GAS CHROMATOGRAPHY OF POLYMERS, Volume 10

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Journal of Chromatography Library - Volume 10

GAS CHROMATOGRAPHY OF POLYMERS

JOURNAL OF CHROMATOGRAPHY LIBRARY Volume 1 Chromatography of Antibiotics 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 High-Performance Liquid Chromatographic Methods by N. A. Parris Volume 6 Tsotachophoresis. Theory, Instrumentation and Applications by F. M. Everaerts, 3. 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-Layer Chromatography edited by A. Zlatkis and R. E. Kaiser Volume 10 Gas Chromatography of Polymers by V. G. Berezkin, V. R. Alishoyev and I. B. Nemirovskaya

Journal of Chromatography Library

- Volume 10

GAS CHROMATOGRAPHY OF POLYMERS

V.G. Berezkin, V.R. Alishoyev and I.B. Nemirovskaya Institute of Petrochemical Synthesis, Acurleiny of Sciences of tlie U.S.S.R., Moscow

ELSEVIER SCIENTIFIC PUBLISHING COMPANY AMSTERDAM - OXFORD - NEW YORK 1977

ELSEVIER SCIENTIFIC PUBLISHING COMPANY 335 Jan van Galenstraat P.O. Box 211,Amsterdam, The Netherlands Distributors f o r the United States and Canada: ELSEVIER/NORTH-HOLLAND INC. 52, Vanderbilt Avenue New York, N.Y. 10017

ISBN : 0-444-41514-9 Copyright 0 1977 by Elsevier Scientific Publishing Company, Amsterdam 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 publisher, Elsevier Scientific Publishing Company, Jan van Galenstraat 335, Amsterdam

Printed in The Netherlands

Contents Foreword. by N . S. Nametkin

........................................

IX

Preface to the English edition

..........................................

XI

Introduction

......................................................

1. Basic principles of gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Examples of applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of complex mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of physicochemical quantities .......................... Preparation techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GCequipnient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample injectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Katharometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flame-ionization detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theoretical concepts of the GC separation process ........................ Eddy diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Resistance t o mass transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Qualitative and quantitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Qualitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Standard compound method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method using tabulated data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Method of several phases ....................................... Computational methods and correlation ratios . . . . . . . . . . . . . . . . . . . . . . Physicochemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preliminary treatment of the sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quantitative analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Internal normalization method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Absolute calibration method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Internal standard method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 . Gas chromatographic methods for the analysis of monomers and solvents . . . . . . . Specific features of the GC analysis o f impurities .......................... Methods for improving the sensitivity of the determination of impurities . . . . . . . Increasing the size of the sample being analyzed ........................ Use of high-sensitivity detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of non-isothermal methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concentration methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparative elution chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi11

1 6 6 7 7 9 10 12 12 12 13 13 20 21 21 22 22 23 23 24 24 25 .25 26 28 28 28 29 33 34 41 41 43 44 47 47

VI

CONTENTS

Chromatography without a carrier gas (high-concentration GC) . . . . . . . . . . Frontal chromatography...................................... Methods for demasking impurities against the background of the main component Utilization of selective sorbents and selective detectors . . . . . . . . . . . . . . . . . . Analytical reaction GC .......................................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . The study of polymer formation reactions ............................... Investigation of polymerization reactions ............................... Study of copolymerization reactions ................................... Study of polycondensation reactions ................................... References ....................................................... 4 . Determination of volatile compounds in polymer systems

...................

Direct analytical methods ............................................ Multi-stage methods for determining volatile components . . . . . . . . . . . . . . . . . . . Determination of volatile components in polymer solutions . . . . . . . . . . . . . . . Application of extraction methods .................................. Application of methods for separating volatile impurities from polymers . . . . Specific features of the GC analysis of solvents. monomers. plasticizers and stabilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47 48 51 51 52 52 59 59 72 76 80 85 87 93 93 100

102 103 108

5 . Study of the kinetics and mechanisms of chemical transformations of polymers at elevated temperatures ............................................ Static methods for studying chemical conversions of polymers . . . . . . . . . . . . . . . Dynamic methods for studying polymer conversion processes . . . . . . . . . . . . . . . . Periodic GC analysis of volatile products ............................. Automatic analysis of volatile products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Use of GC detectors for the continuous analysis of volatile products . . . . . . . . Impulse pyrolysis of polymers ..................................... Specific features of procedures for studying thermooxidative degradation . . . . . . Application of GC in studying the degradation of polymers . . . . . . . . . . . . . . . . . . References .......................................................

113 114 116 116 121 124 127 131 140 140

6 . Reaction gas chromatography of polymers ............................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

145 156

7 . Pyrolysis gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Equipment and experimental procedure ................................. Identification of polymers ........................................... Determining the composition of polymer systems (mixtures and copolymers) .... Non-analytical applications of pyrolysis GC .............................. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References ........................................................

159 160 175 179 183 189 190

CONTENTS

VI 1

8. Inverse gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Identification of polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of the molecular weight of oligomers ....................... Investigation of the themiodynamics of interaction of volatile compounds with polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The study of phase transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Investigation of the kinetics and equilibria of chemical reactions . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

206 209 215 221

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

225

195 196 199

This Page Intentionally Left Blank

Fore word The extensive use of polynieric materials in industry, construction, space technology, the home and inany other areas is typical of the progress made in the second half of this century, and therefore books dealing with methods for the investigation o f polymer chemistry and for testing polymeric materials are of great importance. This book by V. G. Berezkin and his co-workers V. R. Alishoyev and I. B. Nertiirovskaya is concerned with the application of gas chromatography t o polymer chemistry. The following features of the book should be particularly noted. (1) The book is up t o current scientific and experiniental standards. It generalizes and systematizes a huge body of information scattered over a great number of publications (the bibliographical references in the book exceed 900). The text covers all of the basic fields of application of gas chromatography in polymer chemistry. (2) The text is clear and critically oriented, and the advantages and disadvantages o f the methods described are emphasized. This greatly facilitates the choice of the best method for a particular problem. (3) The book is meant for experimentalists, and the presentation adopted by the authors niakes it useful and valuable both to specialists working in the field of analytical gas chromatography and to researchers who lack appropriate experience. The latter group should first study Chapter 1, which expounds the fundamentals of the chromatographic method, covering sufficient ground for practical application. The book is of interest t o a wide circle of specialists engaged in polymer chemistry. This book was first published in Russian in the U.S.S.R.in 1972. It met with wide acclaim and was out of print in a very short time. I hope that this enlarged English edition will also be well received by readers throughout the world.

CorrespondingMember of the U.S.S.R. Academy of Sciences, Doctor of Chemical Sciences, Head ofA. V, Topchiev Institute of Petrochemical Synthesis, Academy of Sciences of the U.S.S.R.

