ISBN 0-444-98732-0 1. Air-Pollution-Measurement. 2. Air-Analysis. 3. Gas chromatography. I . Drugov, lu. S. (IuriT Stepanovich) II. Title. 111. Series. TD890.647 1991 628.5’3’0287-d~20

90-3981 CIP

ISBN 0-444-98732-0 (Vol. 49) ISBN 0-444-41616-1 (Series)

@ Akademische Verlagsgesellschaft Geest G Portig K.-G., Leipzig, 1991 Licensed edition for Elsevier Science Publishers B. V., 1991 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the copyright owner. Printed in Germany

Contents

Chapter 1 Introduction

References

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1

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Chapter 2 Air as an object of analysis . . . . . . . . . . . . . . . . . . . . . . . .

2.1. Atmospheric air and sources of atmospheric pollution 2.2. General characteristics of air pollutants . . . . . 2.3. Specific features of air as an object of analysis . . . References . . . . . . . . . . . . . . . . . . .

Chapter 3 Gas chromatography in the analysis of air pollutants

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3.1. General considerations . . . . . . . . . . . . . . . . . . . . . 3.2. Peculiarities of the gas chromatographic analysis of impurities . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 20 23

Chapter 4 Detectors for the gas chromatographic determination of impurities

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4.1. 4.2. 4.3. 4.4. 4.5.

4.6. 4.1. 4.8. 4.9. References

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Principal characteristics of detectors for gas chromatography Flame ionization detector. . . . . . . . . . . . Themionic detector . . . . . . . . . . . . . Photoionization detector . . . . . . . . . . . . Electron-capture detector . . . . . . . . . . . . Flame photometric detector . . . . . . . . . . . The thermal energie analyser (TEA) detector . . . . . Detectors for direct identification of impurities . . . . Hall electrolytic detector . . . . . . . . . . . .

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Chapter 5 Collection and pretreatment of samples for chromatographic analysis

5.1. 5.2. 5.3. 5.4. 5.4.1. 5.4.2. 5.4.3. 5.4.4.

Sampling into containers . . . . . . . . . . Use of absorption of contaminants in sample collection . . . . Cryogenic concentration of contaminants Use of adsorption for contaminant concentration . . Activated coaland carbon adsorbents . . . . . . Porous polymer adsorbents . . . . . . . . . Sorbents used in gas-liquid chromatography . . . Silica gel . . . . . . . . . . . . . . .

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25 28 28 29 30 30 31 31 31 33

35 35 31 40 46 48 50 55 51

VI Molecular sieves . . . . . . . . . . . . . . . . . . . . . Aluminium oxide . . . . . . . . . . . . . . . . . . . . 5.5. Multilayer sorption traps . . . . . . . . . . . . . . . . . . 5.6. Sorbents for passive sampling . . . . . . . . . . . . . . . Chemisorbents . . . . . . . . . . . . . . . . . . . . . 5.7. Trapping of solid particles and aerosols . . . . . . . . . . . . 5.8. Preparation of concentration tubes containing solid sorbent . . . . . 5.9. Influence of sampling conditions on the efficiency 5.10. of sorption of pollutants . . . . . . . . . . . . . . . . . . Properties of analysed compounds . . . . . . . . . . . . . . 5.10.1. Properties of the sorbent used . . . . . . . . . . . . . . . 5.10.2. Air flow-rate . . . . . . . . . . . . . . . . . . . . . . 5.10.3. Air sample volume . . . . . . . . . . . . . . . . . . . . 5.10.4. Concentration temperature . . . . . . . . . . . . . . . . . 5.10.5. Air humidity . . . . . . . . . . . . . . . . . . . . . . 5.10.6. Coadsorption . . . . . . . . . . . . . . . . . . . . . . 5.10.7. Concentration in the flow . . . . . . . . . . . . . . . . . . 5.10.8. Desorption of pollutants from the sorbent . . . . . . . . . . . 5.11. Solvent extraction . . . . . . . . . . . . . . . . . . . . 5.11.1. Extraction in Soxhlet apparatus and ultrasonic field . . . . . . . . 5.11.2. Thermal desorption . . . . . . . . . . . . . . . . . . . . 5.11.3. Influence of experimental conditions on the completeness of 5.12. recovery of pollutants . . . . . . . . . . . . . . . . . . . 5.12.1. Temperature . . . . . . . . . . . . . . . . . . . . . . 5.12.2. Humidity . . . . . . . . . . . . . . . . . . . . . . . 5.12.3. Coadsorption . . . . . . . . . . . . . . . . . . . . . . 5.12.4. Desorption duration . . . . . . . . . . . . . . . . . . . 5.12.5. Choice of solvent . . . . . . . . . . . . . . . . . . . . 5.13. Increase in desorption efficiency . . . . . . . . . . . . . . 5.13.1. Solvent mixture . . . . . . . . . . . . . . . . . . . . . 5.13.2. Two-phase desorption systems . . . . . . . . . . . . . . . 5.13.3. Desorption methods . . . . . . . . . . . . . . . . . . . 5.14. Choice of sampling method . . . . . . . . . . . . . . . . . 5.15. Metrological aspects of air pollutant determination by gas chromatography References . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.5. 5.4.6.

Chapter 6 The reactive-sorption method and its application for concentrating pollutans . . . . . . . .

6.1. 6.2. 6.3. 6.3.1. 6.3.2. 6.3.3. 6.3.4. 6.4. 6.4.1. 6.4.2. 6.5. 6.6. 6.7. 6.7.1.

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70 71 72 7a 78 80 80 80 81 83 83

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91 96 96 97 98 98 100 100 100 102 103 . . . . 108 113 . . .

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Concentration of pollutants on solid adsorbents . . . . . . . . Reactive-sorption concentration . . . . . . . . . . . . . Use of the reactive-sorption method to enhance the reliability of the determination of pollutants . . . . . . . . . . . . . . . . Determination of aggressive gases . . . . . . . . . . . . . Determination of nitrogen oxides . . . . . . . . . . . . . Determination of sulphur fluorides . . . . . . . . . . . . Determination of hydrocarbons . . . . . . . . . . . . . . Diminution of coadsorption of pollutants . . . . . . . . . . Dynamics of sorption and coadsorption . . . . . . . . . . . Diminution of coadsorption . . . . . . . . . . . . . . . . Improvement of chromatographic separation . . . . . . . . . Moisture removal . . . . . . . . . . . . . . . . . . . Identification of pollutants . . . . . . . . . . . . . . . . Determination of hydrocarbons . . . . . . . . . . . . . .

58 58 59 59 62 65 67

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123 123 124 125 126 127 127 130 133 134 135 135

VII 6.7.2. Determination of oxygen-containing compounds . . 6.7 3 . Determination of halogen-containing compounds . . 6.7.4. Iktermination of sulphur-containing compounds . . 6 7.5. Determination of pilrogen-containing compounds . 6.7.6. Determination of inorganic compounds . . . . . 6.8. Conclusion . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

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Chapter 7 Quantitative methods for the determination of impurities

150

7.1. Preparation of standard mixtures . . . . . . . . . . . . . . . . 7.1.1. Static methods . . . . . . . . . . . . . . . . . . . . . . . 7.1.2. Dynamic methods . . . . . . . . . . . . . . . . . . . . . . 7.1.2.1. Methods used to dilute (mix) streams . . . . . . . . . . . . . . . 7.1.2.2. Diffusion method (use of diffusion cells) . . . . . . . . . . . . . . 7.1.2.3. Exponential dilution flask method . . . . . . . . . . . . . . . . 7.1.2.4. Diffusion method using permeation tubes (ampoules) . . . . . . . . . 7.1.2.5. Other methods . . . . . . . . . . . . . . . . . . . . . . . 7.1.2.6. Preparation of standard aerosol mixtures . . . . . . . . . . . . . . 7.2. Detector calibration . . . . . . . . . . . . . . . . . . . . . 7.3. Calculation of impurity concentrations . . . . . . . . . . . . . . 7.4. Detection limits for air contaminants . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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150 150 151 151 152 152 153 157 159 161 161 162 163

Chapter 8 Practical application of gas chromatography to the determination of air pollutants . . . . . . . . . . . . . . . . . . . . .

165

Carbon oxides . . . . . . . . . . . . . . . . . . Halides and their derivatives . . . . . . . . . . . . . . Nitrogen-containing compounds . . . . . . . . . . . Sulphur- and phosphorus-containing compounds . . . . . . Metals and their derivatives . . . . . . . . . . . . . . Low-boiling hydrocarbons . . . . . . . . . . . . . . Aromatic hydrocarbons . . . . . . . . . . . . . . . Polyaromatic compounds . . . . . . . . . . . . . . . 8.9. Organic oxy compounds . . . . . . . . . . . . . . . 8.10. Amines and nitro compounds . . . . . . . . . . . . 8.11. Odorants . . . . . . . . . . . . . . . . . . . . 8.12. Halogenated hydrocarbons . . . . . . . . . . . . . . 8.13. Freons . . . . . . . . . . . . . . . . . . . . . 8.14. Chlorine-containing pesticides, polychlorobiphenyls and dioxins . 8.15. Bis(ch1orornethyl) ether . . . . . . . . . . . . . . . 8.16. Vinyl chloride . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . 8.1.

8.2. 8.3. 8.4. 8.5. 8.6. 8.7. 8.8.

Conclusion Subject Index

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165 167 . . . . 170 . . . . 172 . . . 176 . . . 178 . . . 179 . . . 180 . . . 186 . . . . 190 . . . 194 . . . 195 . . . 196 . . . . 197 . . . 198 . . . 199 . . . 2Q3

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IX

JOURNAL OF CHROMATOGRAPHY LIBRARY A Series of Books Devoted to Chromatographic and Electrophoretic Techniques and their Applications Although complementary to the Journal of Chromatography, each volume in the Library Series in an important and independent contribution in the field of chromatography and electrophoresis. The Library contains no material reprinted from the journal itself. ~~~

Other volumes in this series Volume 1

Chromatography of Antibiotics (see also Volume 26) by G. H. Wagman and M. J. Weinstein

Volume 2

Extraction Chromatography edited by T. Braun and G. Ghersini

Volume 3

Liquid Column Chromatography. A Survey of Modern Techniques and Applications edited by Z. Deyl, K. Macek and J. Janak

Volume 4

Detectors in Gas Chromatography by J. SevEik

Volume 5

Instrumental Liquid Chromatography. A Practical Manual on HighPerformance Liquid Chromatographic Methods (see also Volume 27) by N. A. Parris

Volume 6

Isotachophoresis. Theory, Instrumentation and Applications by F. M. Everaerts, J. L. Beckers and Th. P. E. M. Verheggen

Volume 7

Chemical Derivatization in Liquid Chromatography by J. F. Lawrence and R. W. Frei

Volume 8

Chromatography of Steroids by E. Heftmann

Volume 9

HPTLC-High Performance Thin-LayerChromatography edited by A. Zlatkis and R. E. Kaiser Volume 10 Gas Chromatographyof Polymers by V. G. Berezkin, V. R. Alishoyev and I. B. Nemirovskaya

Volume 11 Liquid ChromatographyDetectors (see also Volume 33) by R. P. W. Scott Volume 12 Affdty Chromatography by J . Turkova Volume 13 Instrumentation for High-Performance Liquid Chromatography edited by J. F. K. Huber Volume 14 Radiochromatography.The Chromatography and Electrophoresis of Radiolabelled Compounds by T. R. Roberts

X Volume 15 Antibiotics. Isolation, Separation and Purification edited by M. J. Weinstein and G. H. Wagman Volume 16 Porous Silica. Its Properties and Use as Support in Column Liquid Chromatography by K. K. Unger Volume 17 75 Years of Chromatography-A Historical Dialogue edited by L. S. Ettre and A. Zlatkis Volume 18A Electrophoresis. A Survey of Techniques and Applications. PartA: Techniques edited by 2. Deyl Volume 18B Electrophoresis. A Survey of Techniques and Applications. Part B: Applications edited by 2. Deyl Volume 19 Chemical Derivatization in Gas Chromatography by J. Drozd Volume 20 Electron Capture. Theory and Practice in Chromatography edited by A. Zlatkis and C. F. Poole Volume 21 Environmental Problem Solving using Gas and Liquid Chromatography by R. L. Grob and M. A. Kaiser Volume 22A Chromatography. Fundamentals and Applications of Chromatographic and Electrophoretic Methods. Part A: Fundamentals edited by E. Heftmann Volume 22B Chromatography. Fundamentals and Applications of Chromatographic and Electrophoretic Methods. Part B: Applications edited by E. Heftmann Volume 23A Chromatography of Alkaloids. Part A: Thin-Layer Chromatography by A. Baerheim Svendsen and R. Verpoorte Volume 23B Chromatography of Alkaloids. Part B: Gas-Liquid Chromatography and High-Performance Liquid Chromatography by R. Verpoorte and A. Baerheim Svendsen Volume 24 Chemical Methods in Gas Chromatography by V. G. Berezkin Volume 25 Modern Liquid Chromatography of Macromolecules by B. G. Belenkii and L. Z. Vilenchik Volume 26 Chromatography of Antibiotics Second, Completely Revised Edition by G. H. Wagman and M. J. Weinstein

XI Volume 27 Instrumental Liquid Chromatography. A Practical Manual on HighPerformance Liquid Chromatographic Methods Second, Completely Revised Edition by N. A. Parris Volume 28 Microcolumn High-Performance Liquid Chromatography by P. Kucera Volume 29 Quantitative Column Liquid Chromatography. A Survey of Chemometric Methods by S. T. Balke Volume 30 Microcolumn Separations. Columns, Instrumentation and Ancillary Techniques edited by M. V. Novotny and D. Ishii Volume 3 1 Gradient Elution in Column Liquid Chromatography. Theory and Practice by P. Jandera and J. ChuraEek Volume 32 The Science of Chromatography. Lectures Presented at the A. J. P. Martin Honorary Symposium, Urbino, May 27-31, 1985 edited by F. Bruner Volume 33 Liquid Chromatography Detectors. Second, Completely Revised Edition by R. P. W. Scott Volume 34 Polymer Characterization by Liquid Chromatography by G. Glockner Volume 35 Optimization of Chromatographic Selectivity. A Guide to Method Development by P. J. Schoenmakers Volume 36 Selective Gas Chromatographic Detectors by M. Dressler Volume 37 Chromatography of Lipids in Biomedical Research and Clinical Diagnosis edited by A. Kuksis Volume 38 Preparative Liquid Chromatography edited by B. A. Bidlingmeyer Volume 39A Selective Sample Handling and Detection in igh-Performance Liquid Chromatography. Part A by R. W. Frei and K. Zech Volume 39B Selective Sample Hndling and Detection in High-Performance Liquid Chromatography. Part B by K. Zech and R. W. Frei Volume 40 Aqueous Size-Exclusion Chromatography by P. L. Dubin

XI1 Volume 41A High-Performance Liquid Chromatography of Biopolymers and Biooligomers. Part A: Principles, Materials and Techniques by 0. Mikes Volume 41B High-Performance Liquid Chromatography of Biopolymers and Biooligomers. Part B: Separation of Individual Compound Classes by 0. Mikes Volume 42 Quantitative Gas Chromatography for Laboratory Analyses and On-line Process Control by G. Guiochon and C. L. Guillemin Volume 43 Natural Products Isolation. Separation Methods for Antimicrobials, Antivirals and Enzyme Inhibitors edited by G. H. Wagman and R. Cooper Volume 44 Analytical Artifacts. GC, MS, HPLC, TLC and PC by B. S. Middleditch Volume 45A Chromatography and Modification of Nucleosides Analytical Methods for Major and Modified Nucleosides HPLC, GC, MS, NMR, W and FT-IR edited by C. W. Gehrke and K. C. T. Kuo Volume 45B Chromatography and Modification of Nucleosides Biological Roles and Function of Modification edited by C. W. Gehrke and K. C. T. Kuo Volume 45C Chromatography and Modification of Nucleosides ModiFIed Nucleosides in Cancer and Mormal Metabolism Methods and Applications edited by C. W. Gehrke and K. C. T. Kuo Volume 45D Chromatography and Modification of Nucleosides Comprehensive Database for RNZ and DNA Nucleosides Chemical, Biochemical, Physical, Spectral and Sequence edited by C. W. Gehrke and K. C. T. Kuo Volume 46 Ion Chromatography: Principles and Applications by P. R. Haddad and P. E. Jackson Volume 47 Trace Metal Analysis and Speciation edited by I. S. Krull Volume 48 Stationary Phases in Gas Chromatography by H. Rotzsche Volume 49 Gas Chromatography in Air Pollution Analysis by V. G. Berezkin and Yu. S . Drugov