NIKOLAI SLRGEYEVICH NAMETKIN


Preface to the English edition At present, gas chromatography is the most widespread method for the analysis of organic compounds. A very extensive and important field of application of gas chromatographic methods is polymer chemistry and technology, although polymers cannot be analyzed directly by gas chromatography. This book reflects the basic principles of the application of gas chromatography in polymer chemistry: monomer and solvent analysis, the study of polymer formation processes, investigations of the disintegration of high-molecular-weight compounds and the study of polymers by pyrolytic and inverse chromatography. Chapter 1 deals with the fundamentals of the gas chromatographic method. With this information, those who have no previous knowledge of gas chromatography and have not used it can apply the methods described in this book in their practical work. Despite the wide application of gas chromatography in research and analysis in polymer science and industry, many of the existing solutions are, unfortunately, far from optimal. This is due to many reasons, one of which is the tremendous output of information, scattered over many, often inaccessible, publications. When writing the book, the authors attempted to carry out a systematic rationalization of the extensive published material. They hope that the use of the text will considerably facilitate the choice of the best method of investigation for a particular problem and thus permit valuable infonnation on substances and processes in polymer chemistry to be obtained. This English edition contains many additions reflecting progress in the application of gas chromatography in polymer chemistry since the Russian edition was published. Chapters 4-6 were written by V. G. Berezkin and I. B. Nemirovskaya, Chapter 7 by V. R. Alishoyev and V. G. Berezkin and Chapter 3 by V. G. Berezkin with the participation of Yu. B. Amerik. The other parts of the book were written by V. G. Berezkin, who also planned the arrangement and did the general editing. The authors thank A. N. Genkin for a number of valuable remarks on Chapter 8 and B. M. Kovarskaya for a useful discussion of Chapter 5. The authors are deeply grateful to Corresponding Member of the U.S.S.R. Academy of Sciences K . V. Chmutov and Correspon’ding Member N. S. Nametkin for their support and encouragement.

V. G . BEREZKIN V. R. ALISHOYEV I. B. NEMIROVSKAYA


Introduction The success of any scientific research depends largely on the rational choice and use of experimental methods. Many modern processes in industry have become possible only as a result of the development of new, efficient methods of control. In chemistry, which is the science of the structure and transformation of substances, the central experimental problem often consists in determining the composition of complex mixtures, trace impurities that contaminate the main substance and physicochernical characteristics of substances. The achievements in chemistry and the chemical industry during the past two decades are due in many respects t o the vigorous development of gas chromatography (GC), the wide use of which has led t o revolutionary changes in methods of organic and gas analysis and in many physicochemical methods. This development is attributed t o the following features of GC. ( I ) GC is a universal method; in a single analysis, it is possible to determine the qualitative and quantitative composition of a complex mixture containing up t o several hundred volatile components. ( 2 ) High-sensitivity detectors, which are used for recording the results of chromatographic separations, permit the determination of harniful impurities present in concentrations as low as I O F - 1O-I" %. (3) GC is a rapid method that readily lends itself t o automation. (4) GC can be used successfully for determining both equilibrium distribution values and kinetic and diffusion characteristics o f the systems under investigation (volatile standards and solid or liquid stationary phases). GC is also widely applied in polymer chemistry, although under ordinary conditions high-molecular-weight compounds are not volatile, and the chromatography of polymers at high pressures is in its early days. This is due to the fact that in polymer chemistry manv important processes (for instance, polymer formation or degradation reactions) involve, as initial reactants or reactions products, low-molecular-weight substances that can be successfully analyzed by classical GC methods. GC methods are not always used in their optimal version, however, and they are applied very non-uniformly t o different problems in polymer chemistry. GC is used most widely in those fields where the forms of its application are traditional. Thus, GC is the basic method for determining impurities in monomers and solvents for polymerization and is widely applied in studyingvolatile degradation products. It is used much less for investigating the thermodynamics of the interactions of,standard volatile compounds with high-molecular-weight compounds by the method of inverse GC. Pyrolysis GC, in which the polymer system under investigation is characterized by means of the volatile pyrolysis products, is probably the only example of a method developed jointly by researchers working in the fields of GC and polymer chemistry, a method widely used for the identification of polymers, the quantitative analysis of copolymers and the determination of their structures. There is no doubt, however, that in the near future other modifications of GC will be developed specially for polymer studies. The authors hope that this book will arouse interest in its readers and will promote a wider application of GC methods in polymer chemistry. The authors have systematized and generalized the different applications of GC methods in polymer chemistry and have attempted to outline some future prospects for development of these methods in this important field.


Chapter I

Basic principles of gas chromatography The development of gas chromatographic (CC) methods has led to revolutionary changes in analytical chemistry and also in experimental methods of physical chemistry and methods for the separation of volatile compounds, as GC has a number of important advantages over other methods [ l ] . Firstly, GC is universal and can be applied to the analysis of a very wide range of substances from hydrogen isotopes to oligomers and metals, with boiling points within the range -250-1000°C. The method enables one to obtain, in a single analysis, information not only on a single substance but also on the contents of all (or most) of the components present in a complex mixture. Secondly, the practical application of the method is simple, the equipment used being of a standard type with automatic recording of the results of an analysis. Thirdly, GC ensures a high efficiency of separation within a relatively short analysis time (1 -30 min). The development of GC is still continuing. About 1500-2000 papers are published annually in this field, and the total number of publications up to 1970 was about 20,000 [2]. A study of the published information indicates the major role of gas chromatography among methods for the analysis of organic compounds and gases. Thus, for instance, in 1974, the papers in the principal analytical journals (Analytical Chemistry, Zeitschriff fur Analytische Chemie, Analyst (London),Zhurnal Atialiticheskoi Khimii and Zauodskaya Laboratoriya) were distributed as follows: all types of chromatography, 58.8%; GC, 34.6%;spectral methods, 25.7%; electrochemical methods, 6.6%; other methods (colunietric, gravimetric and others), 8.9%. Thus, in the analytical chemistry of organic compounds and gases, the total number of papers on GC exceeds those on spectral methods, and one in three papers is based on GC. The development of CC began after James and the Nobel Laureate Martin published the first work on gas-liquid chromatography [3] . Chromatography as a general method of separation, however, was first discovered in 1902 by the Russian botanist Mikhail Seniyonovich Tsvet (1 872-1919), who proposed an adsorption chromatographic method of separation in the liquid phase and described its application to the analysis of chlorophyll in plants [4]. The basic units of the chromatograph are the chromatographic column and the detector. The column (Fig. 1 .I, C) separates the test mixture into its components and the detector (Fig. 1 . l , D) records (in the carrier gas flow) the concentrations of the separated Components. The results of the separation are recorded automatically. Fig, 1.I shows schematically the separate stages of the chromatographic separation of a three-component mixture, illustrating the positions of the chromatographic zones in the column at definite time intervals, and the relationship between the separation process and the recorded chromatogram. At the moment of injection of the test mixture, the zones of all three substances are located at the head of the column. Under the action of the flow of carrier gas, the components of the mixture begin to move along the column Keferences p. 29

:

BASIC PRINCIPLES 01;GC

2

(a)

C

i

t

t

t

i

t

1:ig. 1.1. Schematic representation of the Chromatographic separation of a three-component mixture. (a) Dynamics of chromatographic process (position of chromatographic zones in column at definite time intervals); (b) recorded chromatogram.

at different speeds, which are determined by the nature of the compounds being separated and the type of sorbent used. The first t o be eluted from the column is component 1 (sloping hatching) then component 2 (vertical hatching) and finally component 3 (horizontal hatching). Separation in GC is based on different distributions of the molecules of the components being separated between the mobile gas phase and the stationary phase. A dynamic equilibrium is established between these phases for each component of the test mixture. Under the action of the flow of carrier gas, the components of the test mixture move along the chromatographic column with different speeds. The speed of this motion depends, for each component, on its distribution constant between the gas and stationary phases. The speed of motion of a chromatographic zone is inversely proportional to the distribution constant, i.e., readily sorbable Components move along the sorbent layer more slowly that sparingly sorbable components. A quantitative description of the elution process in GC can be obtained most readily by kinetic treatment of the e!ementary processes of the motion of the molecules of the

BASIC PRINCIPLES OF C C

3

test compounds in the column. It is assumed that the following conditions are fulfilled in a chromatographic separation: (1) the molecules of the test compounds are in dynamic equilibrium between the gas and stationary phases, and this equilibrium is independent of the presence of other components in the sample; (2) the molecules of the test conipounds move along the column only in the gas phase; ( 3 ) the carrier gas velocity, the temperature and the properties of the sorbent are constant along the length of the column and across its section, and the pressure drop can be neglected. In the course of separation in the column, a definite proportion of the niolecules of a given coniponent is in the gas phase at any instant, namely nm/(n, + n m ) , where nm and n, are the numbers of molecules of the given component in the mobile and stationary phases, respectively. Consequently, if the total retention time of this type of molecule in the column is t R , the average retention time of a molecule in the gas phase is tRn,,/(n, + nm). As the molecules are moving along the column only in the gas phase, the molecules of the test compound will pass through a Column of length L , with a mean carrier velocity zl, within a time f, (n,/n, n,). Therefore, the following equation holds:

+

Using eqn. 1.1, one can determine the retention time as the average time for the passage of the molecules of the test substance from the head to the end of the column:

L n tR = Y - ( l + 2 ) Id

m

Considering that the gas hold-up is L/ii and nJnm = K,(V,/V,), where K , is the distribution constant of the substance between the liquid and gas phases and V,/V,, is the ratio of the volume of the stationary phase t o that of the mobile phase in the colurnn, we obtain, after simple rearrangements:

or

where tN is the net retention time and F is the volume velocity of the carrier gas as measured at the column temperature. Eqn. 1.4 can be used to obtain an expression for the net retention volume, V N . The retention volume is the volume of the carrier gas necessary to elute a compound from the column under given conditions:

V,=t,F=K,V, (1.5) where VN is the net retention volume, VM is the hold-up volume of the column and VR is the retention volume (in the absence of a pressure drop). Hence, under standard experimental conditions, the retention volume (adjusted in ternis of the hold-up volume of the column) is directly proportional to the distribution I/,=

V,

-

References p. 29

BASIC PRINCIPLES OF GC

4

constant of a given compound between the mobile and stationary phases and is a value characteristic for a given compound. This means that each substance, independent of its concentration in the sample, will be eluted from the column after a definite time, which is characteristic of the substance. The retention time is the same type of constant for a given compound as other widely used characteristics such as the boiling point and specific gravity. Eqn. 1.5 substantiates the use of GC in the qualitative analysis of the components of a mixture and in measuring distribution constants. Fig. 1.2 exhibits the relationship between the distribution constant and the retention values of the substances being analyzed. Fig. 1.2a shows the distribution isotherms of substances A and B, and Fig. 1.2b indicates the position of the chromatographic zones in the column a certain time after the beginning of the analysis. Substance B is sorbed better than substance A by the stationary phase (KO,,, KD,*). Therefore, most of the molecules of B will be in the stationary phase and a smaller proportion in the carrier gas flow. For substance A, we have the opposite situation. Therefore, the zone of substance A will move along the column faster than the zone of substance B. At the column outlet, the separated components of the test mixture proceed, in the carrier gas flow, t o the detector, the response of which is proportional to the concentration or flow-rate of the components of the test substance in the carrier gas. The detector readings are recorded automatically by an electronic potentiometer. The diagram obtained, which reflects the results of the chromatographic separation, is called a chromatogram. A typical chromatogram of a hydrocarbon mixture is shown in Fig. 1.3. On the basis of the chromatogram one can determine the qualitative composition of the mixture analyzed. Let us consider the basic elements of a chromatogram [5]. The baseline is the portion of the chromatogram (for instance, between peaks 1 and 2) obtained when only the carrier gas is eluted from the column. A chromatographic peak is the portion of the chromatogram corresponding to the detector signal when one or several components are eluted from the column. The retention time ( t R ) is the time elapsed from the moment the sample is injected into the column to the appearance of the peak maximum. The hold-up time (tM)is the retention time of a compound that is not sorbed by a given stationary phase. The adjusted retention time It;) is the total retention time less the hold-up time ( t i = t R - tM).The peak width 010) is the segment of the zero line obtained (a)

I

(b) A

0

a

Fig. 1.2. Distribution isotherms (a) and chromatographic separation (b) of a two-component mixture of A and B. a and c are the concentrations of the compounds analyzed in the stationary and mobile phases, respectively.


5

\ 1 IU-..L',L 12

8

4

, 0

Tlme (min)

Fig. 1.3. Parameters of a chromatogram. Separation conditions: chromatograph, Tsvet-4, column 200 x 0.4 cni filled with 10%Apiezon K o n Chronlosorb P; temperature, 6 5 ° C . Peaks: 1 = air; 2 = cyclohexadiene; 3 = cyclohexane; 4 = rnethylcyclohexane. The designations of the parameters are explained in the text.

by interpolation of the baseline in the interval from the beginning to the end of the peak. In chromatographic practice, use is generally made of the peak width at half-height (/A, s), which is easier to determine from the chromatogram. The peak height ( h ) is the distance from the peak maximum to its base, measured in a direction parallel to the detector signal axis. The peak area (S) is the area enclosed between the line bounding the peak and its base. Qualitative analysis in CC is carried out on the basis of measurement of the retention times (or retention volumes). It is more convenient to identify unknown components by using relative, rather than absolute, retention values. The relative retention time (or References p. 29

6


relative retention volume), r,,,,

is determined by the equation

VN , A - K D,A tR,B-tMB vN9B KO,E Relative retention values are determined exclusively by the distribution coefficients of a given compound and the compound used as the standard; they are independent of such experimental conditions as the carrier gas velocity, the amount of the stationary phase, the column length and the sample size. The literature contains a great number of experimental data on relative retention values of various compounds on different stationary phases (see, for instance, refs. 6 and 7). Peaks of unknown components of the mixture being analyzed are identified by comparing the relative retention times of the maxima of these peaks, which are determined directly from the chromatogram, with the tabulated values for known compounds. In developing the chromatographing procedure, after the problem of the chromatographic separation of the final sample components has been solved, their quantitative determination must be carried out. The size of the signal given by the detector used in GC is directly proportional to the concentration of a component in the carrier gas under constant experimental conditions: - "R.A

r~~~-

= 'R,A-

tM,A -

tlR,B

h(t) = l/Ric(t)

(1 *7)

where h is the detector signal, c(t) is the concentration at time t and Ri is a constant. Therefore the amount, i, of the component being analyzed is directly proportional to the area of its chromatographic peak:

q i= J c(t)dt = J Rih(t)dt = R, J h(t)dt = R , S

(1.8)

The content of the jth component in the mixture can be calculated by the following equation:

KS..100

p. = 1 1 ZKiS, This is the simplified scheme for the interpretation of chromatograms. In conclusion, we shall give some examples that characterize the main types of application of the method.