Chapter 1

Introduction

“We have reached the stage in the history of mankind where any human activity should be commensurate with nature’s potential. Man has already extinguished hundreds of species of animals and birds, destroyed up to two thirds of the forests that only recently had covered the Earth and disturbed many natural balances in the biosphere. It may seem that these facts are self-evident. However, in spite of the ever-growing concern over environmental pollution, millions of tons of various ecologically harmful substances are released annually into the atmosphere and oceans and are deposited in the outer layers of the Earth’s surface. The dustiness of the upper atmospheric layers and the contents of carbon monoxide and dioxide, organofluorine compounds and other substances are still growing; an oil film fatal to living organisms is being spread over the surface of the seas and oceans and is not shrinking. Civilization, which previously was only a blessing to mankind, today is showing its seamy side.” These concise characteristics of the vital problems facing mankind and urgently calling for Atmospheric pollution is a global solutions were given by Academician A. P. VINOGRADOV. problem. As the transfer of air and water masses is independent of the will of governments and the issuing of visas, the problems can only be solved if all governments will actively participate in their elucidation and observe international agreements. To study these problems and to develop optimal measures for environmental monitoring and control in various parts of the planet it is necessary effectively to determine the concentrations of pollutants. What is a pollutant? According to the definition adopted by the United Nations Organization, a pollutant is a substance found in an improper place at an improper time in an improper amount. Industry and transport in the form of various effluents and agriculture through fertilizers and pesticides are continuously increasing the number of actual and potential pollutants and the task of identifying pollutants therefore becomes more and more complicated each year. The definition of a harmful substance is of great importance. A harmful or noxious substance is a material which, while in contact with a human organism (under working conditions or everyday life), can cause disease or health problems that can be detected by modem methods both when in contact with the substance and in later periods of life of present and future generations [l].To characterize the permissible concentrations of harmful substances, the following values are used. The threshold limit value (TLV) is the concentration (in air) of a material to which most workers can be exposed daily without adverse effects. These values are established (and revised annually) by the American Conference of Governmental Industrial Hygienists and are time-weighted concentrations for a 7- or 8-h working day and a 40-h working week. For most materials the TLV may be exceeded to a certain extent, provided that there are compensatory periods of exposure below the value during the working day (or in some instances during the week). For a few materials (mainly those which produce a rapid response), the limit is given as the ceiling concentration (i. e., the maximum permissible concentration) that should never be exceeded, the threshold between safe and dangerous concentrations [l]. The time-weighted average (TWA) applies to the expression of permissible levels for occupational exposure. Exposure above the TWA is permitted, provided it is compensated for by equivalent excursions below the level during the working day or shift. In some national lists, the magnitude, duration and frequency of permissible excursions above these averages are specified [l]. 2 Berezkin, Gas Chrom.-BE

2

1. Introduction

In the U.S.S.R., the maximum allowable concentration (MAC) of a harmful substance in the air of the working zone is usually used as the main criterion. The MAC denotes a concentration that will not cause, in the course of work for 8 h daily or for any other period of time but not more than 41 h per week throughout the working life of an individual, any disease or deviation in the health status as detectable by the available methods of investigation, during the working life or during the subsequent life of the present or the following generations [l]. At present one of the vital problems is to protect the environment from pollutants that are the products of human enterprises. When they enter the air, water or soil, toxic substances (industrial poisons) pose a real threat to the normal existence of man, animals and plants. Industrial and transport development, the growing density of the population, the penetration of man into the stratosphere and space, the intensification of agriculture (application of pesticides), transportation of petroleum products, burial of dangerous chemical and radioactive wastes on the bottom of the oceans and the continuing nuclear arms tests-all these contribute to the constantly growing global pollution of our environment. Nowadays up to one million different chemical compounds of anthropogenic origin are constantly present in the biosphere, and the number is steadily growing. Annually almost 250000 new chemical substances are synthesized all over the world and many of them become potential pollutants. Air pollution causes grave concern because without air life on Earth would be impossible. According to the definition of the World Health Organization (WHO), air pollution takes place in those instances when a pollutant (or several pollutants) are contained in the atmosphere in an amount and during a period that harm or may harm people, animals, plants or property or can cause unaccountable damage to the health and property of people. The main sources of atmospheric pollution are discharges from industrial establishments and also processes of fuel combustion or evaporation (thermal power plants, internal combustion engines, etc.), forest fires and volcano eruptions. As a result of meteorological processes air pollutants spread over considerable distances in the atmosphere, leading to the global air pollution of our planet. At present the composition of air in rural and industrial areas is basically the same (they differ only in the quantitative composition of the pollutants). With these conditions it is a priority task (especially for industrially developed countries) to fight atmospheric pollution. The reasonable utilization of natural resources and the protection of nature, the establishment of the state sanctuaries and national parks, an increase in green plantations, a reduction in industrial discharges into the atmosphere and the development of wasteless chemical technologies are the main routes for solving the ecological problems for the benefit of mankind. However, it is impossible to solve the whole range of problems connected with the protection of the atmosphere and other parts of the environment without effective air quality control systems. The necessity to develop comprehensive methods of detecting various toxic substances in the atmosphere is universally recognized. The global pollution of the atmosphere and oceans and the significance and difficulty of tackling this problem have necessitated wide international cooperation on environmental protection. There exist numerous international programmes aimed at protecting various segments of the environment from pollution of air, water, soil and biota. Most of these programmes are being successfully carried out under the auspices of UNO, WHO, UNESCO and WMO (World Meteorological Organization). By the 1970s chemists and analysts had long realized the necessity to adopt measures for air pollution control and to develop reliable and effective analytical techniques for identifying and measuring industrial poisons in the atmosphere and in the workplace air. Rapid, sensitive and selective techniques for detecting micro-impurities of toxic organic matter, inorganic gases and aerosols of heavy metals in the air have been developed. Some countries have sanctioned standard methods (obligatory for internal application) to control the contents of the major air pollutants such as carbon monoxide, sulphur dioxide, nitrogen

1. Introduction

3

oxides, hydrocarbons, photooxidants and aerosols of heavy metals. The number of publications on the methods for air pollution analysis has also considerably increased. I n the last decade more than 25 books and about 35000 papers have been published on analytical techniques, methods of concentrating toxic micro-impurities from the air, the identification of pollutants and accurate methods for their determination. This book was written to describe systematically methods of detecting pollution of the atmosphere and the air in industrial areas. The current state of gas chromatographic methods for identifying harmful chemicals in air is described. Both general techniques for the analysis of imputities and practical applications of proposed methods in solving the concrete analytical tasks are discussed. Problems concerning the determination of impurities in air by gas chromatography are reflected in a number of books [2-141. In addition, a particularly important book devoted to applications of liquid chromatography in environmental analysis has been published [15]. It is remarkable for it general approach to the problems discussed and undoubtedly is of interest for those who are tackling similar problems using gas chromatography. However, taking into account the rapid development of this area, its complexity and versatility and the different insights on the same problem by different investigators, we considered it useful to write another book on the determination of impurities in air. We sincerely hope that the book will prove useful to readers and will prompt the further development and practical utilization of gas chromatography in the field of the air pollution control. Finally we thank Dr. H. G. STRUPPEfor valuable discussions and comments.

References (Chapter 1) [l] English-Russian Glossary of Selected Term in Preventive Toxicology. USSR State Committee for Science and Technology, Moscow, 1982. [2] WARK,K.;WARNER,C.F.: Air Pollution, its Origin and Control. New York: Harper and Row 1976. [3] THAIN, W.: Monitoring Toxic Gases in the Atmosphere for Hygiene and Pollution Control. Oxford: Pergamon Press 1980. [4] GRAEDEL,T. E.: Chemical Compounds in the Atmosphere. New York: Academic Press 1978. [S] Permissible Concentration Levels and Approximate Safe Levels of Toxicants in the Environment. Severodonetsk, VNIITBHP, 1978. [6] Technical Conditions and Methodological Instructions for Detection Methods for Air Pollutants. Annotated Index. Severodonetsk, VNIITBHP, 1981. [7] JENNINGS, W. G.; RAPP,A.:Sample Preparation for Gas Chromatographic Analysis. Heidelberg: Hiithig 1983. [8] GROB,R.L.(Ed.): Chromatographic Analysis of the Environment. New York: Marcel Dekker 1975. G. 0.: Controlled Test Atmospheres. Ann Arbor Science Publishers 1980. [9] NELSON, [lo] DRUGOV, Yu. S.; BEREZKIN, V.G.: Gas Chromatographic Analysis of Air Pollution (in Russian). Moscow: Khimya 1981. [ l l ] DRUGOV,YU. S.; BELIKOV,A.B.; DJAKOVA,G.A.; TULCHINSKYJ,V.M.: Methods of Air Pollutants Analysis (in Russian). Moscow: Khimiya 1984. [12] SCHREIER, F. (Ed.): Analysis of Volatiles. Berlin, New York: Walter de Gruyter 1984. [13] METZNER, K.: Gaschromatographische Spurenanalyse. Leipzig: Akademische Verlagsgesellschaft Geest & Portig K.-G. 1977. [14] LEI THE,^.: Die Analyse der Luft und ihrer Verunreinigungen in der freien Atmosphare und am Arbeitsplatz. Stuttgart: Wissenschaftliche Verlagsgesellschaft 1974. 1151 LAWRENCE, J. F.: Liquid Chromatography in Environmental Analysis. Clifton, New Jersey: Humana Press 1984.

2'

Chapter 2

Air As an Object of Analysis

2.1.

Atmospheric air and sources of atmospheric pollution

More than 99.9%of dry atmospheric air consists of nitrogen, oxygen and argon with only about 0.1% remaining for carbon dioxide, krypton, neon, helium, xenon and hydrogen. However, carbon monoxide, ozone, nitrogen oxides and ammonia (0.003-0.25 mg/m3) and also 0.5-1.5 mg/m3 of hydrogen and methane are traceable even in pure air. The small amounts of these gases in air are accounted for by the existence of free ozone in the upper atmospheric layers, by putrefaction and decomposition processes (ammonia, methane, carbon and nitrogen oxides) and by atmospheric phenomena (nitrogen dioxide). All other compounds (solid, liquid or gaseous substances that change the natural atmospheric composition) that enter the air from the other sources (mainly of anthropogenic origin) are classified as pollutants. They include carbon, sulphur oxides, hydrocarbons, various oxidants, aerosols of metals, solid particles (dust, soot, organic aerosols) and radioactive substances. Nitrogen oxides belong to this group of compounds too and are formed in general as a result of human activities. Industry, especially in highly industrialized areas, is the main contributor to air pollution. The major sources of industrial air pollution are thermal power stations (TPS) that burn coal and release soot, ash and sulphur dioxide into the atmosphere, steel plants that release mainly soot, dust, iron oxide, sulphur dioxide and sometimes even fluorides, and cement plants, which are sources of enormous amounts of dust. The large plants producing inorganic chemicals pollute the atmosphere with diverse gases (sulphur dioxide, silicon tetrafluoride, hydrogen fluoride, nitrogen oxides, chlorine, ozone, etc., depending on the technological process. Cellulose plants and petroleum refineries emit gases with unpleasant odours (odorants) into the atmosphere. Petroleum plants (petroleum distilleries, refineries, organic synthesis) pollute the air with hydrocarbons and other organic compounds classes (amines, mercaptans, sulphides, aldehydes, ketones, alcohols, acids, chlorohydrocarbons, etc.). Atmospheric air pollution by the chemical plants may occur for the following reasons [l]: 1.Incomplete yield of the main product (incomplete reaction, losses of the main product and others). 2. Release into the atmosphere of impurities and pollutants during raw material processing (fluoride compounds, natural phosphates and ores; sulphur dioxide and hydrogen sulphide from natural gas, crude oil and coal; arsenic and selenium from the sulphur pyrite during sulphuric acid production, etc.). 3. Losses of substances used in technological processes, e. g., volatile organic solvents, carbon disulphide and hydrogen sulphide used in artificial silk and viscose manufacture, nitrogen oxides during the chamber and tower sulphuric acid production processes and fluoride compounds in the aluminium industry. 4.The escape of odorous substances and oxidation and destruction products as a result of thermo-oxidative destruction, heating or drying processes (in the food, soap, glue and furniture industries, car painting, synthesis and processing of polymers, solvent generation, etc.). In large cities, gases coming from motor vehicle exhausts and the processes of fuel evaporation add considerably to atmospheric pollution. The amount of hazardous substances in exhaust fumes depends to a large extent on the type and maintenance of the engines. A gasoline engine does not significantly increase carbon dioxide levels in the atmosphere. However,

2.2. General characteristics of air pollutants

5

it is a direct source of pollutants such as carbon monoxide (formed as a result of incomplete combustion of the fuel in the cylinders, gaseous alkanes, alkenes and unburnt fuel components, high-boiling polycyclic aromatic hydrocarbons and soot, products of incomplete fuel oxidation (e. g., aldehydes), halocarbons, heavy metals (e. g., lead in ethylated petrol) and nitrogen oxides formed in the process of fuel combustion. Exhaust fumes containing reactive alkenes and nitrogen oxides under the action of solar radiation can participate in photochemical reactions resulting in toxic smog, which is harmful to plants, building materials and living beings.

2.2.

General characteristics of air pollutants

Air pollutants are classified as primary and secondary types. Depending on the source and the mechanism of formation, the former are chemical substances that enter the atmosphere directly from stationary or mobile sources. The latter are formed as a result of the interaction of the primary pollutants in the atmosphere and their reaction with the air components (oxygen, ozone, ammonia, water) under the action of ultraviolet radiation. Frequently, the secondary pollutants, such as substances containing the peroxyacetyl nitrate (PAN) group, turn out to be far more toxic than the primary type. The major part of solid particles and aerosols in the air are secondary pollutants. Conventionally, pollutants were classified into several groups depending on their toxicity, potential danger, spread and emission sources [2]: (1) major (critical) air pollutants-carbon monoxide, sulphur dioxide, nitrogen oxides, hydrocarbons, solid particles and photochemical oxidants; (2) polycyclic aromatic hydrocarbons (PAHs); (3) traces of elements (mainly metals); (4) permanent gases (carbon dioxide, fluoromethyl chloride, etc.); (5) pesticides; (6) abrasive solid particles (quartz, asbestos, etc.); (7) various pollutants with multilateral action on the organism [nitrosamines, ozone, polychlorobiphenyls (PCBs), sulphates, nitrates, aldehydes, ketones, etc.]. All critical pollutants are primary air pollutants. Nitrogen oxides are formed predominantly by the high-temperature fixation of nitrogen and oxygen in power plants and internal combustion engines. Nitrogen oxide is formed during electrical discharges in the atmosphere and is also present in the exhaust fumes of motor vehicles. Annually up to 5. lo7t of nitrogen oxides are emitted into the atmosphere, with 53% of that amount traceable to anthropogenic sources. Ultimately in the atmosphere nitrogen oxides turn into nitrates. Sulphur dioxide is formed from burning fuels with a high sulphur content (coal, oil). This toxic gas comes from stationary combustion sources, such as thermal power stations (85-95%), industrial cites (oil refineries, fertilizers, sulphuric acid and the petroleum chemicals industry) (5-10%) and internal combustion engines (2-7%). Sulphur dioxide, which takes part in the formation of photochemical smog, is considered to be the major air pollutant, dangerous for animals and plants. The total annual emission of sulphur dioxide into the atmosphere amounts to 8 . lo7t (considerably exceeding the contributions from other toxic chemicals) and is continuously growing in proportion to the energy consumption [3]. Carbon monoxide is one of the most hazardous and extremely wide spread air pollutants. It derives its toxicity from its reaction with haemoglobin in the blood. CO is generated by the incomplete combustion of different types of fuel. Forest fires and photochemical conversions of organic substances in the atmosphere serve as natural sources of CO. Up to 25% of carbon monoxide is of anthropogenic origin. A large proportion of the CO in the atmosphere in industrial and residential regions (in the U.S.A. up to 40% of the total atmospheric pollution) is accounted for by transport exhaust fumes. The average CO atmospheric concentration (about near highways and in towns during tends to increase significantly (to up to 3 * rush hours [4].

6

2. Air as an object of analysis

It is assumed that in the future the air pollution from stationary sources by toxic substances such as sulphur, carbon and nitrogen oxides and dust will decrease. However, the gases and vapours of organic substances and heavy metals (lead, cadmium, beryllium, etc.) will still present grave dangers. The concentration of hydrocarbons emitted into the atmosphere from natural sources is slightly higher than 1 mg/m3. The annual emission of hydrocarbons reaches 3. lo8t. Of this large amount, about 50% is traceable to transport, about 15%to industrial and residential fuel burning and another 26% to coal, waste and refuse burning (up to 1 m3 of refuse per person is burned annually throughout the world) and evaporation of fuel and solvents. The “average” automobile exhaust contains up to 400 mg/m3 of alkanes, 120 mg/m3 of acetylene, 200 mg/m3 of aromatics and 300 mg/m3 of alkenes [3]. The solid particles are represented in the atmosphere by dust, sand, ash, soot, volcanic ash and aerosols of organic (high-molecular-weight compounds) and inorganic nature. Frequently the toxicity of solid particles is due to the adsorption on their surface of such dangerous compounds as PAHs and nitrosamines. Photooxidants are formed in the atmosphere when reactive hydrocarbons interact with nitrogen oxides under the action of ultraviolet radiation. As a result, highly toxic chemicals such as peroxyacetyl nitrate and peroxybenzoyl nitrate are formed. Even at a concentration of 0.2 mg/m3 these compounds have a sharp lachrymatory effect, damage plants and destroy rubber. Peroxybutyl and peroxypropyl nitrates are even more toxic. They are unstable, especially at higher temperatures, and decompose to form simpler products, e. g., methyl nitrate and carbon dioxide [4].The atmosphere over most large cities is polluted with oxidants, as their generation is proportional to the development of transport and industry. Another group of pollutants consists of polycyclic aromatic hydrocarbons (PAHs). They can belong to both the primary and secondary air pollutant categories and are usually adsorbed on solid particles. Many PAHs possess marked carcinogenic, mutagenic and teratogenic activity and present a grave threat to the population. PAHs are mainly generated by power plants fuelled by oil and coal and by the petroleum transport and industries. Of several million known chemical compounds, only about 6000 have been tested for carcinogenic activity. It has been established that 1500 compounds recognized as potential atmospheric pollutants are also characterized by marked carcinogenic properties (PAHs, nitrosamines, halohydrocarbons and others). In concentrated industrial areas the amount of PAHs and other carcinogenic substances released into the atmosphere by plants may reach 80% of the total environmental pollutants. Traces of such highly toxic pollutants as arsenic, beryllium, cadmium, lead, magnesium and chromium have been detected in the atmosphere. They are usually present in the form of inorganic salts adsorbed on solid particles. Nearly 60 metals have been identified in the combustion products of coal. Mercury, arsenic, barium, beryllium, bismuth, bromine, cadmium, chlorine, cobalt, copper, iron, lead, manganese, antimony, molybdenum, nickel, selenium, tellurium, thallium, tin, titanium, uranium, vanadium, zinc and zirconium are present in the flue gases of power plants. For most of these elements the flue gases of power plants contribute 75% of the total air pollution level from diverse sources. The major proportion of pollutants enter the atmosphere from coal burning. Of all power plants fired by coal, oil and gas, coal power plants release more than 95% of the solid particles, 85% of the sulphur oxides, 70% of the nitrogen oxides and more than 90% of the trace elements. Lead enters the atmosphere as a result of oil combustion, volcano eruptions, automobile exhausts and various technological processes. Annually nearly 2 . lo5t are emitted into the atmosphere in the form of halides. The annual increase in the mercury content in the environment of industrially developed countries is ca.5%. Metallic mercury and lead and their organometallic compounds are extremely toxic. Mercury enters the atmosphere from volcano eruptions and is released by the chemical, electronic and instrument engineering industries.

2.2. General characteristics of air pollutants

7

Organomercury halide compounds that are formed from metallic mercury and its inorganic salts under the action of microorganisms are especially hazardous to humans. I n the F.R.G. alone the burning of diverse types of fuels results in up to 40 t of mercury entering the atmosphere annually and subsequently settling on soil and water surfaces. In the atmosphere the pollutants accumulate, interact, hydrolyse and oxidize under the influence of humidity and oxygen and also change their composition under the influence of radiation. Hence, the chemical properties of toxicants determine the duration of their residence in the atmosphere. This period is 4 days for sulphur dioxide, 2 days for hydrogen sulphide, 5 days for nitrogen oxide and 7 days for ammonia, while CO and CH4, being inert, remain unchanged for 3 years [ 5 ] . The next group includes the low-activity compounds-permanent gases (Freons and carbon dioxide) with longer residence times. The ever-increasing fuel combustion and forest fires are constant sources of C 0 2 in the atmosphere. In the U.S.A. approximately 2 lo9t of carbon dioxide are emitted annually from burning mineral fuels. Refrigeration units serve as the major source of emission of Freons (fluorochloroalkanes). Permanent gases accumulate in the stratosphere, undergo chain reactions and destroy the ozone layer that protects the lower atmosphere from high-energy solar radiation. According to some workers this is the reason why C 0 2 , not being toxic in the proper sense of the word, causes global changes in the atmospheric temperature. This in its turn results in climatic changes over the planet due to the greenhouse effect. Pesticides are often sprayed from aeroplanes. Organophosphorus pesticides are particularly toxic because, when subjected to photolysis in the atmosphere, they form products that are considerably more poisonous than the initial compounds. Serious diseases such as silicosis and asbestosis may be caused by inhaling so-called “abrasive” particles (silicon dioxide and asbestos). The pollutants in this last category are the reaction products of primary atmospheric pollutants. The most noteworthy are sulphates, nitrates and nitrosamines. Nitrosamines are formed in the atmosphere as a result of the reaction of amines with nitrogen oxides. Nitrosamines have also been detected in tobacco smoke. Other widespread air pollutants, such as PCBs, are also considered to be potentially carcinogenic. They are usually added to pesticides to promote their effect. The emission sources of some of the most important atmospheric pollutants and their concentrations in industrial and rural areas are given in Table 2.1. On entering the atmosphere, most toxic compounds undergo changes under the influence of ultraviolet radiation, humidity, ozone and air oxygen. The reaction products and the initial compounds (primary pollutants) react with each other, sometimes producing more toxic and hazardous compounds (secondary pollutants) [S]. The atmospheric conversions and interactions of pollutants and processes of dilution, deposition, adsorption and absorption do not prevent toxic chemicals from accumulating in the atmosphere and spreading over wide areas [4]. Therefore, the aim of keeping the atmosphere clean necessitates thorough and effective control over the degree of air pollution, which fluctuates significantly in time and space. These fluctuations depend on the specific features of the pollution sources (type of emission source, origin and properties of pollutants, their amounts), meteorological and topographic factors (wind direction and velocity, temperature inversions, atmospheric pressure, air humidity, relief of the locality and distance from the pollution source). In the early 1970s, intensive development of methods for air pollution control and air quality standards were stimulated under the auspices of the United Nations Organization and other international organizations. The unification of units characterizing the degree of atmospheric air pollution became essential. On the recommendations of WHO and IMO (International Meteorological Organization), pg/m3 and mg/m3 were adopted as such unified units [4].

2. Air as an object of analysis

8 Table 2.1. Concentrations of atmospheric pollutants [2] Pollutant

Emission source

Concentration in urban areas (mg/m’)

Concentration in rural areas (mg/m’)

Carbon monoxide Sulphur dioxide Nitrogen oxide Nitrogen dioxide Ozone

Automobile exhaust Burning of oil Combustion (oxidation) - do Photochemical reactions in the atmosphere Natural gas. Rottening process Automobile exhaust - do Photooxidation of alkenes in the air Automobile exhaust - do -

5.0 0.2 0.2 0.1 0.3

0.1

3.0

1.4

0.05

0.07 0.03

0.001 0.001 0.001

0.02 2.0

0.001 0.005

Rottening process - do Incomplete combustion

0.01 0.004 0.05

0.01 0.002 0.001

Methane Ethylene Acetylene Peroxyacetyl nitrate Alkenes (C,-C,) Hydrocarbons (except methane) Ammonia Hydrogen sulphide Formaldehyde

0.002 0.002 0.001 0.01

To fight air pollution, air quality standards should be established (in the U.S.S.R., PCL permissible concentration level) as a basis for further measures to preserve the environment. The existing air quality standards enable atmospheric purification measures to be rationally planned in those areas where the degree of air pollution exceeds the permissible concentration level. In the U.S.S.R., national PCL standards were adopted for the most widespread air pollutants as early as 1951. At present in the U.S.S.R.the concentrations of more than 160 toxic chemicals are monitored in the air in residential areas, and in industrial zones the PCL has been adopted for more than 800 chemicals [6, 71. The pollutant concentrations are also controlled and normalized in other countries (see, for example, the Appendix in the book by JENNINCS and RAPP[S]).

2.3.

Specific features of air as an object of analysis

It must be stressed that polluted air is one of the most complicated objects for analysis, for the following reasons: 1.The atmosphere and the air in industrial zones consist of multi-component mixtures of pollutants belonging to chemical compounds of many different classes. 2. The concentrations of many pollutants are extremely low, often 10-4-10-3%or even lower. 3.The polluted air is a labile system subject to changes under the influence of meteorological conditions and chemical interactions of the pollutants. 4. Frequently, the polluted air is a heterogeneous system containing solid particles and liquid aerosols. Therefore, in order to analyse air pollutants it often becomes necessary to use all available gas chromatographic methods and instrumentation.

References

9

References (Chapter 2) LEITHE, W.: Die Analyse der Luft und ihrer Verunreinigungen in der freien Atmosphare und am Arbeitsplatz. Stuttgart: Wissenschaftliche Verlagsgesellschaft 1974. SAMSFIELD,M.: Energy Sources. 3 (1977) 111. WARK,K.; WARNER, C. F.: Air Pollution, its Origin and Control. New York: Harper and Row 1976. THAIN, W.: Monitoring Toxic Gases in the Atmosphere for Hygiene and Pollution Control. Oxford: Pergarnon Press 1980. GRAEDEL,T. E.: Chemical Compounds in the Atmosphere. New York: Academic Press 1978. Permissible Concentration Levels and Approximate Safe Levels of Toxicants in the Environment. Severodonetsk, VNIITBHP, 1978. Technical Conditions and Methodological Instructions for Detection Methods for Air Pollutants. Annotated Index. Severodonetsk, VNIITBHP, 1981. JENNINGS, W. G.; RAPP,A.: Sample Preparation for Gas Chromatographic Analysis. Heidelberg: Hiithig 1983.

Chapter 3

Gas Chromatography in the Analysis of Air Pollutants

The analysis of a complex mixture of air pollutants requires the technique employed to be highly efficient, highly selective and highly sensitive. In our opinion, gas chromatography meets these requirements most completely. Let us consider this technique in more detail.

3.1.

General considerations

Chromatography is a field of science that studies the motion of a substance (or a group of substances) in a flow of one or several phases moving relative to another one (or other ones)

111. The mobile phase employed in gas chromatography is a gas (vapour). Gas chromatography permits the separation of volatile substances and the determination of their physico-chemical parameters. Its practical application is based on the different retention and band broadening patterns exerted by the substances moving in the gas-phase flow relative to a film of the stationary phase, with the solutes distributed between the stationary (solid or liquid) and gas phases. Gas chromatography is subdivided into two types, depending on the aggregate state of the stationary phase. In “gas-solid” or “gas adsorption” chromatography the latter is a solid, whereas in “gas-liquid” or “gas-liquid-solid’’ chromatography it is a liquid applied as a thin film on a solid support. This chapter is confined to the principles of gas chromatography. For more detailed information the reader is referred to Refs. [2-71. The advantages of gas chromatography are that the separation capacity of the sorbent can easily be changed by choosing the optimum stationary liquid phase (SLP), the symmetry of solute bands and a better reproducibility of sorbent properties. Compounds of different types are used as SLPs, such as hydroxydipropionitrile, diglycerol, squalane, tricresyl phosphate and eutectics [e.g., sodium nitrate (18.2%)-potassium nitrate (54.5%)-1ithium nitrate (27.3%)]. Especially high selectivity is exhibited by complex-forming phases. Thus, silver solution in ehtylene glycol is efficient for the separation of unsaturated compounds including cis and transisomers. SLPs must be thermally and chemically stable and possess low viscosity. Thermally stable selective SLPs include polyethylene glycols (upper temperature limit 225”C), cyanoethylmethylsilicones (275”C), Apiezone (300°C),methylsilicones (350°C), methylphenylsilicones (375°C) and carboranemethylsilicones (400°C). Basic parameters used in gas chromatography to characterize retention and band broadening include the retention volume, VR, the number of theoretical plates (TP) per metre of column length ( N L ) and the height equivalent to a theoretical plate (HETP) (H). The retention volume VR is that of the carrier gas passed through a column from the moment of sample injection to the output of the maximum peak solute concentrhtion. It can be expressed as VR = F C t R ,where tR is the retention time and Fc is the carrier gas velocity at a given temperature and gas pressure in the column. A more invariant measure of retention is the adjusted retention volume, V k , representing the retention volume of a solute minus that of a non-sorbed gas: V ; = ( t R - fM) F, = t R F , (3.1) where fk = tR - fM is the adjusted retention time and tM is the retention time of a component not sorbed by the stationary phase. This parameter ranges from fractions of a millilitre to

3 . 1 . General considerations

11

several litres, depending on the experimental conditions and the column diameter. Another parameter widely used in analytical gas chromatography is the relative retention value, rlJ (relative retention volume or relative retention time),

where V R ,and VKj are the adjusted retention volumes of compounds i and j , respectively, t k , - t R , - t M , t k , = tR, - tM and tk, and fk, are the adjusted retention times of compounds i and j , respectively. The extent of solute band broadening occurring as the solute traverses the column is characterized by the number of theoretical plates defined as N = 5.545(tRlWJ2, where W , is the peak width at half-height (in time units). Other parameters widely used to characterize a column include the specific efficiency, the number of theoretical plates per metre of column length ( N J , and the height equivalent to a theoretical plate (H):

N , = NIL, H = WN, H.N=1

(3.3) (3.4) (3.5)

where L is the column length. These parameters range from 1000 to 20 000 TP/m, depending on the experimental conditions (carrier gas velocity, diameter of sorbent particles). As a chromatographic band moves in the carrier gas flow, its broadening (swamping) occurs. The three basic processes causing band broadening are eddy diffusion, diffusion in the gas phase and interphase mass transfer. Eddy diffusion is independent of the carrier gas velocity and results from the path lengths for individual solute molecules being different owing to a non-uniform sorbent packing and sorbent particles being non-uniform in size. Longitudinal diffusion occurs in the gas phase and is caused by the concentration gradient associated with each chromatographic band. The finiteness of the mass transfer rate leads to the actual solute distribution being different to the equilibrium value, which also contributes to band broadening. The relative contribution of the above factors depends on the carrier gas velocity. Normally, longitudinal diffusion and the finite interphase mass transfer rate are the major factors of broadening at low and high flow-rates, respectively. Variation of the HETP with the linear carrier gas velocity is roughly described by the Van Deemter equation:

H

=A

+ B / u + Cu

(3.6a)

or (3.6b) where A is the eddy diffusion coefficient including multi-flow-path factors, B is the longitudinal diffusion coefficient resulting from solute diffusion in the gas phase along a column, C is the mass transfer coefficient arising from the finiteness of the interphase mass transfer rate, u is the linear carrier gas velocity, 1 and y are constants, d , is the sorbent particle diameter, D, is the solute diffusion coefficient in the gas phase, k ist the capacity ratio (or partition ratio), defined as k = V',JVM, where V M is the dead volume, and dl is the effective thickness of the liquid phase film on the solid support surface. DIrepresents the solute diffusion coefficient in the liquid phase. The Van Deemter equation is helpful in optimizing the separation conditions. Figure 3.1 shows the dependence of the HETP and the role of different band broadening factors on the linear carrier gas velocity.