EXAMPLES OF APPLICATIONS Analysis of complex mixtures GC is widely used for the analysis of complex mixtures, beginning with the separation of methane molecules containing different hydrogen isotopes (Fig. 1.4a [8] ) and ending with high-boiling oligomers of organosilicon compounds (Fig. 1.4b [9] ). Usually, the chromatographic analysis lasts a few minutes, tens of minutes or more rarely, hundreds of minutes. Some mixtures, however, can be analyzed within a few seconds (Fig. 1 . 4 ~[lo]). It should be noted that GC is used not only for laboratory analyses but also for the control and regulation of engineering processes. Fig. 1.4d [ 111 shows a chromatogram

I

EXAMPLES OF APPLICATIONS

of an analysis of a sample collected from a flow of a polymerizate of a synthetic ethylene-propylene rubber (SKEP) using a Soviet-designed process chromatograph (KhP-2 16). The problems involved in the analysis of complex mixtures have been discussed in books [12, 131. Determination of physicocheinical quantities GC methods are widely used for detennining physicocheniical characteristics such as distribution coefficients, activity coefficients, heats of solution, heats of adsorption, adsorbent surface areas, coefficients of diffusion in gas and liquid phases and rate constants of heterogeneous and homogeneous reactions [ 14-17] . Fig. 1.5 [ 181 illustrates two chromatograms of the decomposition products of tert.-butyl hydroperoxide. Chromatograni (a) was obtained at the beginning of the reaction of the decomposition of the peroxide, and chrotnatogram (b) during the course of the reaction. The decomposition of the rut.-butyl hydroperoxide leads to shrinking of the peak of the hydroperoxide (5) and to the appearance of new peaks of the reaction products on the chromatograni (1-3). Chromatographic study of changes in the concentrations of the initial compounds and reaction products with time enables one t o obtain, in most instances, the infomiation necessary for describing the kinetic behaviour of the system. GC methods are also widely used for studies of adsorption phenomena and measurement of the surface areas of soIids. As an exaniple, Table I .1 [ 191 lists data that characteriLe the good agreement between surface areas measured by classical methods and b y GC. GC enables one t o obtain within a much shorter time, results that are comparable in accuracy with those of classical methods. Preparation techniques GC can be used for isolating pure components from a mixture. Automated preparative equipment is available in which sample injection, separation and collection of pre-assigned

rABLk 1 1 COMPARISON or suRr ACL AREAS or CATALYSTS AS D L T L R M I N L D BY CHROMATOGRAPHIC AND STATIC MrTHODS I’ROM ARGON ADSORPTION [ 191 Saniple

Specific surface area (mz/g) Volume niethod (on vacuiini set-up)

Chelate polynicr Molybdenum f o 11 Titanium oxide Silica gel A h niin a Aluniinosilica tc Silicoii-magnesia catalyst __ References p. 29

0.08

-.

4.2 20.3 135 336 414

Difference (%)

Chromatographic method of therriial desorption

0.085 0.01 4.2 20.3 132 332 398 -

1-6.3

0.0 0.0 -2.2 -1.2 -3.9


8

(a)

I

I

I

300

I

320

310

I

I

330

340

Time (mtn)

2

1

k I

l

I

6

Time bet)

I

I

12

2

I

I

I

60

40

20

Time (min)

9

EQUIPMENT

1

40

-

L

1

1

~~

2o

1 1

40

20

1

J

Time i m i n )

Fig. 1.5. Chromatogram of a solution of ferf.-butyl hydroperoxide in the presence of manganese stearate (a) at the beginning of and (b) during the reaction. Conditions: column, 120 X 0.4 cm; sorbent, 30% dinonyl phthalate o n Celite-545; temperature, 50°C. Peaks: 1 = acetone; 2 = fer?.-butanol; 3 = ?err.-butyl peroxide; 4 = toluene (internal standard); 5 = ?err.-butyl hydroperoxide;6 = chlorobenzene (solvent).

fractions is carried out automatically. In order to obtain pure reactants, including monomeric solvents, methods of preparative chromatography are used on an industrial scale [20-231.

GC EQUIPMENT Fig. 1.6 [24] shows schematically a simple gas chromatograph. The carrier gas proceeds from a cylinder (1) through a reducer (2), a pressure regulator ( 3 ) and a flow stabilizer (4) to the reference cell of the detector (6), then through the sample introduction system (7) to the chromatographic column (9), which is located together with

Fig. 1.4. Examples of the application of gas chromatography. (a), Chromatogram of isotopic molecules of methane [8]. Conditions: capillary glass column (47 m X 0.22 m m I.D.), on the inner walls of which was a layer of active silica formed as a result of treatment of the glass capillary with a 10% solution of sodium hydroxide at 100°C; temperature, 77°K; carrier gas, nitrogen-helium (7:3); flow-rate, 1 ml/min. The upper chromatogram was obtained with the use of an ionization chamber as detector and the lower chromatogram with a flame-ionization detector. Peaks: 1 = I4CH,; 2 = CH,,H; 3 = CH,'H,; 4 = CH'H,; 5 = C3H,; I ' = "CH,; 2' = I3CH,; 3' = CH,IH; 4' = CH 12H1 '3 5' = CH,,H,; 6' = C'H,. (b), Chromatograni of high-boiling organosilicon compounds [ 9 ] : CH, 1,-0-SiCH,(C,H,), .Conditions: column, 100 X 0.4 cm; sorbent, (C,H,),CH,Si-fO-Si< C*HS 6.85%PFMS-6 on INZ-600 treated with dimethyldichlorosilane vapour; temperature, 354°C. Peaks: 1, n = 0; 2, n = 1; 3, n = 2; 4, n = 3. (c), Chromatogram of the rapid separation of a gas mixture [ 1 0 ) . Conditions: column, 100 X 0.4 cm; sorbent, 20% molecular sieves CaA o n Celite; temperature, 20°C. Peaks 1 = hydrogen; 2 = oxygen; 3 = nitrogen; 4 = methane; 5 = carbon monoxide. (d), Chromatograms of the analysis of a flow of the polymerizate of an ethylenepropylene copolymer n4th a KhP-216 process chromatograph. Peaks: 1 = ethylene; 2 = propylene. References p. 29

10


----

-7

!----- -1 I

I

4I’

Fig. 1.6. Simple gas chromatograph. For explanation, see text.

the detector in a thermostat (10). The pressure at the column inlet is measured by a pressure gauge (5) and the volume velocity of the carrier gas is checked periodically by a foam meter (1 1). The sample is injected with a syringe (8) into the flow of carrier gas upstream of the column through a sample injector (7). The flow of carrier gas carries the sample to the column where its components are separated into separate zones. The separated substances (chromatographic zones) enter the detector (6), which determines the concentration (or mass flow) of the components in the carrier gas. The detector signal, which is proportional to the concentration (or mass flow), is automatically recorded by a potentiometer (12). Detailed consideration of the separate units can be found elsewhere [25,26]. Sample injectors

In laboratory practice, gaseous and liquid samples are usually injected into the chromatograph with syringes similar to medical syringes. A simple device for introducing the sample into the chromatograph with a syringe is shown in Fig. 1.7a. The syringe needle is introduced through a rubber gasket into a heated evaporator through which a continuous flow of carrier gas is passing. The sample to be injected rapidly evaporates and is transferred to the chromatographic column in the vapour state. In chromatography, gaseous samples are usually injected by means of a system with detachable tubes of known volume [27] , a diagram of which is given in Fig. 1.7b. The same idea was used in developing an automated system for injecting gaseous samples with diaphragm valves [28]. For automatic injection of samples from a liquid flow, use is

EQU IPM I:NT

11

(b)