12

3. Gas chromatography in the analysis of air pollutants

I

_ _ _ - - - - -3 /

I

--

Fig. 3.1. Dependence of HETP on carrier gas velocity and the effect of the main band broadening factors. I Contribution of eddy diffusion; 2 contribution of longitudinal diffusion; 3 contribution of interphase mass transfer

As the solute bands are moved by the carrier gas along the column, two opposing processes occur. The distance between the concentration maxima of successive components governed by the column selectivity increases, which improves the separation, whereas the bands are broadened depending on the column efficiency, which impairs the separation. The separating ability of a column depends mainly on its efficiency and the selectivity of the sorbent used. A quantitative measure of separation of two successive components is the resolution, R , representing a dimensionless value equal to the difference in the retention volumes, AYj = VRi- VRj, divided by the sum of the peak widths at the baseline. The peak resolution is determined by the sorbent selectivity, a(= rij), the column efficiency, N , and the mass distribution ratio, D, (capacity factor, k ) .

R=-.-.-1. ( a - 1 ) 4

01

1+D,

(3.7)

Figure 3.2 gives a graphical description of the relationship between the major physical processes that occur in a capillary column and its chromatographic characteristics, such as selectivity, efficiency and capacity. It illustrates in more detail the role of the individual factors in eqn. (3.7). The carrier gases conventionally used include nitrogen, air, argon, carbon dioxide, hydrogen and helium. The carrier gas must be chemically inert with respect to the solutes and the stationary phase. It must match the detector employed, i.e., cause no decrease in the de-

t

kinetics

compound and

Fig. 3.2. Relationship between column selectivity, efficiency and capacity with the major processes occuring in the column in the course of separation

3.1. General considerations

13

tection sensitivity, and possess a low viscosity and be non-explosive (which is of particular importance for industrial use), sufficiently pure and cheap. The mobile phase in GC is compressible. A pressure drop in a column leads to the expansion of the carrier gas and increases its velocity, which affects V Rand VR.This is why in precise measurements the actual (pure) retention volume, V,, is used, which is calculated allowing for the pressure drop and is thus independent of the latter. VN = jVX (P,IPO)Z - 1 2 (Pi/P0)31

j = - 3.

(3.8) (3.9)

where j is the pressure gradient factor and Piand Po are the inlet and outlet pressures, respectively. Another parameter used in gas chromatography is the specific retention volume, V , , representing the retention volume per unit mass of the stationary phase: (3.10) where Wl is the mass of the stationary phase and T is the absolute temperature in the column. The specific retention volume is used to determine certain physico-chemical parameters such as the activity coefficient. The main purpose of the solid support in gas chromatography is to provide a thin (fractions of a micrometre or less) film of SLP to improve the mass transfer between the mobile and stationary phases. Solid supports include diatomites subjected to special treatment in order to decrease their adsorptive activity, polymeric supports based on polytetrafluoroethylene and inorganic salts. Normally, their specific surface area varies from 0.1 to 1.5 m2/g. In capillary chromatography, the inner walls of a capillary column serve as a solid support. The role played by the solid support in gas-liquid chromatography is not less important than that of the SLP. Retention in GLC is due to the absorption of solutes in the SLP and their adsorption on the SLP/solid support and SLP/carrier gas interfaces [8, 91. The actual retention volume, V N , is governed by the following parameters of the sorbent [9]: VN=K~V~+KglS~+K~K~S~

(3.11)

where K I , KgI and K , are the solute distribution constants in the systems SLP-gas, SLP surface-gas and solid support surface-SLP, respectively; V, is the column volume occupied by SLP and S, and S, are the areas of the gas/SLP and SLP/solid support interfaces, respectively. The contribution of interface adsorption to the retention value varies from 0 to 100%. Therefore, in studying solute interactions with SLPs it is essential that the fraction of the retention volume corresponding to the solvation of a component in the SLP be extracted from the overall value. Procedures have been developed that permit the quantitative characterization of all the main types of solute interactions with the stationary phase, including adsorption on the gas/SLP interface ( K g lin eqn. (3.11)) and the SLP/sblid support interface (K,) and solvation in the SLP ( K J . Gas-liquid chromatography is the basic chromatographic technique used successfully in analysing composite mixtures. Band identification is based on the retention data. The retention of a component in a mixture is compared with that of a standard. Note that given a particular sorbent and identifying a substance by means of chromatography alone one should consider the sum of retention data obtained on columns with SLPs of different types. For chromatographic identification relative retention values are used, which can be evaluated much more precisely (the error decreases by a factor of two or more). At present, the most

14

3. Gas chromatography in the analysis of air pollutants

widely accepted parameters used in analysis and identification are Kovhts retention indices. The retention index system describes the retention behaviour of a compound relative to that of n-alkanes. As the number of carbon atoms in the n-alkane molecule increases by one, the retention index, Zi, is incremented by 100 units:

(3.12) where z and z + 1 are the carbon numbers of n-alkanes eluted before and after the substance i , respectively. V k , , VX,, VR(,+ are the adjusted retention volumes of the substance i and n-alkanes of carbon number z and z + 1 , respectively. The values of retention indices depend markedly on the type of sorbent used. Thus, for example, the retention of ethyl formate on silicone SLPs is characterized by the following retention indices: 487 (OV-1), 605 (OV-17) and 766 (XE-60); the same parameters for ethyl acetate are 506 (OV-l), 632 (OV-17) and 741 (XE-60). These data indicate that the retention index is governed by the natures of both the solute and SLP employed. Parameters other than retention index and relative retention value also used in chromatography. It has been shown that the known relative values can be considered as particular cases of the following general expression [8]:

(3.13) In developing a procedure for the determination of impurities it sometimes seems desirable to calculate the required efficiency of a column when advantage is taken of the stationary phase for which the retention values of analytes are known from the literature. Such a calculation can be accomplished using eqn. (3.7). However, in the last decade, experimental data have been presented in the literature in the form of the retention index rather than separation factor (see eqn. (3.7)). In this instance the calculation should utilize the following equation [lo]: 104(i0ge)2

n = 16R2

b;(AZ)*

+I-

10Zloge b,(AZ)

(3.14)

where n is the number of effective theoretical plates, AZ = Z2- Il is the difference in the retention indices of two sorbates, R is resolution, e is the basis of natural logarithms, b, = log(t:+ l / f L ) and t : + , and t i are the adjusted retention times of two elements of a homologous series of standard compounds of carbon number z + 1 and z, respectively. The efficiency of a chromatographic column depends markedly on its type. For classical packed columns (3-5 mm I.D., 100-200 cm long, sorbent particle diameter 0.1-0.3 mm) it is normally in the range 1000-3000 theoretical plates. Capillary columns are 10-100 times more efficient, the efficiency of the open type being 30 000-100000 theoretical plates [ll-131. At present, the predominant gas chromatographic mode is capillary chromatography. In 1986 ca. 75% of the papers published on gas chromatography in leading journals such as Analytical Chemistry, Journal of High Resolution Chromatography and Chromatography Communications and Journal of Chromatography utilized capillary chromatography. The use of capillary gas chromatography is distinguished by a number of features that are responsible for its rapid development in comparison with packed column chromatography: (1) higher efficiency; (2) speed of determination; (3) high sensitivity (when using a splitless sample injection technique.and the desired components being concentrated at a reduced column temperature or using the “Grob solvent effect” [13-151; (4) better reproducibility of the thermal conditions utilizing temperature programming as a result of a smaller column diameter and weight; and (5) lower sorbent and carrier gas consumption.

15

3.1. General considerations

Table 3.1. Compounds identified in the PAH fraction of street dust from Giza Square, Egypt. Peak numbers refer to Fig.3.3 [17] Peak

Compound

Peak number

Compound

Naphthalene Dimethylnaphthalene Trimethylnaphthalene Trimethylnaphthalene Dibenzothiophene Phenanthrene Anthracene C2-9H-Fluorene C2-9H-Fluorene Methyldibenzothiophene Methyldibenzothiophene Methylphenanthrene Methylphenanthrene Methylphenanthrene Methylphenanthrene C,-Dibenzothiophene C2-Dibenzothiophene Dibutyl phthalate C2-Dibenzothiophene Dimethylphenanthrene Dimethylphenanthrene Fluoranthene Pyrene C,-Phenanthrene C,-Phenanthrene

26 27 28 29 30 31 32 33 34 3s 36

C,-Phenanthrene C,-Phenanthrene C,-Phenanthrene C,-Phenanthrene Me thylfluoranthene Benzonaphthothiophene Benzo[ghi]fluoranthene Benzo[c]phenanthrene Benz[a]anthracene Chrysene/triphenylene Methylbenz[a]anthracene or methylchrysene Phthalate ester Phthalate ester Benzo[b]fluoranthene and benzoblfluoranthene Benzofluroanthene Benzo(e1pyrene Benzo[a]pyrene Perylene Indeno[l,2,3-cd]pyrene Dibenzanthracene Benzo[ghi]perylene Anthanthrene Dibenzopyrene

number 1

2 3 4

5 6 7 8 9 10 11

12 13 14 1s 16 17 18 19 20 21 22 23 24 25

37 38 39 40 41

42 43 44 4s 46 47 48

Capillary chromatography is very popular for air pollutant determinations and readers are referred to specialized books (11-131. Currently, capillary columns rely mainly on bonded and immobilized phases. The use of immobilized liquid stationary phases (1) enhances the lifetime of the columns because chemically immobilized phases will be incapable of curling into large drops, which usually results in a sharp decrease in column efficiency and (2) increases the detection sensitivity because the immobilized phases permit the utilization of a concentration procedure o n sample/ solvent injection, e.g., using the “Grob solvent effect”. The capillary column materials and the coating materials are equally important, for the following reasons: (1) the surface of the internal column walls which serve as a solid carrier in capillary chromatography can frequently produce a detrimental effect (especially in pollutant analysis) on the chromatographic process as a result of irreversible and catalytic conversions of the compounds treated on the internal capillary surface (see, e.g., ref. [16]); and (2) the polymeric coating of fused-silica columns has a limited temperature stability. Urban air contains a variety of harmful aromatic hydrocarbons. Figure 3.3 [17] shows a chromatogram for the fraction of PAHs in urban dust (Egypt). It is natural that such a complex mixture can only be separated by the use of high-efficiency capillary gas chromatograPhY.

16

3. Gas chromatography in the analysis of air pollutants

37

39

42 41

'

I

35f80

Fig. 3.3.

120

1

160

I

200

I

24 0

I

I

280

315

L

"C

Chromatogram of the fraction of polycyclic aromatic hydrocarbons contained in street dust

~71.

Column: 21 m x 0.32mm I.D.; SLP, SE-52; temperature programme, 35 to 80°C (temperature balistically programmed), 80 to 315°C at 5 K/min. Compounds identified as in Table 3.1

Capillary columns are equally efficient in determinations of nitroaromatic compounds, which is a difficult problem. Table 3.2 shows the concentrations of this important class of organic compounds which were detected in atmospheric dust in Tokyo [18]. The use of capillary columns allows the applicability of gas chromatographic techniques to be greatly extended for environmental control purposes. The adsorption of solutes on the interfaces results in the relative retention value in the general case depending not only on the ratio of distribution constants of the solute and standard, but also on the adsorptive properties of the SLPisolid support and gadliquid interfaces. Therefore, the relative retention time and retention index are not constants for a compound when adsorption occurs. The chromatographic constant of compound i is the invariant retention index, Ioi, corresponding to the solute interaction with the SLP only, which can be evaluated in terms of the following equation [9, 191:

I, = lor + aik,,

(3.15) (3.16)

where a is a constant defining the adsorption of the solute on the sorbent used, k,,is the capacity ratio of a standard, k,, = VX,,/V,, Vk,, is the adjusted retention volume of a standard, V , is the retention volume of a non-sorbed component, Kli is the distribution constant of

17

3.1. General considerations Table 3.2. Nitroarenes identified in airborne particulates in Tokyo air [18] Compound

1 2 3 4 5

6 7 8 9 10 11 12 13 14 15 16 17 18 19

o-Ethylnitrobenzene pEthylnitrobenzene pDinitrobenzene 2,6-Dinitrotoluene 2,4-Dinitrotoluene 1-Nitro-2methylnaphthalene 4,6-Dinitro-rn-xylene 3,s-Dinitro-o-xylene 1,5-Dinitronaphthalene 1,3-Dinitronaphthalene 5-Nitroacenaphthene 2-Nitrofluorene 9-Nitroanthracene 4,4’-Dinitrodiphenyl 2,s-Dinitrofluorene 1-Nitropyrene 2,7-Dinitrofluorene 4-Nitro-pterphenyl 6-Nitrochrysene

Relative retention.) Phenylmethylsilicone

Carbowax 20M

Reference

Sample

Reference

Sample

0.340 0.432 0.564 0.611 0.696 0.795 0.796 0.841 1.13 1.16 1.18 1.29 1.30 1.51 1.61 1.68 1.75 1.77 1.90

0.346 0.439 0.568 0.606 0.690 0.792 0.798 0.841 1.13 1.16 1.17 1.28 1.30 1.51 1.60 1.68 1.75 1.78 1.90

0.391 0.470 0.745 0.727 0.773 0.783 0.811 0.847 1.46 1.49

0.388 0.474 0.739 0.727 0.780 0.786 0.817 0.843 1.46 1.50

Concentration (ng/m3)

Mutagenicity

0.056 0.009 0.017 0.006 0.024 0.005 0.011 0.078 0.057 0.12 0.11 0.050 0.27 0.071 0.19 0.14 1.5 0.16 0.27

-

+ + + k + f + + + + + + i+ + + +

*) Internal standard: Benzo[qquinoline (retention time = 16.61 min for 5 % phenylmethylsilicone column and 27.46 min for Carbowax 20M column). Analysis of nitroarenes in airborne particulates was carried out under the following conditions. Neutral organic fraction was prepared from the particulates collected from 7 500 m3 air by a high-volume sampler. After adding 1.0 pg of benzo[flquinoline to a benzene solution of the neutral organic fraction (0.5 d ) , 3 pl auf the solution were injected on to the capillary gas chromatograph (HP 5880A) by a splitless injection mode. Identification was made by comparison of relative retentions of sample peaks with those of 40 reference substances. Quantitative determination was performed using calibration graphs for each nitroarene identified. Concentrations indicate the values obtained with the phenylmethyl silicone capillary column. The values obtained with the Carbowax 20M capillary column were as follows; 0.047, 0.009, 0.027, 0.005, 0.021, 0.006, 0.014, 0.083,0.045 and 0.083 ng/m3 for the compounds numbered 1,2,3,4,5,6, 7, 8 , 9 and 10, respectively.

compound i between the SLP and the gas and K I ,and Klc,+1) are the distribution constants of two standard compounds of carbon number z and z + 1, respectively, with K , , 5 K l i < Klc,+l). To evaluate Zoimeasurements are performed on several columns differing in SLP content on the solid support. The contribution of adsorption to the retention index value may be appreciable (several to a few tens percent). In capillary chromatography, adsorption also affects the retention values [16]. At the usual low pressures, the role of the carrier gas lies mainly in transferring solutes along the column. At higher pressures, however, the non-ideality of the gas phase begins to count, which alters the distribution of the solute between the mobile and stationary phases. The chromatographic mobility of many substances increases. 3

Berezkin, Gas Chrom-BE

18

3. Gas chromatography in the analysis of air pollutants

Fig. 3.4. Schematic diagram of a gas chromatograph. 1 High-pressure carrier gas source; 2 carrier gas preparation unit; 3 sample injection unit; 4 chromatographic column; 5 thermostat; 6 detector; 7 recorder; 8 minicomputer for instrument control and data processing

High-pressure gas chromatography (HPGC) is a technique intermediate between gas and liquid chromatography. HPGC has the following advantages over the latter: (1) the possibility of directed variation of the solute retention volumes by varying the pressure over a wide range, (2) rapidity of the analysis owing to the lower viscosity of the mobile phase and higher diffusion coefficients, (3) the possible application of universal highly sensitive detectors employed in gas chromatography. However, the HPGC instrumentation and experimental procedures remaining fairly complicated, which restricts the large scale application of this technique. A particularly interesting variant is chromatographic separation with the gaseous mobile phase in the supercritical state. Supercritical fluid chromatography (SFC) has made it possible to separate porphyrins, which could not be analysed at high temperatures owing to their thermal instability. The use of carbon dioxide and ammonia in the supercritical state proved efficient in separating compounds with molecular weights as high as 40000. The unusual characteristics of HPGC prompted the following classification of gas chromatography according to the state of the mobile phase: (1) GC at mobile phase pressures of 1-25 atm and (2) GC with the mobile phase at high pressures (100-500 atm), including the supercritical state. A schematic diagram of a gas chromatograph is shown in Fig. 3.4. The carrier gas is continuously supplied from the high-pressure cylinder I to the carrier gas preparation unit 2, where it is further purified and the required parameters of the mobile phase (pressure, velocity) are maintained throughout the experiment. The carrier gas then passes to the sample input system 3, which normally in laboratory chromatographs is an independently thermostated cylindrical flow chamber provided with a self-sealing thermostable rubber septum through which a sample (1-10 pl) is injected from, e.g., a syringe (9) into the carrier gas flow at high temperature. In the input system the liquid sample rapidly vaporises and the carrier gas flow transfers it to the chromatographic column 4 located in a thermostat. The separation is usually run at 20-400°C. Special thermostats allow the separation to be performed at temperatures as low as liquid nitrogen boiling point, which is mainly used in separating the isotopes of low-boiling gases. Analytical separations are usually performed on 0.5-5 m X 0.5-4 m m I.D. packed columns and 10-100 m x 0.2-0.5 mm I.D. open capillary columns. The columns are packed with sorbents consisting either of a non-volatile liquid (stationary phase) supported as a thin coating on a solid macroporous carrier with low specific surface area (0.5-2 m2/g) or of a solid with a developed surface (50-500 m2/g). The weight of the stationary phase is normally 2-20% of that of the solid support. The mean diameter of the sorbent particles is usually 0.1-0.4 mm, but fine fractions are used when filling the column. The separation of the components of a mixture occurs in a chromatographic column as a result of the solute bands moving at different velocities. The bands are passed by the carrier gas flow to a detector 6 whose signal, which is proportional to the analyte masses, is recorded

3.1. General considerations

19

continuously by a recorder 7. A microcomputer 8 serves for instrument control and data processing. Gas chromatography is a hybrid technique: separation of a mixture into components occurs in the chromatographic column, and quantitative analysis of the eluate is performed by the detector. Hence separation and quantitative (and, frequently, qualitative) analysis are separated in time and space. Therefore, the final result of the analysis is determined by the characteristics of both the column and the detector. The detector plays a very important role in gas chromatography. Now mostly differential detectors are used whose signal is proportional to the instantaneous value of the solute concentration or mass flow. The most useful types are the katharometer (thermal conductivity detector) and flame ionization, electron-capture and flame photometric detectors. The highly sensitive ionization and flame photometric detectors are employed in the analysis of trace impurities and very small samples and in capillary chromatography. The analytical potential of gas chromatography can be substantially expanded by employing several detectors. This makes it possible to determine quantitatively and qualitatively the composition of unresolved bands containing two or more compounds, provided that the selectivity of their detection by the detectors employed is different. The use of the mass spectrometer for detection has led to the development of highly e f i cient combined gas chromatography-mass spectrometry (GC-MS). A gas chromatograph with a packed or capillary column is connected to a mass spectrometer via a molecular separator serving to concentrate the separated bands and partially remove the carrier gas. The ion sources of a number of mass spectrometers equipped with powerful high-vacuum pumps sometimes allow the direct connection of a chromatographic column to a mass spectrometer without a molecular separator, with the solutes passing directly from the column to the ion source for ionization. The flow of charged particles is then supplied to a mass analyser where separation according to mass-to-charge ratio ( m l z ) occurs. Each substance is characterized by its specific mass spectrum, reflecting its structure. The analyte is identified by comparing its mass spectrum with standard spectra measured on pure samples or available in the literature. The mass spectrometer can be used first as a highly selective detector for taking chromatograms at a fixed mass-to-charge ratio and second for the rapid monitoring of a full mass spectrum during chromatographic analysis. Computers are widely used for data processing. The sensitivity of a mass spectrometer is l O - ” - l O - ” mg/s. GC-MS is currently one of the most universal and informative instruments and it is widely used in the analysis of air pollutants. Figure 3.5 demonstrates the relationship between the various parameters and the criteria of chromatographic separation for gas chromatography. It is made in the form of two quasicircumferences one of them includes criteria of chromatographic separation, another one units the parameters of chromatographic experiment. A similar approach was described by BERRIDGE for HPLC [20]. The inner circumference indicates the following major criteria: R,, peak resolution; t , , time of separation; AP, pressure difference on the column (the pressure drop); S, response factor (peak height) for a bulky (1) and a smaller sample (2). Intermediate parameters (dotted circumference) are k’, capacity factor; a,selectivity factor; N , column efficiency (number of theoretical plates); 7 , carrier gas viscosity; ps, volume fraction of the carrier gas component with displacing properties; d , , column diameter; T, column temperature; u , carrier gas linear velocity; d , , size of solid support (adsorbent) particles; L, column length; P,,content of SLP on the solid support; MCS, weight ratio between the mixed stationary phase components. The data in Fig. 3.5 indicate a complex relationship between the various parameters and the criteria of the chromatographic method. It is recommended that the relationships are used in the compilation of the programme for the optimization of a chromatographic separation.

20

3. Gas chromatography in the analysis of air pollutants

Fig. 3.5. Relationship between the various characteristics of chromatographic separation

Gas chromatography allows one to carry out qualitative and quantitative analyses of organic and inorganic compounds whose vapour pressures at the temperature of the column exceed 0.001-1 mm Hg and which are thermally stable at the decomposition temperature. This method is widely used for determination of compounds present in samples in trace amounts (10-4-10-8%).

3.2.

Peculiarities of the gas chromatographic analysis of impurities

The difficulties associated with the gas chromatographic determination of impurities are mainly due to the fact that when in very low concentrations the latter show a behaviour different to that exerted by macroscopic amounts of the same substances. A detailed discussion of this problem has been given elsewhere [21-231. Organic pollutants are often preconcentrated at reduced temperatures in order to analyse them in air (see, e.g., ref.24). An interesting example of determining Arctic hydrocarbon air pollutants at ppt levels was described by Norwegian researchers [25]. The detection limit was 1-5 ppt. The Arctic air was shown to contain various gases of this type (C2-C4). The 'main problem to be faced in the determination of trace impurities (10-4-10-8%)in air is potential analyte losses that may accompany any step of the gas chromatographic analysis. The disappearance of trace impurities can occur when taking a sample (sorption by container walls, incomplete consumption and chemical reactions in the concentrator, etc.), during the desorption of analytes from the trap (inefficient desorption), when injecting the samples into the chromatograph (e.g., sample decomposition in the vaporizer at high temperature) and owing to sorption on or chemical reactions with the sorbents and the inner surfaces of the

3.2. Peculiarities of the gas chromatographic analysis of impurities

21

chromatographic equipment. In the latter instance we are dealing with phenomena similar to the adsorption of traces of isotopes in radiochemistry. The concentration of the active sites in the sorbent becomes comparable to that of the analytes and therefore adsorption, which is insignificant at normal concentrations, is important at micro concentrations. Overestimated results may arise from analyte vapour adsorption on the walls of the sampling valve. There are various ways of preventing such losses, but the most useful involves continuous conditioning of the column and the whole system with analytes in order to saturate active sorbent sites and the equipment surfaces. Such conditioning may take a long time (several hours), especially when analysing reactive inorganic gases or unstable and readily hydrolysed substances (nitrogen and sulphur oxides, ozone, halogens, mixed halogen compounds, hydrides, etc.) even if all the parts of the chromatograph are made of materials resistant to corrosion (nickel, PTFE, glass, etc.), and inert sorbents (PTFE, graphite, polymers based on polytrifluoromonochloroethylene) are utilized for separation [26]. Frequently, the preliminary sorbent treatment takes about 20-30 h (e.g., for nitrogen dioxide) and must be repeated (1-2 h) before each set of analyses. It should always be borne in mind that the absence of peaks of the impurities being analysed (especially when analysing reactive, unstable and readily hydrolysed substances) by no means indicates their absence in the sample or an undetectable concentration. This may be due to irreversible sorption or chemical reactions undergone by the impurities (hydrolysis, pyrolysis, reactions with sorbents, etc.). Such interactions can be minimized by taking advantage of cryogenic gas chromatography. By analysing corrosive and reactive compounds (ozone, nitrogen, halogen and sulphur oxides, fluorides, etc.) at low temperature (below - 20°C) or with a programmed increase from - 100°C until the elution of one of the components (e.g., sulphur or nitrogen dioxide) one can successfully separate the impurities of corrosive substances in the presence of moisture and low-boiling compounds (CO, C 0 2 , N2, 0 2 , etc.) without losses. One of the problems attending the analysis of impurities is the drying of air dehumidification, which is particularly important when dealing with large samples. As the amount of moisture accumulated in traps is several orders of magnitude greater than that of the concentrated impurities, analyte losses often result which requires the solutions being analysed to be strongly diluted. A widely used means of drying air consists in passing it through a dehydration plug filled with absorbents or chemicals that selectively absorb water. One of the best absorbents of this type are molecular sieves 3A, which absorb water but allow organic and inorganic impurities except ammonia and methanol to pass through [27]. The interaction of analytes with the chromatographic equipment and sorbents results in insufficient resolution when analysing trace amounts of flavours and many other compounds of natural and biological origin. One of the means of increasing the sensitivity of such analyses is to utilize packed or capillary columns and vaporizers made of fused silica, which decreases the possibility of irreversible adsorption. Another helpful approach consists in injecting a sample of thermally unstable compounds or large volume of diluted solution directly into the chromatographic column. The purity of the solvent used for the extraction of impurities form the sorbent following sample concentration is of great importance. The solvent must not contain impurities at concentration comparable to and eluted within a short time interval of the analyte substances. It is desirable that these sorbents (e.g., chlorobenzene) should be purified of impurities by ordinary distillation. Of no less importance is the purity of the carrier and diluent gases employed for the preparation of standard mixtures and the calibration of detectors. The thorough purification of cylinder nitrogen used for sorbent vaporization and trace compound analysis by GC and GC-MS involves five traps connected in series [28]. Two of them are filled with a mixture of Carbopak, activated carbon and Chromosorb W, the latter being coated with Apiezon. The

22

3. Gas chromatography in the analysis of a h pollutants

third, fourth and fifth traps are filled with Zeolyte 3A, silica gel impregnated with 20% sulphuric acid and Carbosieve S , respectively. Neglect of the above requirements may cause appreciable errors and make the results of the analysis useless. One of the significant sources of errors can be the glass-wool used as an insert or plug in chromatographic columns and splitters. The treatment of used glass-wool with n-pentane, dichloromethane, gaseous hydrogen chloride, and carbon disulphide followed by gas chromatography recovered traces of previously analysed hydrocarbons, phthalates and organic acid salts at levels of 0.3-0.005 mg/g. It is advisable for the glass-wool to be pretreated with gaseous hydrogen chloride and then extracted for 24 h with dichloromethane in a Soxhlet apparatus. It is also recommended that its surface be silylated. Another frequently occurring source of errors is gases evolved from the septa and vaporizers of chromatographs, which give rise to ghost peaks that distorts the results. The composition and concentration of these gases have been established to depend on the septum material, temperature and the period during which the septum was used in the chromatograph. The best properties are exhibited by septa made of perfluorinated elastomers and some silicone rubbers. At any step in the analysis of impurities one should take into account the possibility of thermal and exclusive desorption of the components previously adsorbed on the column or their exclusion from subsequent samples by more polar compounds ( e g , water and acids). Competitive sorption is also observed when concentrating impurities from air, the possible exclusion of one compound by others creating conditions that are unfavourable for sorption of the former, as occurs with non-polar impurities in the presence of polar substances. When the concentration of impurities in a trap containing a sorbent increases by a factor of 102-105and the molecules are within the range of the action of the sorbent, the possibility of chemical (catalytic, heterogeneous, etc.) transformation of the sample increases sharply. Sample heating during thermal desorption (150-250°C) further increases this possibility owing to potential pyrolysis and other interactions of the concentrated compounds. This substantially distorts the results of analysis and frequently makes them useless under these conditions. Catalytic reactions lead to losses of styrene when it is being concentrated on charcoal, and the analysis of atmospheric air containing alkenes and halogens (chlorine and bromine) performed in the presence of ozone results in the formation of halogenated hydrocarbons on the column sorbent. Thus, the presence of butenes and chlorine gives rise to 2,3-dichlorobutene and such reactions can occur on both porous polymers and carbon-containing sorbents. To avoid false peaks of halogenated hydrocarbons, glass-fibre impregnated with 10% sodium thiosulphate solution is installed before the sorbent. The false peaks of halogenated hydrocarbons change with the ozone and nitrogen oxide concentrations in the atmosphere. When concentrating toxic impurities from flue gas on Tenax, the sorbent undergoes partial transformation into 2,6-biphenyl-p-quinone under the action of sulphur dioxide and nitrogen oxide. The analysis of sulphur and nitrogen oxides contained in industrial emuents is accompanied by the sulphonation and nitration of porous polymeric sorbents (Tenax and XAD 21, which affects their capacity and selectivity (especially with polar sorbents). Gas chromatography has shown ihat extremely toxic N-nitrosodimethylamines may be formed in a Tenax concentration trap during sorption from air through the interaction of dimethylamine present in the air with nitrogen oxides and ozone in the presence of traces of moisture [29]. This is a serious obstacle to the concentration and quantitation of N-nitroso compounds. The false peaks of the latter are also formed when impurities are concentrated on other sorbents (charcoal, silica gel, alumina, Florisil and the sorbents treated with aqueous KOH solution, ascorbate or phosphate-nitrate buffer). The formation of “false” N-nitrosamines is particularly intensive with activated carbon and silica gel.

References

23

Account should also be taken of the direct interaction of the sorbent in the trap with the impurities being analysed, which leads to losses of the latter. This can be exemplified by the absorption of nitrogen dioxide impurities by Porapak Q (accompanied by sorbent nitration) or irreversible interaction between sulphur-containing compounds and activated carbon..The concentration of ozone and chlorine dioxide impurities is not safe because when concentrated these compounds are explosive [26]. When employing the thermal desorption of impurities from sorbent-containing traps it is essential that the temperature applied is not too high (150°Cis the optimal value), as certain compounds undergo isomerization at 250°C to yield “false” compounds of higher molecular mass. Thus, C1-C3 hydrocarbons give cis-2-butene and cis-2-pentene under these conditions 1301. It should be noted that the adsorptive activity of the inner walls of capillary columns, including the quartz type, may cause irreversible adsorption of polar analytes [31, 321. The above peculiarities in the determination of impurities should be taken into account when analysing air pollutants.

References (Chapter 3) BEREZKIN, V. G. ; GAVRICHEV, V. S.; KOLOMNETS, L. N.; KOROLEV, A. A,; LIPAVSKII, V. N.; NIKITINA, N. S.; TATARINSKII, V. S.: Gazovaya Khromatografiya v Neftekhimii (Gas Chromatography in Petrochemistry), Moscow: Nauka 1975, p. 17. GROB,R. L. (Ed.): Modem Practice of Gas Chromatography. New York: John Wiley 1985. KATZ,E. (Ed.): Quantitative Analysis Using Chromatographic Techniques. Chichester: John Wiley 1987. SUPINA,W. R.: The Packed Column in Gas Chromatography. Bellefonte, Pennsylvania: Supelco 1974. S. A,: Contemporary Practice of Chromatography. Amsterdam: Elsevier POOLE,C. F.; SCHUETI’E, 1984. LEIBNITZ, E.; STRUPPE, H. G. (Ed.): Handbuch der Gaschromatographie. Leipzig: Akademische Verlagsgesellschaft Geest & Portig K.-G. 1984. KISELEV,A. V.; YASHIN,Y. I.: Gas-Adsorption Chromatography. New York: Plenum Press 1969. BEREZKIN, V. G.: J. Chromatogr. 98 (1974) 477. BEREZKIN,V. G. In: GIDDINGS, J. C.; GRUSHKA, E.; BROWN,P. R. (Eds.): Advances in Chromatography, vol. 27. New York: M. Dekker 1987, pp. 1-35. BEREZKIN,V. G.; RETUNSKY, V.N.: J. Chromatography 330 (1985) 71. JENNINGS, W.: Gas Chromatography with Glass Capillary Columns. New York: Academic Press 1980. JENNINGS, W.: Comparisons of Fused Silica and Other Glass Columns in Gas Chromatography. Heidelberg: Huthig 1981. LEE,M.; YANG,F.; BARTLE, K.: Open Tubular Column Gas Chromatography. New York: Wiley-Interscience 1984. GROB,K.; GROB,G.: J.Chromatogr. Sci. 7 (1969) 584. GROB,K.; GROB,G.: J. Chromatogr. Sci. 7 (1969) 587. BEREZKIN, V. G.: Gas-Liquid-Solid Chromatography (in Russian). Moscow: Khimiya 1985. MASHALY, M.; SANDRA, P. In: VIIIth Intern. Symp. on Capillary Chromatography. Vol. 1. Riva del 0. (Ed.): Heidelberg: Huthig 1987, p. 476. Garda, May 19-21, 1987. SANDRA, MATSUSHITA, H.; IIDA,Y.: J. HRC and CC. 9 (1986) 710. BEREZKIN, V. G.: J. Chromatogr. 159 (1978) 359. BERRIDGE, J. C.: Techniques for the Aromated Optimization of HPLC Separations. Chichester: John Wiley 1986. BEREZKIN, V. G.; TATARINSKII, V. S.: Gas-Chromatographic Analysis of Trace Impurities. New York: Consultants Bureau, 1973.