3

14

Pig. 1.7. Sample injectors. (a), Sample injector,for introducing samples with a syringe: 1 = carrier gas inlet; 2 = rubber seal; 3 = heater; 4 = to chromatographic column. (b), Injector forgaseous samples: 1 = Ilow of gas to be analyzed; 2 = Ilow of carrier gas; 3 = chromatographic column; 4 = saniple injection loop of known volume. (c), Automatic sample injector with moving rod forliquid samples: 1 = seal; 2 = membrane seal; 3 = moving rod; 4 = injection volume; 5 = body; 6 = sample flow; 7 = flow of carrier gas.

made of sample introduction systems with a moving rod (Fig. 1 . 7 ~ )The . role of the calibrated sample injection volume is played by the channel in the rod. As the rod moves, a definite volume of the liquid sample filling the calibrated channel is transferred from the flow of sample into the flow of carrier gas, where the liquid sample is evaporated and the vapour is carried to the chioinatographic column by the flow of carrier gas. Capillary colunins require only very small samples for analysis. This problem is usually solved not by designing miniature sample introduction systems, b u t by using the flow division method. Thus, only a very small portion of the carrier gas containing part of the sample is directed t o the capillary column. References p. 29

12


Columns In a chromatographic column, the components of a mixture are divided into separate zones. At present, two basic types of chromatographic column are used, namely packed and capillary columns [29,30]. The packed columns can be subdivided into preparative columns (diameter greater than 10 mm), analytical columns (diameter 3-6 mm) and capillary packed columns (diameter 0.5-2.0 mm). The length of the column with the packing is 0.8-10 m. Capillary columns (diameter 0.2-0.6 mm) are generally used without a packing, the inner walls being coated with a film of the liquid stationary phase. The length of capillary columns is 20-100 m. In spite of their high efficiency, capillary columns are used much more rarely than packed columns because high-efficiency capillary columns with reproducible characteristics are more difficult to prepare. The material of construction of chromatographic columns must be resistant to adsorption and catalytically inert. In most instances, columns made of stainless steel, glass, polymers and copper are used. Packed columns are filled with narrow fractions of either solid adsorbents with a developed surface or solid supports whose surface is coated with a layer of the liquid stationary phase. In order to reduce the adsorption and catalytic activity, mineral solid supports are treated with the vapour of dimethyldichlorosilane or hexamethyldisilazane, which deactivate the hydroxyl groups of the surface. Diatomite supports are used most often and for polar compounds good results are obtained by using inert polymer supports. Detailed information on solid supports is given elsewhere [31,32]. Detectors

A detector determines quantitatively the concentration (mass flow) of the test components in the carrier gas after they have been separated in the chromatographic column. The characteristics of the detector largely determine the accuracy and sensitivity of the entire analysis and the detector is therefore one of the most important units of the chromatographic installation. Hence “the history of development of gas chromatography is to some extent the history of development of the detector” [33]. Two types of detectors are widely used in GC: concentration detectors, whose readings depend on the concentration of the substance in the carrier gas, and mass (flow) detectors, whose readings are determined by the rate of feed (mass flow) of the test substance carried to the detector in the flow of carrier gas. The readings of the concentration detector depend only slightly on the flow-rate, whereas those of the flow-rate detector change sharply with the flow-rate. An example of the first type of detector is the katharometer, and of the second type the flameionization detector. We shall now consider the different types of detectors. Katharometer The principle of operatir. of the katharometer is based on the change in the electric resistance of the sensor (filament, coil) in relation to the heat conductivity of the gas leaving the GC column. The heat conductivity of the gas in the low-concentration range

THEORETICAL CONCEPTS OF THE SEPARATION PROCESS

13

depends linearly on the concentration of the eluted substances in the flow of carrier gas. Therefore, the resistance of the sensor changes linearly with the concentration of the detected substance in the flow of gas leaving the column. Use is generally made of the differential method, in which the working and reference cells of the detector are wired as a Wheatstone bridge, and a flow of pure carrier gas passes through tlie reference cell. When pure carrier gas passes through both cells, the bridge is in equilibrium. When a zone of a substance is eluted from the column, the composition of the gas mixture in the working cell changes, and so do the temperature and resistance of the sensor wire, and the potentiometer records the imbalance of the bridge, which is proportional t o the concentration of the substance in the carrier gas. The main advantage of the katharometer is its universality. The katharometer can be used for detecting permanent gases, various inorganic compounds (including such aggressive coinpounds as nitrogen dioxide, hydrogen chloride and fluoride gases, if a katharometer of special design is used) and vapours of organic compounds. In quantitative calculations, it is necessary to take into account'that the detector signal depends on the type of compounds being examined [34-391. Wide use is also made of the gas density balance [40-421, the sensitivity of which is slightly lower than that of the katharometer. The gas density balance, however, has the following advantages over the katharometer: (1 ) no preliminary calibration is necessary for qualitative analysis; (2) analysis of more aggressive gases is possible, because the vapours of the test substances do not come into contact with the sensitive elements; and (3) readily available gases are used as carrier gases.

E'larn~-ionizutb~~ detector The flame-ionization detector [43,44] is widely applied for detemiining organic coinpounds and especially impurities. The principle of operation is based on a sharp decrease in the electric resistance of a hydrogen flame when trace amounts of organic compounds, which forni ions in tlie course of oxidation, are introduced into it. These ions are collected at the electrodes, one of which is usually a burner nozzle. The very low ionization current that then arises is amplified and recorded by a potentiometer. A pure hydrogen flame usually generates a background current of the order of 10-"-10-12 A; when the test organic substances are introduced into the flame, currents of 10-12-10-7A are generated. Fig. 1.8 shows a flame-ionization detector of the type DIP-2. In recent years, selective ionization detectors [45-481 have been increasingly applied: the electron-capture detector for determining halogen-containing compounds [49], the themionic flame detector for phosphorus- and nitrogen-containing compounds [SO] and the mass spectral detector for most compounds [ S l , 521.

THEORETICAL CONCEPTS OF THE GC SEPARATION PROCESS

I n GC, when the Lones of the test compounds are nioved along the sorbent layer by the flow of carrier gas, two opposite effects occur simultaneously; the distance between the concentration maxima of the chromatographic zones of adjacent components References p. 29

BASIC PRINCIPLES O F GC

14

I

36 mrn

I

Fig. 1.8. Flame-ionization detector, type DIP-2: 1 = body; 2 = electrode/burner; 3 =diffuser for air supply; 4 = electrode/collector; 5 = upper detachable cup; 6 = air inlet orifice; 7 = inlet of eluate with hydrogen.

increases (this effect improves the separation) and so does the width of the chromatographic zones (this effect impedes the separation). The theory of GC explains the relationship between separation and the experimental parameters, and also the observed regularities in the two basic chromatographic characteristics, i.e., the retention value and peak broadening.


15

The retention volume (see eqn. 1.5) is directly proportional to the distribution constant of the test c o n ~ p o u n dbetween the liquid and gas phases and t o the volume of the liquid stationary phase in the column. This equation has been used for determining the distribution constant of organic con~poundsin the gas-liquid system. The values of the distribution constants are in good agreement with the corresponding values obtained by the static method [53], thus supporting the validity of eqn. 1.5, which is the basic equation of GC. It should be noted, however, that in the general case, when the distribution isotherpi is non-linear, the distribution constant in the retention volume equation is a function of the concentration. In this instance the retention volume also depends on the concentration, and this effect leads t o the formation of asyminetrical Chromatographic zones. When determining retention values in CC it is necessary to take into account the compressibility of the carrier gas [3], due to which the flow-rate, pressure and density of the carrier gas vary according t o a definite law along the length of the column. Let us determine the retention value, making an allowance for the compressibility of the carrier gas. Taking into account eqn. 1.3, we can write the following expression for the retention time: (1.10)

To calculate the integral, we shall use Boyle’s law: UP

(1.1 1)

= UOPO

and Poiseuille’s l a w (1.12) where u and p are the linear velocity and pressure of the carrier gas at some point x in the chromatographic column, uOand p o are the linear velocity and pressure, respectively, of the carrier gas at the column outlet, kpem,,is the permeability constant and 77 is the viscosity of the carrier gas. Expressing u and dw from eqns. 1.1 1 and 1.12, we can calculate the integral (1.13) where p i is the pressure at the column inlet. The value of the term kpenn./77u0p0can be found from the equation (1.14)

or kpenn. =

2L

- 77UOPO P’- P i References p. 29

(1.15)


16

Thus (1.16) and hence (1.17) or (1.18) where j is the pressure-gradient correction factor of the gas: (1.19) tk is the adjusted retention time taking into account the pressure drop in the column. The corrected retention volume,yi, and the corrected gas hold-up volume of the column, pM, can be calculated by similar equations: = VRj

v$=

(1.20) (1.21)

VMj

V i is the retention volume that would have been measured if the carrier gas were incompressible. Because the correction for compressibility is less than unity, the corrected retention volume is less than that observed at the column outlet. Fig. 1.9 shows the dependence of the pressure-gradient correction factor on the ratio of the pressures at the inlet and outlet of the column. It can be seen that the pressuregradient correction factor greatly depends on this pressure ratio. As an illustration, let us consider the determination of the corrected volume from the experimental results. As a result of experimental measurements, the following experimental parameters were determined: retention time of the test substance, 6.0 min; retention time of helium (non-sorbable component), 0.5 min; carrier gas velocity measured at 25°C (298°K) and at an atmospheric pressure of 760 mmHg, 50 ml min; column temperature, 100°C (373°K); water vapour pressure at 25"C, 24 mmHg; gauge pressure at the column inlet, 900 mmHg. First, we shall determine the true velocity in the chromatographic column. In measuring the carrier gas volume velocity, Fo, with a foam velocity meter, one must take into consideration the correction for the water vapour pressure, p H I O ,at the measurement temperature, T,, and the correction for the gas volume due to the difference between the column temperature, T,, and that of the velocity meter, T , : (1.22) In this instance: F O = 5 O (298 =)

[l-

(g)]=60.5cm2/min


17

c i

1

3

3

-b L-

4

5

6

PI /Po

Fig. 1.9. Dependence of the correction for the compressibibty of the carrier gas o n the ratio of the pressure at the inlet and outlet of the column and o n the pressure drop. The pressure at the inlet is 1 atm.

Let us calculate the retention volume, V A = fRFo= 6.0 min X 60.5 ml/min = 363 ml, and the corrected retention volume, taking into account that p i / p o = 1.18, j = 0.98 and = j V R = 334 ml. Subtracting the corrected gas hold-up volume (V; = jFdM = 29.6 ml) from the corrected retention volume, we obtain the value of the net retention volume, VN = 304 ml. Absolute retention values are used mainly in determining physicochemical quantities (activity coefficient, distribution constant, etc). As noted above, for identification purposes use is made of relative retention values or functions of relative retention values. The second basic characteristic of the chromatographic process of separation of a compound is the broadening of its chromatographic zone. A sufficiently general and formal description of zone broadening was given by Martin and Synge [54] on the basis of the theory of theoretical plates. In this'theory the chromatogi iphic column is considered as a system consisting of a set of successive sections. Each section is a theoretical plate in which equilibrium of the test compound between the liquid and gas phases is established instantaneously. The chromatographic process is simulated as two operations that are repeated many times, namely (1) instantaneous transfer of the mobile phase from a given plate to the next plate in the absence of mass exchange between the phases, and (2) establishment of equilibrium of the test compound between the gas and stationary phases on each plate. For instance, let a column contain five theoretical plates (Fig. 1 .lo) [ 5 5 ] . In the initial position, each plate is filled with a gas, and the zero plate contains a sample of the test compounds A and B, one of which (A) is not sorbed in the stationary phase, while the other (B) is sorbed; the mass distribution ratio D, = 1, i.e., half of the molecules of substances B are in the mobile phase and the other half in the stationary phase. We introduce into the column a volume of pure carrier gas equal to the volume of the gas phase of the plate. The gas phase of the zero plate (together with substances A and B References p. 29


18 ........ o c .......

I

0 0

, .

.. ..

.. ..

..

..

O Q 0.3

3

. . . . . . . . . . . .

.. .. .. ..

.... . . o o

0 0 ' 0 0

. . . . . . .

. . 0 "

I

I

I

I

I

I

I

0 0

. . . . . .

1

..

. . . . . . .

.. . " 0 0 0

.. .. .. ..

.. ..

....

..

..

0 0

0 0

0 0

..

Pig. 1.10. Schematic representation of the chromatographic process based o n the theory of theoretical plates for compounds A (not retained by the stationary phase) and B (retained by the stationary phase). 0-4, successive stages of the chromatographic process. The plot depicts the distribution of concentrations of compounds A ( 0 ) and B ( 0 ) after five stages of gas phase transfer. Upper part of column, gas phase; lower part, liquid stationary phase. Q = Amount of substance analyzed; 1 = distance from the column inlet.

which are located in it) will go to the first plate, the gas phase of the first plate to the second, and so on. In the zero and first plates the substance will be distributed between the two phases in equilibrium. This process will be repeated upon injection of each new portion of carrier gas. After four elementary volumes of carrier gas have passed through the column, the substance will start to be eluted from the column. The elution curve of substance B is shown in Fig. 1.1 1 [ 121 . The asymmetry of the elution curve is due to the excessively small number of plates (N). At N > 50, the peaks are already almost symmetrical, and at N > 100 the chromatographic zones correspond to the Gauss equation. The solution of the problem leads to the following equation for the chromatographic zone based on the theory of theoretical plates: (1.23) where n is the number of theoretical plates in the column, q is the size of the sample analyzed, and V is the volume of the carrier gas that has passed through the column. From eqn. 1.23, it follows that

I

' R C

c

max.

u"u

max.

v; (2?T)f

(1.24)

In estimating the number of theoretical plates in the column, the real process is compared with the above-described ideal process of separation, which yields the same results. The characteristics of zone broadening in the theory of theoretical plates is the number of theoretical plates of the chromatographic column.

.

T H E O R r T I C A L CONCI PTS O F T H E SEPARATION PROCFSS

19

I 010

“v

rig. 1 . 1 1. Elution curve tor n column of five theorctical plates calculated o n tlie basis of the theory of theoretical plates. cy = Fraction of substance emerging from column; r i v = number of volumes of carrier gas passed through column and corresponding to one theoretical plate.