24

3. Gas chromatography in the analysis of air pollutants

Yu.S.; BEREZKIN, V. G.: Gas Chromatographic Analysis of Pollution Air. (in Russian). DRUGOV, Moscow: Khirniya 1981. JENNINGS, W.G.; UP, A,: Sample Preparation for Gas Chromatographic Analysis. Heidelberg: HUthig 1983. BRETIZLL, T.A.; GROB,R. L.: Intern. Lab., Apr. 1985, p. 30. SCHIDBAUER, N. S.; OEHME,M.: Journal HRC and CC. 8 (1985) 404. ANVAER,B. I.; DRUGOV,Yu.S.: Gazovaya Khromatografiya Neorganicheskikh Veshchestv (Gas chromatography of inorganic compounds). Moscow: Khimiya 1976. BEREZKIN, V. G.; DRUGOV,Yu.S.; GORYACHEV, N. S.: Zhurnal analyticheskoi khirnii. 37 (1981) 319. WESTRICK, T. J.; LAMPARSKI, L. L.: Anal. Chem. 53 (1981) 22. BERKLEY, R.E.; PELLIZZARI, E.D.: Anal. Lett. A l l (1978) 327. CRIP, S.: Ann. Occup. Hyg. 23 (1980) 47. PURCELL, J . E.: Chrornatographia 15 (1982) 546. BLOMBERG, L.G.: Journal HRC and CC 7 (1984) 232.

Chapter 4

Detectors for the Gas Chromatographic Determination of Impurities Gas chromatography is a hybrid method (11 in which a column serves for separation and a detector for the quantitative determination of the separated components. The determination is effected in flow of carrier gas to which, as a rule, the detector is insensitive. In this way, the chromatographic separation enables one to simplify the problem of quantitative determination; instead of the initial problem of determining the composition of a multi-component mixture, the detector after chromatographic separation solves a number of simpler problems such as the determination of the concentration of one of the constituents of the mixture to be analysed as a binary mixture with the carrier gas. The characteristics of the detector essentially determine the most important overall paand rameters of the chromatographic technique. Therefore, as pointed out by ZHUKHOWTSKI TURKEL’TAUB. “The history of progress in gas chromatography is to a certain degree the history of detector development” [2]. The problems of detection have been considered in a number of excellent books [3-51. Moreover, this problem has been discussed in detail in some books covering general aspects of gas chromatography [5-81. The selectivity of determination is a function of the properties of both the column (sorbent) and the detector. In practice, it is usually advantageous to use a highly sensitive detector that selectively reveals contaminants of interest (hazardous or otherwise important for industrial hygiene) and producing no response to the major substance. In some instances it is expedient to use directed chemical conversions to change the sensitivity to contaminants of interest and to the major component. A book has been devoted to the use of chemical methods in the analysis of impurities [9].

4.1.

Principal characteristics of detectors for gas chromatography

The most important detector characteristics are sensitivity and detection limit together with the linearity and response time of the detection system. Sensitivity is one of the most significant parameters of a chromatograph. Increasing the sensitivity allows one to lower the detection limit for admixtures and to improve the separation in some instances. Under otherwise equal conditions (the degree of back-diffusion, decrease in concentration of components due to dilution with the carrier gas, etc.), the sensitivity of a gas chromatograph is determined by the sensitivity of the detection system employed. Therefore, we shall consider below the sensitivity of detection systems. It should be noted that sometimes high sensitivity of a detection system cannot be effectively used owing to unfavourable characteristics of the chromatograph. These may be insufficient, e.g., to meet the requirements of precision of thermostating, stability of gas flow-rate or thermal stability of the sorbent, which causes a significant noise signal. At present only differential detectors are employed in gas chromatography. They measure an instantaneous concentration (concentration-dependent detectors) or a mass flow-rate (mass flow-rate-dependent detectors) of a substance of interest in the carrier gas flow. The chromatographic peak area of each component of a mixture is proportional to its concentration in the sample and to the corresponding sensitivity of the detector.

4. Detectors for the gas chromatographic determination of impurities

26

[lo]and later by HAThe above classification of detectors was proposed by KALMANOVSKY LASZ [ 111.

For a concentration-dependent detector, the relationship of the output signal E , to concentration is

E, = S,C

(4.1)

where S, is the sensitivity of the detector and C is the instantaneous concentration of the substance in the volume of the detector chamber. For mass flow-rate-dependent detectors, the relationship of the mass flow-rate of the substance to the output signal Ej is

Ej = Bj

(4.2)

where B is the sensitivity of the detector and j is the mass flow-rate of the substance through the detector: j = dq/dt

(4.3)

where q is the amount of the substance reaching the sensor and r is time. The sensitivity of a gas chromatographic detection system is the ratio of the change in its output signal to the corresponding change in concentration or mass flow-rate of the substance being analysed in the detector input. The sensitivity of a concentration-dependent detector is expressed as

where Sfis the arithmetic mean of the peak areas found in five independent measurements (cm*), A , the sensitivity of the recording unit (mV/cm), Q the carrier gas flow-rate (cm3/min), M the ratio of the detector signal to the input signal of the recorder, q the volume of a control substance (cm3), e the density of the control substance under conditions of detection (mg/cm3)and fi the chart speed of the recorder (cm/min). The sensitivity of a mass flow-rate-dependent detector is determined (in C/mg) by

where A l is the sensitivity of the measuring circuit, i.e., the ratio of the signal of an electrometer amplifier to the length of the recorder scale (Ncm) and qm is the mass of the control substance loaded into the column (mg). In chromatographic measurements, the minimum measurable signal Eminis usually taken as twice the fluctuation noise signal 6, that is, Emin= 26.

(4.6)

To estimate the value of weak signals that can still be detected by a particular chromatograph, the term “detection limit” is used. The detection limit of a gas chromatographic detection system is actually the minimal concentration or mass flow-rate of the desired substance that can be reliably measured over the noise. In chromatography the detection limit is defined as the concentration or a mass flow-rate that induces a chromatographic peak twice as high as the fluctuation of the noise level. The detection limits for a gas chromatograph and the corresponding detection system usually coincide, although the former may sometimes exceed the latter if the chromatograph is of insufficient quality.

27

4.1. Principal characteristics of detectors for gas chromatography Table 4.1. Detector response as a function of amount of the substance to be analysed (taken from ref. 10)

Type of detector

Kind of signal peak area

Concentrationdependent

s-

Mass flow-ratedependent

S. J = A 14 .

S,'q

f-Q

peak height (for Gaussian peaks) h, = 2.51 -.

hj =

2.51 A j f i Q ' 4 VR

For a concentration-dependent detector the detection limit (%) is

Cmin= 26 lOO/S,e.

(4.7)

For a mass flow-rate-dependent detector the detection limit (mg/s) is j,, = 26/B. (4.8) As follows from eqns. (4.7) and (4.8), among detectors with identical noise levels, that having the highest sensitivity will always have the lowest detection lilhit. If the noise levels are different, a detector of lower sensitivity can nevertheless have a better detection limit. Within the linear range of a detector its sensitivity corresponds to slopes of straight lines E, = ScC or Ej = Bj plotted as E, versus C or Ej versus j,respectively, the minimal measurable signal being Emin= 26. The detection limit can be decreased either by increasing the sensitivity or by decreasing the noise level. When investigating the possibility of detecting low concentrations of a substance, one should bear in mind that its concentration in a sample should exceed the detection limit in proportion with the dilution of the substance during separation caused by back-diffusion. The detector response as a function of the amount of the substance to be analysed is given in Table4.1 [lo]. The detection limits of mass flow-rate- and concentration-dependent detectors are interrelated as follows: jmIn = QCmin.

(4.9)

Detection systems usually deviate from linearity with increasing signal. The detector signal is considered to be linear if the deviation of the true signal from ideal linearity does not exceed 5%. The maximum signal corresponding to the linear range of measurement can therefore be determined as (4.10) where El is the detector signal corresponding to the linear range limit at ideal linearity and Em is an actually measured signal. As the minimal measurable concentration is equal to the detection limit, the linear range of measurement (also called the linear dynamic range) can be expressed as the ratio of the input signal causing a 5% deviation of the output signal from linearity to the lowest detectable signal. Errors due to the lack of linearity for detection extending beyond the linear range can be markedly reduced by using correction of the results based on a previously performed calibra-

28 ThI5

4. Detectors for the gas chromatographic determination of impurities

Error of peak-height measurement

Error of peak-area measurement

(%I

(%)

0.085

0 1.5

0.170 0.255 0.340

6.1 11.4 16.7

0.428

21.8

0

0 0 0.5

Table 4.2. Experimental error as a function of the ratio of the detector time constant to the peak width

1 .o 3.0 4.8

tion. However, even in this instance the experimental error will be higher than that for measurements made in the linear range owing to the decrease in detector sensitivity. The dynamic properties of a detection system are determined by its response time (inertia). The response time results from contributions of various elements of the detector and the measuring circuit, depending on the type of the detection system. To a first approximation, a detection system can be considered as a first-order aperiodic unit that is characterized by a single time constant T. For a stepwise variation of the input signal, the output signal should in this instance change as E=Ek(l-e-'?

(4.11)

where E is the instantaneous amplitude of the output signal and Ek is the final amplitude of this signal at the new amplitude of the input signal. The time constant T is the time period during which the amplitude of the output signal reaches 0.632 of its final value with stepwise variation of the input signal. Distortions due to the inertia of the detection system are increased when the rate of variation of the input signal increases. Table4.2 gives values of errors in the results of analyses due to inertia of the detection system as a function of the ratio between the rate of peak alterand the detector time constant T (121. ation (represented by peak width

4.2.

Flame ionization detector

The operation of this detector is principally based on the phenomenon of the appearance of charged particles in a hydrogen burner flame if traces of organic compounds are present. This ionization process results in a sharp increase in the electric current between the burner jet playing the role of one electrode and a second electrode located above the flame. The strength of the induced current is proportional to the flow-rate of organic material through the hydrogen burner flame of the detector. The flame ionization detector was proposed and DEWAR[13]. It is highly sensitive and practically universal, 30 years ago by MCWILLIAM but its sensitivity decreases in the series hydrocarbons > esters > alcohols > acids. The detector is suitable for detecting organic substances at concentrations of lo-'- lo-"%.

4.3.

Thermionic detector

The thermionic detector is a flame ionization detector in which the flame is continuously fed with traces of an alkali metal salt (potassium, rubidium, caesium), and is characterized by high selectivity and sensitivity. Various modifications of the thermionic detector have been

29

4.4. Photoionization detector

Table 4.3. Selectivity of the photoionization detector (solvents producing no positive response for UV lamps having different radiation energies) (taken from ref. 17)

w lamp radiation energy (em 9.5

10.2

11.7

H20 CH3OH CZHSOH CH,Cl CHzC12 CHClS CCI4 Cl2HCCHClz Freons CSHl2

HZO CHSOH CHjCl CH3Cl CHzClz CHCI, CCI4 ClzHCCHClz Freons

HZO Freons (some)

C6H14

ClH15

described, mostly differing in the method of delivering the alkali metal ions into the hydrogen flame. Thermionic detectors for the detection of phosphorus- and nitrogen-containing com[14] and by WELLS[15], respectively. In pounds were proposed by KARMENand GUIFFRIDA one of the widely used detector modifications the source of metal ions is an electrically heated ceramic cylinder with the walls covered with a rubidium salt. Inside this cylinder a low-temperature plasma is formed, the conductivity of which is determined by the concentration of nitrogen- or phosphorus-containing compounds entering the detector [16]. In comparison with the flame ionization detector, the sensitivity of the thermionic type is approximately 50 times (and more) and 500 times higher for nitrogen- and phosphorus-containing compounds, respectively. The selectivity of this detector for these compounds with respect to organic substances (e.g., hydrocarbons) is as high as ca. 1 X lo4.

4.4.

Photoionization detector

The operating principle of the photoionization detector [17-191 is based on absorption of ultraviolet radiation by the sample substance followed by its dissociation into an ion and an electron (ionization): A + hv+A++ewhere A is a molecule of the substance to be analysed, A+ the ionized molecule, e - an electron and h v a photon with energy above the ionization potential of the molecule. The ionization results in a large increase in the conductivity of gas in the detector chamber, proportional to the concentration of the substance therein. A photoionization detector consists of an ultraviolet lamp (the source of ionizing radiation) located adjacent to an ionization chamber containing a pair of electrodes. Application of an electric potential to the electrodes enables one to quantitate the ions produced by absorption of the ultraviolet light by the molecules of a sample compound. To ionize molecules of a sample substance, UV lamps with a radiation energy of 9.5-11.7 eV are available. Using a lamp with an energy of 11.7 eV makes it possible to analyse compounds of various classes including aliphatic hydrocarbons. It is also possible to use lamps of lower radiation energy but sufficient to ionize compounds of analytical interest. In this interest compounds having

30

4. Detectors for the gas chromatographic determination of impurities

higher ionization potentials will not be detected, and the photoionization detector will show selective properties [20]. As the photoionization detector is concentration dependent and non-destructive, it is advisable to use it connected in series with other gas chromatographic detectors, e.g., a flame ionization detector. Taking into account the features of the photoionization detector, it is expedient to use it in capillary chromatography. The detection limit of the photoionization detector for organic species is 10-100 times that of the flame ionization detector. The detector has a sufficiently wide dynamic range of 1 X lo’. Table 4.3 [17] shows some solvents used with photoionization detectors that produce no positive detector response for photoionization sources having different radiation energies. These solvents often represent the main substance (carrier) in sample mixtures.

4.5.

Electron-capture detector

The electron-capture detector consists of a chamber with a pair of electrodes to measure an ionic current and an electron-emitting radioactive source. Owing to collisions with molecules of the carrier gas which flows through the detector chamber after the chromatographic column, the emitted electrons are retarded and turn into a number of slow thermal electrons. Application of an electric potential to the electrodes of the chamber will thus induce an appreciable background current. On passing through the detector, the “electron-trapping” compounds react with electrons to form anions and the electric current across the chamber sharply decreases, the decrease being proportional to concentration of the compound in the chamber. The electron-capture detector is widely used to detect polyhalogen-containing compounds, including many pesticides and polychlorinated biphenyls, polyaromatic compounds, nitriles, organo metallic and sulphur-containing compounds. The detector is also used for other compounds that are quantitatively converted into derivatives capable of being detected by the electron-capture detector with high sensitivity (e.g., see ref. 9). The electron-capture detector was proposed nearly 30 years ago by LOVELOCK and LIPSKY [21]. The bases of the theory and practical application of this detector, especially in the field of environmental protection and biomedicine, were covered in a book [22]. It has been shown that addition of a “dopant” (e.g., nitrogen oxide) to the carrier gas could dramatically increase the sensitivity of the electron-capture detector to those compounds which usually produce very low signals [23, 241.

4.6.

Flame photometric detector

The flame photometric detector can be considered as a flame emission photometer. The compounds to be analysed are burnt in a hydrogen-enriched flame which is similar to a low-temperature plasma. Under these conditions, atoms and simple molecules are undergoing a transition to a state of higher energy. During the reverse transition to the ground state the excited particles emit characteristic radiation. Optical filters used in the detector enable one to select a characteristic optical line, the radiation of which is amplified by a photomultiplier. The flame photometric detector is used for the determination of sulphur- and phosphoruscontaining compounds. The selective sulphur radiation is due to S2 emitting at a wavelength of 394 nm. Phosphorus is detected at 526 nm, the wavelength determined by HPO. The selectivity of phosphorus and sulphur detection with respect to hydrocarbons is 10s:l and 104:1, respectively. The sensitivity of a flame photometric detector is 1 X lO-”g/s for sulphur and

31

4.9. Hall electrolytic detector

1 X lo-’’ gls for phosphorus. The flame photometric detector was proposed by BRODYand CHANEY (251. The so-called double detector modification is a frequently used variant [26].

4.7.

The Thermal Energy Analyser (TEA) Detector

The TEA detector is a chemical emission-based, selective and highly sensitive device [27], used for the specific determination of N-nitroso compounds. A stream of carrier gas from the chromatographic column flows into a catalytic pyrolyser where the N-nitroso compounds are thermally decomposed to produce two radicals, nitrosyl and an organic radical. The two radicals are transported by the carrier gas flow into a reactor fed with ozone. Ozone reacts with the nitroso radicals, giving rise to electron-excited molecules of nitrogen oxide. The excited molecules return to the ground state, emitting near-infrared radiation, which is detected by a photomultiplier. The selectivity and sensitivity of the detector are sufficiently high, e.g., the detection limit for N,N-dimethylnitrosoamine is 100 pg.

4.8.

Detectors for direct identification of impurities

Detectors of this type include the highly sensitive and superselective Fourier transform infrared and mass spectrometric detectors. Gas chromatographs equipped with these detectors are frequently considered as special hybrid instruments (abbreviated to GC-FTIR and GCMS, respectively) and function mostly as micropreparative rather than analytical devices. In contrast, the analytical function in most instances is fulfilled by the detectors. The detection limit for the GC-FTIR system is 20-40 ng, some modifications of the instruments being even more sensitive [28-301. The GC-MS system is generally characterized by higher sensitivity and selectivity [31-341.

4.9.

Hall electrolytic detector

Various electrochemical detectors used in gas chromatography have been described. However, the simplest and most reliable is, to our knowledge, the Hall electrolytic conductivity detector [35-371. The electrolytic detector was first introduced in gas chromatography by COULSON[38]. The operation of this type of detector includes, first, oxidation (reduction) of nitrogen-, sulphur- or halogen-containing compounds to give stable inorganic compounds and, second, their dissolution in a stream of water followed by measurement of the electrolytic conductivity of the resulting aqueous solution.

Elemental composition

Generated products

of analysed compounds

oxidative method

reductive method

COZ HX NO2 (low yield) SOz and SO3

CH4 HX NH3

~

Carbon Halogen (X) Nitrogen Table 4.4. Products of catalytic conversion of organic substances prior to electrolytic detection

Sulphur Phosphorus

p4010

HzS PH3

32

4. Detectors for the gas chromatographic determination of impurities

Table 4.5. Major detectors for gas chromatography Detector

Field of application

Sensitivity (g/s)

Selectivity

Linear dyPrice namic range

1

2

3

4

5

6

Flame ionization

broad (CH-containing organic compounds) limited (selective detection of nitrogen- and phosphorus-containing organic compounds)

2 x 10-12

low

1 x 10'

medium

10-13

high 1 x 105 (N/C = lo4, PIC = 104)

medium

Thermionic flame ionization

(nitrogen) 5 x 10-14 (phospho-

mS) Electron-capture

Photoionization

Flame photometric

Electrolytic

limited (selective detection of organic molecules capable of capturing electrons, e.g., pesticides, polychlorinated biphenyls, polyaromatic hydrocarbons, nitriles, sulphur-containing compounds) broad (selective detection of the compounds whose ionization potential is lower than the radiation energy of the UV lamp employed

5X high (e.g., for carbon tetrachloride)

1 x lo2to 1 x 105

medium

2 x 10-13

1 x 10'

medium

limited (selective detection of sulphur- and phosphorus-containing organic compounds and some other elements) limited (selective detection of halogen-, sulphur- and nitrogen-containing organic compounds)

1 x 10-11 (sulphur) 1 x 10-12 (phosphorus) 5 x 10-13 (chlorine) 3 x 10-12 (sulphur) 3 x 10-12 (nitrogen) detecton limit 100 pg for dimethylnitrosamine)

103 (sulphur) 105 (phospho-

medium

Thermoenergetic

limited (selective detection of nitrosamines)

GC-FTIR

universal, selective

GS-MS

universal, selective

detection limit 200 pg4 0 ng detection limit 1 pg1 ng

high (determined by the ratio of radiation energy to ionization potential) high (SIC 104 P/C 105)

-

NS)

high (Cl/C 106 N/C lo6 SIC 105)

-

high

106

(chlorine) lo4 (nitrogen) 104 (sulphur) 106

medium

much higher than the medium price

high, changeable

104

very high

high, changeable

105-106

very high

References

33

The products generated during the catalytic conversion of organic substances are presented in Table 4.4. The data indicate the limits of practical use for this detector. The detector is equipped with a closed water circulation system which includes a column of an ion-exchange resin to remove dissolved ions. The selectivity of the Hall detector for nitrogen and chlorine with respect to carbon is better than 1 X lo6, and for sulphur being 5 x lo4.The sensitivity of this detector to nitrogen, chlorine and sulphur is 2 X 5 x and 2 x g/s, respectively. The Hall detector is used for the determinations of pesticides, herbicides, polychlorinated biphenyls and trichloromethane. Finally, it should be noted that the detectors described above may be successfully applied to the problem of air pollution control. For the trace analysis of complex mixtures it is advisable to employ a set of detectors that differing in selectivity and to modify chemically the compounds to be analysed. The principal specifications of gas chromatographic detectors are summarized in Table 4.5.

References (Chapter 4) 111 ZOLOTOV, Yu.A.: Essays on Analytical Chemistry (in Russian). Moscow: Khimiya 1977. A. A.; TURKEL’TAUB, N. M.: Gas Chromatography (in Russian). Moscow: Gostop[2] ZHUKHOVITSKI, techizdat 1962. [3] DAVID,D. J.: Gas Chromatographic Detectors. New York: Interscience 1974. J.: Detectors in Gas Chromatography. Amsterdam: Elsevier 1976. [4] SEVCIK, [S] KATz, E. (Ed.): Quantitative Analysis Using Chromatographic Techniques. Chichester: John Wiley 1987. V. G.; TATARINSKII, V. S.: Gas-Chromatographic Analysis of Trace Impurities. New York: [6] BEREZKIN, Consultants Bureau 1973. [7] LEIBNITZ,E.; STRUPPE,H. G. (Eds.): Handbuch der Gaschromatographie. Leipzig: Akademische Verlagsgesellschaft Geest & Portig K.-G. 1984. (81 POOLE,C. F.; SCHUE-ITE, S. A,: Contemporary Practice of Chromatography. Amsterdam: Elsevier 1984. V. G.: Chemical Methods in Gas Chromatography. Amsterdam: Elsevier 1983. 191 BEREZKIN, V. 1.: Works on Chemistry and Chemical Technology. (in Russian). Gorky: NII of [lo] KALMANOVSKY, Chemistry, GSU 3 (1960) 545. I . : Anal. Chem. 36 (1964) 1428. [ l l ] HALASZ, V.I.; ZHUKHOVITSKI, A.A.: J. Chromatogr. 18 (1965) 243. [12] KALMANOVSKY, [13] MCWILLIAM, I. G.; DEWAR,R. A,: Nature 181 (1958) 760. A,; GUIFFRIDA, L.: Nature 201 (1964) 1204. [14] KARMEN, [15] WELLS,C.: USFDA Pesticide Workshop, Kansas City, MO: May, 1966. P.: J. Chromatogr. Sci. 15 (1977) 53. [I61 KOLB,B.; AYER,M.; POSPISIL, J . N.; FORD,J.; JARAMILLO, L.; BECKER, J. H.; HEWITT,G.; MARSHALL, J. K.; ORISHUK, F.: [17] DRISCOLL, American Laboratory 11 (1978) 137. J . ; KRYSL,S.: Chromatographia 6 (1973) 375. [18] SEVCIK, J.N.: J. Chromatogr. 134 (1977) 49. [19] DRISCOLL, J. N . : J . Chromatogr. Sci. 6 (1982) 375. [20] DRISCOLL, [21] LOVELOCK, J . F.; LIPSKY,S. R.: J . Amer. Chem. SOC.82 (1960) 431. A,; POOLE,C. F.: Electron-Capture Detector. Theory and Practice in Chromatography. Am[22] ZLATKIS, sterdam: Elsevier 1981. [23] SIMMONDS, P.G.: J. Chromatogr. 166 (1978) 593. [24] PHILLIPS, M. P.; SIEVERS, R. E.; GOLDEN, P. D.; KUSTER,W. C.; FELSENFELD, F. C.: Anal. Chem. 5 1 (1979) 1819. S. S.; CHANEY, J. E.: J. Gas Chromatogr. 4 (1966) 42. [25] BRODY, P.L.; HOWE,R.L.; SHUMAYS, A.A.: Anal. Chem. 5 0 (1978) 345. [26] PAITERSON, 4

Berezkin. Gas Chrom -BE

34

4. Detectors for the gas chromatographic determination of impurities

[27] FINE,D. H.; ROUNBEHLER, D.P.: J. Chromatogr. 109 (1975) 271. [28] ERICKSON, M.D.: Appl. Spectrosc. Rev. 15 (1979) 261. G.; HUSMANN, H.; PODMANICZKY, L.; WEEKE,F.; RAPP,A. In: SCHREIER, P. (Ed.): Anal[29] SCHOMBURG, ysis of Volatiles. Berlin, New York: Walter de Gmyter 1984, p. 121. K.-H.; HERRES,W.: GIT-Supplement 5/84 Chromatographie, p. 33. (301 KUBECZKY, A.M.; SIMPSON, C.F.: J. Phys. 13 (1980) 1131. [31] GREENWAY, [32] TENNOEVERDE BRAUW, M. C.: J. Chromatogr. 165 (1979) 207. W. H.: J. Chromatogr. Sci. 17 (1979) 2. [33] MCFADDEN, [34] JENSEN,T. E.; KAMINSKY, R.; VEETY,B. D.; WOZNIAK,T. Y.; H m s , R. A.: Anal. Chem. 54 (1982) 2388. [35] HALL,R. C.: Crit. Revs. Anal. Chem. 8 (1978) 323. S O . ; GAGE,D.R.; KAGEL,R.A.: J. Chromatogr. Sci. 19 (1981) 358. [36] FARWELL, [37] HALL,R. C.: J. Chromatogr. Sci. 12 (1974) 152. D. M.: Adv. Chromatogr. 3 (1966) 197. [38] COULSON,

Chapter 5

Collection and Pretreatment of Samples for Chromatographic Analysis The sampling and identification of contaminants in complex mixtures is one of the most important aspects of modern analytical chemistry of air pollutants [l]. The recovery of trace amounts (up to lo-' mg/m3) of various chemical compounds from complex physico-chemical systems such as air is a difficult analytical problem [l,31. The collection and pretreatment of samples may involve some losses or chemical transformations of the sample, resulting in changes in the initial sample composition [l,2, 6-91, which may significantly affect the end results of the determination. Undesirable changes in the sample composition may also occur during concentration of the sample and its recovery from the concentrator (e. g., by thermal desorption), pretreatment of the sample, etc. [8-121. Qualitative changes in the composition of a processed sample may result from the use of high temperatures, the presence of oxygen and other oxidants, a considerable (1000-fold and more) increase in the concentration of all the components of a sample during the enrichment process, the presence of highly reactive, readily polymerizable or unstable compounds, etc. [ l , 8, 121. The collection of a sample of air and its concentration always involve a certain risk of changes in the sample composition. Irreversible processes that occur early in the analytical procedures should receive particular attention [8, 11, 121. The use of sophisticated instrumentation, inert sorbents and highly effective columns in subsequent stages of the analysis cannot give accurate information if the sample was pretreated incorrectly [8]. Sample pretreatment problems include collection of a representative sample, concentration of target components, transport of concentrated contaminants from the sampler, chemical modification of the sample and introduction of target components on to a chromatographic column. The problem of sample pretreatment for chromatographic separation and determination has been considered in great detail in reviews [2, 7, 111, books [8, 9, 121 and in separate chapters of books devoted to general chromatography (see, e.g., refs. 1 and 10). Determination of contaminants in workplace atmospheres usually requires a sample collection stage which lasts 4-8 h. This is necessitated by the requirements of industrial hygiene, specific technological aspects and the cyclic nature of operations [l, 121. The extensive employment of protracted collection procedures [l,10, 121 and the increase in the duration of air flow through a sorbent from 10-30 min to.severa1 hours have necessitated the reconsideration of the requirements of sample collection systems [l,8, 9, 121. Monitoring increases the number of parameters such as temperature, humidity, air flow-rate and the presence of contaminants that need to be considered [ll, 121 and enhances their importance. At present, methods of collection of various types of harzardous substances from air are largely confined to the collection of polluted air into samplers, absorption of contaminants by a solvent, their freezing out, sorption in tubes packed with a sorbent and trapping of solid particles and aerosols by filters [l, 11, 121.

5.1.

Sampling into containers

Contaminated air samples are collected in containers for monitoring gaseous (at normal temperatures) substances [l-51. Usually this procedure does not involve sample pre-enrichment.

36

5. Collection and pretreatment of samples for chromatographic analysis

Fig. 5.1. Schematic diagram of the equipment for the detection of peroxyacetyl nitrate (PAN) and its homologues in air [24]

The containers are composed of various reservoirs (vessels) made of stainless steel, glass or plastics. The air sample is passed through the container at a low rate (0.1-0.2 l/min) and sampled into a preevacuated vessel; the container can be also filled using a nipple device. An aliquot of the air in the container is then introduced into the chromatographic evaporator by means of a gas syringe [ l , 3, 51. This, the simplest sampling technique, allows the direct analysis of air pollutants with small relative errors (5-10%) as it does not require pollutant preconcentration [13]. The limitations of the container technique are as follows: restricted range of analytes; the risk of adsorption (chemisorption) of the compounds on the walls of the container and gas syringe; restricted range of concentrations to be determined, depending on the detector sensitivity; possible occurrence of chemical reactions in the container during sample storage (in the presence of air containing moisture and oxygen), especially with reactive gases; impossibility of obtaining a representative sample owing to the presence of compounds of low volatilitv in air. Adsorption on the internal walls of the container can be minimized by preconditioning the surface with suitable chemical reagents [l]. The detection limit can be lowered by subsequent concentration of the pollutants in the container, e. g., on solid sorbents or an inert material [14]. In the latter instance the air containing, e. g., vapours of solvents used in industry of lacker and paint) is displaced from the heated container (125°C) by a current of nitrogen (20 ml/min) into a liquid nitrogen-cooled trap (10 cm X 2 mm) filled with glass beads (30-50 mesh). Following trapping of the pollutants, the trap is heated to 200°C and the compounds are displaced by the carrier gas and analysed in a chromatograph [14]. However, this procedure is rarely used. A more popular technique is the direct analysis of gases in a container using highly sensitive detectors. It has been applied in the analysis of very low concentrations of Freons (10-4-10-5mg/m3) in air using an electron-capture detector [15, 161, formaldehyde using a photoionization detector [17], hydrocarbon gases (C,-C,) using a flame ionization detector [18], carbon monoxide and dioxide and other low-boiling gases (10-100 mg/m3) using a katharometer [18, 191, gaseous odorants such as hydrogen sulphide, ammonia, amines and mercaptans [20], vinyl chloride, vinyl acetate and other gases [ 19-23]. This type of analytical procedure can be readily represented schematically. The equipment for analysing peroxyacetyl nitrate and its homologues in air is shown in Fig. 5.1. This compound can be detected in contaminated air accumulated in a 3 ml miniature loop-like container using an ECD electron-capture detector at a 13 ppb by volume concentration [24].

37

5.2. Use of absorption of contaminants in sample collection

r

I

.