Using eqn. 1.23, one can propose a method for detemiining the number of theoretical plates. The peak width at a height c = c,,,,,/e can be calculated from the equation (1.25) Solving this equation, we obtain

n=2

(

v:: v; ve

(1.26)

-~

Considering that V g = V e = (1/2)be, where b, is th width of th chromatographic peak at a height h,,a,./e, we have (1.27) I n practice, the following equation is usually used for calculating the number of theoretical plates : (1.28) where x is the chart distance from the moment of sample injection t o the emergence of the peak maximum and)’ is the peak width at half-height. The efficiency of a chromatographic column increases with the length of the column used. Therefore, a inore invariant value is the height equivalent to a theoretical plate (HETP), o r h). h = L/n

(1.29)

The concept of the chromatographic process in the theory of theotetical plates is rather fomial; this theory does not consider the actual causes of chromatographic zone broadening. Van Deeniter er al. [56] and Klinkenberg and Sjenitzer [57] developed a velocity theory in which Lone broadening in a packed column is attributed to a number of kinetic causes. References p. 29

20


v (rnl/rnin)

Fig. 1.12. Dependence of HETP on linear velocity of carrier gas. 1 =Contribution to HETP from molecular diffusion; 2 = contribution from mass transfer; 3 = contribution from eddy diffusion.

According to the velocity theory, the dependence of h on u is expressed by the equation h = A + (B/u) iCu

(1.30)

The first term of this equation ( A ) reflects the contribution to HETP from eddy diffusion, the second (Blu) from the molecular longitudinal diffusion and the third (Cu) from the resistance to mass transfer. Fig. 1.12 illustrates the dependence of HEPT on the carrier gas velocity and shows the contributions corresponding to each process that causes zone broadening. Further development of the theory of spreading in chromatography was carried out by Giddings [ 5 8 ] . We shall now consider in more detail the separate groups of processes that lead to zone broadening .

Eddy diffusion In any packed column, zone broadening is due, in particular, to the many possible routes, of different lengths, by which the molecules of the test substance move along the column in the flow of carrier gas, Le., to the multiplicity of channels along which the carrier gas moves in the packing. Therefore, depending on the length of the route, some molecules will reach the end of the column earlier and others later compared with the average molecule hold-up time in the column. Thus, the multi-route progress of the carrier gas through the packing layer leads to broadening of the chromatographic zone. This cause of broadening is called eddy diffusion. In Van Deemter’s equation, eddy diffusion is expressed by the term A , and A = 2hdp

(1.31)

where A is a coefficient characterizing the shape of the particles and the uniformity of their packing in the column, while dp is the average diameter of the sorbent grains. The value of A is independent of the nature of the test compound, its retention value and the nature of the carrier gas. A more rigorous description of eddy diffusion was given by Giddings [ 5 8 ] .

THEORETICAL CONCEPTS OF T H E SEPARATION PROCESS

21

Molecular diffusion

In the course of separation, there is always a concentration gradient in the gas phase of a chromatographic zone and molecular diffusion therefore always takes place in the gas phase, leading to peak broadening. The contribution from molecular diffusion is reflected in Van Deeniter’s equation by the second term (1.32) where y is the obstruction factor, which takes into account the sinuosity of the diffusion paths in the packing and Dg is the diffusion coefficient of the test substance in the carrier gas. The factor y is equal to or less than unity [59]. Molecular diffusion depends on the properties of both the test substance and the carrier gas. When the principal cause of broadening is the longitudinal molecular diffusion, it is expedient to use dense gases for reducing the zone broadening, i.e., it is preferable to use nitrogen or argon rather than hydrogen or helium as the carrier gas. Resistance to mass transfer During the motion of a chromatographic zone along the column, the front edge of the zone is predominantly characterized by the process of sorption, i.e., transfer of molecules from the gas to the stationary phase. After the maximum, the opposite phenomenon is observed, namely desorption takes place at the rear edge, i.e., transfer of molecules to the test compound from the stationary to the gas phase. Both of these processes occur rapidly, although not instantaneously. Therefore, the zone of the substance in the gas phase slightly leads the zone of the substance in the stationary phase, which also contributes to peak broadening [60]. The third term of Van Deemter’s equation reflects the contribution from this process to the HETP: (1.33) where k = KV,/Vg is the extraction coefficient, d, is the thickness of the liquid stationary phase (LSP) film and D, is the diffusion coefficient of the test compound in the LSP. The value of C depends on various factors, the most important being the thickness of the LSP fdm. The value of C increases directly proportionally to the square of this thickness. It should be noted that the value of h , although it is an important characteristic of the column, which defines the broadening of a chromatographic zone, cannot be regarded as the only value that determines the possibility of solving a particular analytical problem. In developing an analytical procedure, the problem very often reduces to the separation of at least two compounds with similar properties. For a quantitative assessment of the separation of the chromatographic zones of two compounds present in a mixture in similar concentrations, a number of separation criteria have been proposed that are a function of the difference between the retention References p. 29

22


values and the widths of the chromatographic zones. The IUF'AC Committee [61] recommended the use of the following value as a separation criterion: R , = 2Y10, - Y*)

(1.34)

where y is the chart distance between the peak maxima of compounds 1 and 2 and y A andyB are the peak widths of compounds 1 and 2 at the base of the peaks. The value of R , varies from 0 to "0; the peaks are completely separated at R , = 1 . In the Soviet literature, the quantity K 1 is usually adopted as a separation criterion [33], where

K = 1/2R, The peak resolution is determined by the sorbent selectivity, a (a= f R , J f R , J , column efficiency, (N), and the mass distribution ratio, D, (see ref. 62):

(1.35)

the

(1.36)

(1.37)

where fi is the volume ratio of the mobile to stationary phase in the column, K is the distribution constant and a,K and N refer to the second, dower component. The development of a satisfactory separation procedure can often be reduced to the determination of the optimal conditions under which the value of the separation criterion for a pair of compounds that are difficult to separate would be the highest. The dependence of the separation coefficient on the experimental parameters is considered in detail elsewhere [33].

QUALITATIVE AND QUANTITATIVE ANALYSIS The advances made in GC were due largely to the development of efficient identification methods, the characteristic feature of which is a wide use of a combination of various physical and chemical methods for identifying the peaks in a chromatogram [48,63-651. The general scheme of application of some widely used identification methods in GC is shown in Fig. 1.13.' Qualitative analysis Qualitative analysis often includes the following stages: (1) preliminary preparation of the sample, ( 2 ) chromatographic separation with the use of chemical reactions and selective detectors, (3) isolation and physicochemical study of separate fractions, (4) GC re-examination of separation fractions. Thus, in order to determine the composition of a test mixture, bothchromatographic methods based on the measurement of the retention values and methods based on the physicochemical properties of the test components are applied. In the following sections we shall consider the principal' methods of identification.

QUALITATIVE A N D QUANTITATIVE ANALYSIS

-

r

3

-

7

4

23

5

-

J

1;ig. 1.1 3. General scheme of qualitative analysis in gas chromatography. 1 = Sample to be analyzed; 2 = preliminary preparation of sample (separation, chemical treatment, physicochemical investigation); 3 = chromatographic separation, chemical analysis; 4 = selective detectors; 5 = physicochernical study of separated fractions; 6 = gas chromatograph; 7 = re-cxamination of separate fractions.