I I

I I

I L

‘ 4

5

3

.4

2

.6

bl

2 Fig. 5.2. Device for atmospheric air sampling [25]. 1,2 Flexible plastic panels; 3 sampling chamber; 4 elastic helical helico-conical spring; 5 self-sealing plug stopper for air inlet; 6 safety plate (against inadvertent piercing of the lower panel)

For the analysis of gaseous air pollutant mixtures, the most frequently used containers are bags made of laminated plastics such as PTFE, Lavsan, nylon, polyethylene, polypropylene, PVC, polyamide, etc., and fitted with a valve or nipple for filling [19-231. One of these devices is depicted in Fig. 5.2. It is intended for the rapid sampling of contaminated air and is composed of a small rectangular package with sealed edges and is made from a thin and durable inert material (polyethylene terephthalate) [25]. Inside the package there is a tension spring 4 made of stainless steel, which is compressed prior to sampling. The package has a small opening for intake of air, with a self-sealing stopper 5 made of PTFE (Fig. 5.2). During sampling the stopper 5 is removed and a moderate negative pressure (vacuum)) is generated inside the package due to extension of the spring, the ambient air is admitted into the package and the stopper is replaced. Plastic containers are light, very durable, and the polymeric film is almost insensitive to many reactive gases such as chlorine, ozone, nitrogen, sulphur oxides, and ammonia [20-231. This allows storage of the sample gases in such containers after completion of the sampling for as long as 10-14 days without any significant changes [20,23]. In bags made of laminated aluminium-coated plastics, polyethylene, nylon or PVC the stored gases decompose by no more than 1% per day [23]. Container-type sampling method is optimal for the extraction of pollutants from air provided that the sample is composed of gaseous compounds only and that a highly sensitive chromatographic detector of the electron-capture, flame photometric, photoionization or thermionic type is used. In these instances the detection limit may be as low as 10-4-10-s mg/m3.

5.2.

Use of absorption of contaminants in sample collection

Absorption of toxic contaminants by a solvent is often used in air sampling [l, 51. The advantages of the method lie in the possibility of the simultaneous concentration of contaminants in a wide range of sample substances (except aerosols and solid particles) and a high selectivity of determination associated if an appropriate solvent is chosen. In addition, the method of

38

5. Collection and pretreatment of samples for chromatographic analysis

determination is simplified as the sample is analysed in the liquid form irrespective of the analytical method. The drawbacks of the method involve the impossibility of obtaining a representative sample in the presence of aerosols and solid particles and, frequently, a low sample enrichment in the analysis of low concentrations. The latter circumstance is due to the high dilution of the sample, as 5-10 ml of the solvent solution are used for sample collection. This difficulty may be partially overcome by repeated concentration of contaminants by evaporation of the solvent under vacuum, selective extraction of contaminants from the solution or a chemical reaction. One should bear in mind, however, the possibility of losses of sample or changes in the sample composition arising from side-reactions. Sample collection is usually performed by passing the polluted air through a glass vessel (absorber) with several millilitres of an organic or inorganic solvent, depending on the sample composition. The flow-rate may vary over a wide range, from 0.1 to 30 l/min [26]. The most effective method of absorption of contaminants from air is the use of an absorber with a porous plate, which increases the absorption of contaminants owing to the large surface area (small air bubbles) compared with the conventional design of an absorber. A frittedglass filter should not have excessively large pores. The most efficient absorbers with respect to the volume of the solvent (the smaller the amount of the liquid, the greater is the sample concentration, all other conditions being equal) are those with a fritted-glass plate welded along the entire diameter. The capacity of such vessels reaches 30 l/min [26]. When selecting a solvent and a flow-rate, one should remember that the use of volatile organic solvents and a flow-rate above 1 l/min leads to considerable losses of absorbent owing to evaporation. As a result, changes in the collection efficiency occur early in the collection procedure and the errors in quantitative determinations arising from unaccounted for changes in the solvent volume may reach 50% or more [27]. This effect may be reduced by cooling the absorber, but in general highly volatile solvents (ethanol, acetone, chloroform, etc.) are not recommended. The extent of absorption depends on the contaminants being determined and on the absorbent, their concentrations, the air flow-rate, the temperature of the absorber, its design and some other factors. A proper choice of the solvent may permit selective sample collection, for instance, by absorbing in distilled water compounds that are soluble in it. In this way it is possible to separate inorganic from organic substances, C1-CI alcohols from hydrocarbons, aldehydes from organic sulphur compounds, water-soluble amines from phenols, etc., in the process of collection [l]. The effect of selective absorption may be achieved by the use of other polar solvents, e. g., dimethylformamide, n-butyl acetate or polyethylene glycol. Particularly effective is absorption based on chemical reactions of the absorbed substances with the solvent. Chemical absorption allows an extremely selective sampling (depending on the nature of the solvents, acting as reagents). Thus, trapping of small concentrations of organosulphur compounds from air in the form of a mercury complex, decomposition of this complex and determination of the recovered compounds by gas chromatography proved effective [28]. The air was pumped for 24 h at a volume flow-rate of 600 l/h through an absorber containing HgClz solution, which was then placed on a water-bath (60°C), concentrated HCl was added and then a flow of helium was passed through the absorber. The sulphur compounds recovered were collected in a trap cooled with liquid nitrogen and, after subsequent warming, they were analysed on a Durapak OPN/Porasil C (80-100 mesh) column (2 m X 2 mm I. D.) using a flame photometric detector ( t = 200°C). For the rapid determination of hydrazine, methylhydrazine and 1,l-dimethylhydrazine, used as high-energy propellants, in the working area atmosphere, an air sample was absorbed in acetone, the hydrazines being quantitatively trapped and subsequently derivatized to stable compounds [29]. The solution obtained was analysed on a chromatograph using a thermionic detector. The detection limit was 4 ppb.

5.2. Use of absorption of contaminants in sample collection

39

Concentrate

* I

Sample Collection

Sample Preparation

Fig. 5.3. Sampling and treatment for headspace analysis of aromatic hydrocarbons in urban air [46] Derivatization of a sample on collection in order to convert compounds of interest into a form convenient for chromatographic determination and concentration in order to ensure adequate selectivity and precision is common in air sampling [30].The absorption of trace amounts of hydrogen fluoride by 0.1 N alkali solution [31],hydrogen chloride by an aqueous solution of 7-oxabicyclo[4.l.0]heptane [32],ozone by a 1% solution of stilbene in ethyl acetate at -20°C [33],nitrogen dioxide by aniline [34]and nitrogen oxide by a solution of copper bromide and p-chloroaniline in acetonitrile [35],considerably decreases the detection limit for these reactive toxic substances. A similar method is used in the selective absorption of organic contaminants [30].Small concentrations of CI-C4 carbonic acids are absorbed by water [36],an alkali solution [37],sodium carbonate [38]or methanol [39].Trace amounts of aldehydes and ketones were collected from air in a saturated solution of 2,4-dinitrophenylhydrazinein 0.1 N sulphuric acid [40],nitrosoamines were absorbed by a 1 N solution of KOH [41],dimethylformamide by distilled water [42]and, in the determination of lower aliphatic amines by gas chromatography the compounds were derivatized with dilute HC1 [43].Dilute sulphuric acid is a good absorbent for the collection of small concentrations of ethylene oxide [44]and a mixture of hydrochloric and acetic acids was used for the concentration of diisocyanates and chlorine derivatives of diaminophenylmethane [45].The derivatization of contaminants is usually to 96-98% complete [45],and the analysis of the reaction products by gas chromatography using plasma ionization or thermionic detection allows the determination of mg/m3 concentration [41,451.Traces of chlorophenols may be similarly detected after their acetylation or fluorination using electron-capture detection. The absorption of contaminants by organic solvents may increase their concentration in the sample about 10-fold. The enrichment of the sample may be even higher if not the liquid solvent but the equilibrium gas phase above the solution is analysed. A 20-25 1 air sample is passed through the absorber with several millilitres of acetic acid until complete saturation occurs (Fig. 5.3). The solution obtained is neutralized by an alkali and, on achieving interphase equilibrium, the gas phase is analysed by gas chromatography using a plasma ionization detector. The addition of the alkali to the solvent increases the concentration of the pol-

40

5 . Collection and pretreatment of samples for chromatographic analysis

lutants (aromatic hydrocarbons) in the gas phase. The method permits the determination of alkylbenzenes in urban atmospheres (a 9 1 sample) after absorptive (vapour-phase) concentration at levels of 0.01-10mg/m3 with an accuracy of 7-14% and a detection limit of 0.008-0.002 mg/m3 [46]. Vapour phase concentration of contaminants is used to enrich the sample for gas chromatography of various samples, including air pollutants. Absorption of the contaminants and the analysis of the gas phase have been used to determine trace amounts of acetic acid [38], C2-Cs carbonic acids [47], formaldehyde, chloroprene, benzene and acrylonitrile in air [48]. Afther the vapour phase concentration of SO, it was determined on a Teflon column containing Chromosorb 108. An electrolytic conductimetric detector gave a detection limit of several ng/ml with a standard deviation of 0.05 ng/ml [49]. The direct determination of small amounts (from ppb to ppm) of formaldehyde in air proved possible after the absorption of its vapour by a 1%solution of Na,CO, [50]. The chromatographic separation was carried out on a Porapak T (80-100 mesh) column (2 m x 3 mm I. D.) at 95°C using a helium ionization detector. To detect trace amounts of hexafluoro-2-propanol in air the sample was collected in two or three absorbers each containing 20 ml of distilled water [51]. The absorbers were cooled with ice. The chromatographic analysis of 5 pl of the solution obtained on a 3 m column containing 10%Sylar 1OC on Chromosorb with temperature programming from 40 to 210°C using a plasma ionization detector gave a detection limit of 0.03 ppm, the volume of the sample being 120 1. In the analysis of the C2-Cs aliphatic alcohols [52] and phenol [53] mixed with other waterinsoluble organic compounds the pollutants were absorbed by distilled water. To decrease the volatility of the liquid phase and increase its selectivity, it is expedient to use not the pure water in the absorber, but solutions of crystal hydrates or melts of crystal hydrates. In the latter instance the absorber temperature should not be higher than the melting point of the crystal hydrate. High selectivity with aqueous solutions and crystal hydrate melts has been reported [54].

5.3.

Cryogenic concentration of contaminants

This method of concentration involves freezing out toxic species from polluted air at temperatures significantly lower than the boiling points of the sample substances [l]. The cryogenic concentration of contaminants consists in pumping air through a cooled trap (condenser) with a sufficiently large surface area. Metal capillaries or steel and glass tubes packed with an inert material (glass-wool, metal shavings or spirals, copper shot, glass beads, etc.) to increase the condensing (cooling) surface are used as cooling traps. Various coolants and mixtures (Table 5.1) are used, depending on the required temperature of the trap. The freezing-out technique is indispensable for the analysis of unstable or reactive com-

Cryogenic system ice-water ice-sodium chloride dry ice-acetone liquid oxygen liquid air liquid nitrogen

Temperature obtained ("C) 0 - 16 - 80 -183 - 192 - 195

Table 5.1. Coolants used for cryogenic trapping of air contaminants

5.3. Cryogenic concentration of contaminants

41

pounds. The cryogenic concentration of C102 in a glass column packed with glass-wool at -80°C can be used to separate this toxic and highly unstable substance from ozone, chlorine, nitrogen oxides and hydrogen chloride [55]. A similar technique was used to freeze out CO and COz from the complex mixture of toxic components of cigarette smoke and to concentrate small amounts of hydrogen sulphide in traps cooled by liquid argon [56]. The same coolant was used [57] for trapping odorants such as sulphur-containing compounds, lower aliphatic amines, carbonyl compounds, hydrocarbons, lower aliphatic monoalcohols and phenols from air in the vicinity of cellular concrete production facilities. With the use of a flame photometric detector the method can be applied at ppb levels. A concentrator containing glass shot may efficiently sorb small amounts of HIS, SOz, COS and CS2 and also volatile mercaptans, sulphides, disulphides and sulphoxides after being cooled by liquid oxygen [58]. After thermal desorption these compounds were analysed on a capillary column 30 m long containing SE-30 silicone with temperature programming from 70 to 100°C using a flame photometric detector. Ultramicroscopic concentrations of Freons and nitrous oxide were trapped from the stratosphere at liquid nitrogen temperature [59]; at - 130°C tetraalkyllead compounds are effectively frozen out from ambient air [60]. The use of a cryogenic trap has been used in studies of the distribution of halohydrocarbons (fluorochloromethanes and -ethanes) in the stratosphere after concentration of the contaminants by freezing out and application of a column (3 m x 2 mm I. D.) packed with OV-101 on Chromosorb W HP with temperature programming from -50 to +60"C using an electron-capture detector [61]. Concentration of contaminants is even more effective in cooled traps packed with a sorbent [62]. In this instance deep freezing of a trap is not needed and dry-ice suffices, except for low-boiling gases (e. g., CO and, occasionally, C,-C3 compounds). On concentration in a trap containing Silochrome C-80 at -78"C, up to mg/m3 of hydrogen sulphide can be determined in ambient air. Using a plasma ionization detector, hydrocyanic acid was concentrated from exhaust gases in a trap containing Porapak Q cooled by dry-ice [63] and organolead compounds were completely trapped at the same temperature using a short column packed with silicone stationary phase on Chromosorb W [64]. To concentrate and determine CO by gas chromatography at the level of lo-' mg/m3 it is necessary to cool the concentrator containing molecular sieves 5A by liquid nitrogen [65]. The cryogenic concentration of small amounts of highly volatile organic compounds (hydrocarbons and chlorohydrocarbons) has been discussed [66]. A trap containing active charcoal cooled by liquid nitrogen to - 120°C absorbed C2-C6 organic compounds from exhaust gases. Dry-ice sufficiently cooled a packed trap to concentrate very small amounts of vinyl chloride [l], highly carcinogenic N-nitroso compounds [l] and traces of Freons in a concentration tube containing Carbopak B [67]. Deeper cooling was employed for the concentration of C1-C4hydrocarbons, which were then determined at the ppb level using plasma ionization detection [68]. Sometimes low temperatures are used for the intermediate concentration of Contaminants giving a 1000-fold or greater enrichment of the sample [I, 681. This technique is especially frequently used in the gas chromatographic-mass spectrometric determination of pollutants [ l , 691. It consists in trapping the sample substances on a sorbent, thermal desorption of the concentrated substances on to a liquid nitrogen-cooled metal capillary (or the primary section of the analytical capillary column) and subsequent capillary column analysis. An effective method of concentrating toxic organic compounds from an air sample has been described [70]. The air is passed through a trap (6 cm X 1.5 cm) containing Tenax GC (35-60 mesh) and the concentrated contaminants are desorbed at 270°C and collected in a nickel trap (75 cm X 0.5 mm) cooled by liquid nitrogen. The sample is desorbed at 250°C and the recovered substances are analysed on a capillary column containing SE-30 on Carbowax 20M with temperature programming from 25 to 240°C at a rate of 3-4 K/min.

42

5. Collection and pretreatment of samples for chromatographic analysis Adsorption rube

Detect Recorder

4'-I;;

"V

I,

I I

I

I

Cold Trap

I1 I

1I

integrator

I

I

I

I

Cold F O P

Fig. 5.4. System for two-stage concentration, thermal desorption and gas chromatography of air pollutants [71]

Repeated cryogenic concentration of pollutants desorbed from a sorbent trap allows a more compact sample to be injected into a chromatographic column as thermal desorption from such a cryogenic trap takes much less time than with a sorbent-filled trap (5-10 and 20-100 s, respectively). This results in improved pollutant desorption kinetics and chromatographic separation of the sample components [l].The process of such a two-stage pollutant concentration followed by gas chromatographic determination of the isolated components is illustrated in Fig. 5.4. This technique has been applied to the determination of CI-C4 organohalogen compounds in sea and continental air at ppb levels with the use of an ECD [71], the detection of trace amounts of volatile organic species in the atmosphere [72] and the determinationp'f low concentrations (ca. 3 ng per component in a sample) of c6-cI5 aliphatic and aromatic hydrocarbons and c]-c6chlorohydrocarbons [73]. In the latter instance, in the analysis of air (70 1) at 100% humidity, the analytical results were more reproducible for the C1-CIscompounds and worse for the lighter (