Staridard compound method

This method is based on the introduction into the test mixture of standard substances that are assunied to be already present in the mixture. The coincidence of the retention times is usually the basis for identifying the peak of the test compound as the standard coinpound. I t should, however, be noted that this condition is not sufficient for the qualitative identification of a compound, because identical (or very similar) retention times may characterire several substances. The reliability of this method increases with the use of more efficient columns and columns with different phases, the nature of which detenniiies the sequence of emergence of the coniponents and their retention values. As many compounds are not readily available, in piactical chromatography reaction methods are used for the preparation of standard mixtures [66]. Metliod using fabiilafed data

The determination of the qualitative composition of a mixture by this method is based on a cornparison of expeiiinentally detemiined retention values of peaks with tabulated retention values for known coinpounds. Standard compounds with tabulated retention values are introduced into the test mixtuie; in order to increase the reliability of the method, several control measurements must be made for corn pounds with diffeient structures, the presence of which i n the sample being analyzed is assumed, so as to establish the identity of the chromatographic properties of the given column with that used for tabulating the data on the retention values. Tabulated retention values have been published [6, 71.

Refcrenccs p. 29

24


Method of several phases In this method, an unknown mixture is analyzed, not on a single column, but on several columns with different phases [66]. This technique increases the reliability of the chromatographic identification of the substance and permits the type of compound being analyzed to be determined. Fig. 1.14 [67] shows the logarithmic dependence of the retention times of various compounds for two stationary phases, paraffin oil (tR,J and tricresyl phosphate (tR,2).It follows from the data presented that compounds of the same type are characterized by straight lines that do not pass through the origin. This is an important regularity, which is utilized for determining the type of test compound.

Computational methods and correlation ratios When tables of retention values lack data on some compounds;it may be useful to use correlation equations that relate the logarithm of the retention values with the properties of the test compounds [3] (for instance, the number of carbon atoms or the boiling point). In many instances, in order to determine retention values, one can use computational methods based on the additive scheme [68-701. For instance, eqn. 1.38 is valid for the retention values of alkanes:

rii

log V = Enii

(1.38)

where riiis the increment of the logarithm of the retention value corresponding to a definite combination of bonds (the structural element) and nii is the number of structural elements of type ij in the molecule of the compound. The additive scheme is based on the assumption that the molecular interaction of the test compound with the liquid stationary phase can be regarded as an interaction of a sum of definite structural elements of the mdecule, each of which is characterized by a definite contribution to the retention value. Fig. 1.15 depicts the relationship between the calculated and experimental values of the logarithms of the retention times of alkanes. The agreement is satisfactory. cc d

B

I

loo lo-

1-

0

P

/p /,$7O

-

!

A0 1

I

I

‘6

I

O8

$;,:

/ il0I, O6

,

QUALITATIVE A N D QUANTITATIVE ANALYSIS

6

3

25

0

Fig. 1.16. Separation of mixture of hexafluoroben7ene ( I ) , propyl chloride (2) and n-heptane (3) with the use of a selective flarne-emission detector. (a), Chlorine; (b), fluorine; (c), carbon.

Physicochemical methods This group of methods is based on joint utilization of chromatographic and physicochemical methods. The application of selective detectors, which record only compounds of one or several definite classes, pennits information on the nature of the test compounds t o be obtained; this information, coupled with the chromatographic data, enables one reliably t o identify the components of the text mixture. As an example Fig. 1 .I6 [71] shows three chromatograms of the same mixture (hexafluorobenzene, propyl chloride and ti-heptane, recorded with flame-emission detector. Each chromatogram was obtained by recording the intensity of a definite spectral line which was selective for chlorine-, fluorine-, or carbon-containing compounds. This technique made it possible t o establish the elemental composition of the compounds being analyzed. Methods of analytical reaction GC are also widely applied for identification purposes [64, 721. these methods use chemical reactions in a unified chromatographic scheme for analytical purposes. The deduction method [ 7 3 ] is often used for group identification. In this method, two chromatographic analyses of the initial mixture are carried out: one is an ordinary analysis without the use of chemical reactions, while the other utilizes in the chromatographic scheme a reactor containing an absorbent (reagent) that forms non-volatile compounds with certain classes of chemical substances. Therefore, the chromatogram of the second analysis shows no peaks of the reacting compounds (the chromatogram of the second analysis can be obtained from that of the first analysis by deducting the peaks of the reactants), which indicates that they belong t o compounds of a definite class. This method was first used for determining the content of unsaturated compounds in hydrocarbon mixtures. Unsaturated compounds were absorbed in the reactor by concentrated sulphuric acid deposited on silica gel.

Preliminary treatment of the sample

I n thus method, both chemical and physical techniques can be used. For instance, after a reaction of a mixture of fatty acids with bromine, compounds with one unsaturated References p. 29

26


bond are determined by displacement of the peaks on the chromatogram, and compounds with several double bonds are determined by the disappearance of their peaks, as the introduction of bromine into the molecule of an organic compound sharply reduces its volatility. Extraction is also used as a subtraction method. For example, an efficient method for identifying alcohols is preliminary extraction with propylene glycol of components of a sample dissolved in carbon tetrachloride. Propylene glycol efficiently extracts alcohols, but not aldehydes, ketones, hydrocarbons or esters, which remain in the carbon tetrachloride solution. Acids, phenols and amines, which are also readily soluble in propylene glycol, can be removed by treatment with an alkali or acid. The above general identification methods open up wide opportunities for establishing the composition of unknown mixtures. Quantitative analysis An important stage in chromatographic analysis is the qualitative interpretation of chromatograms, as a result of which the quantitative contents of the components in the test mixture can subsequently be determined. The accuracy of the results obtained depends on a number of factors, in particular on the method of analysis selected, the characteristics of the detector used, the method of calibration and calculation and the nature of the components being analyzed [74]. In accordance with eqn.l.8, the amount of a substance in a chromatographic zone is directly proportional to the area of the chromatographic peak in the chromatogram. In this connection we shall consider methods for determining the area of a chromatographic zone, assuming that it is a Gaussian curve. (1) The area of a chromatographic zone is commonly expressed as the product of the peak height, , and the peak width at half-height:

(1.39)

A more general expression for determining the area of chromatographic peaks, which permits the calculation of the area of partially separated peaks, has been suggested [75] : = K8

hmax. P6

(1.40)

If 0 = 0.5,0.75 or 0.9, then K , = 0.941, 1.66 or 2.73, respectively. (2) The area of a chromatographic zone can be determined as the product of the peak height and the retention time [76] : (1.41) (3) The area of a chromatographic zone is proportional to the height of the chromatographic peak: (1.42)

27

QUALITATIVE AND QUANTITATIVE ANALYSIS

Therefore, the amount of a substance in a chromatographic zone is directly proportional to the following values, which are determined directly from the chromatogram: hmau.pe ; hmax,xm,, or h,,,,,. However, if in the first method the coefficient of proportionality between the amount of substance and the value being measured is largely determined b y the metrological characteristics of the detector and its opening conditions, in the second and third methods this coefficient also depends on the conditions of the chromatographic experiment. In particular, in the second method the value of the area depends o n the number of theoretical plates, which, in general. changes from one substance to another. In the third method, the coefficient of proportionality depends on the retention value and separation efficiency. It should be noted that with large samples, deviations from the above linear dependence also occur and the retention times change. These deviations must occur in all instances when the ratio of the volume of the test sample to the peak width (in volume units), as measured with a vanishingly small sample siLe, exceeds 0.4 [77]. When using ‘manual’ methods for increasing the accuracy, the peak width must be measured by means o f a measuring magnifying glass with reticule divisions of 0.1 mni [78] . In recent years, electronic integrators have been widely used in chromatography [79-811. The use of integrators considerably reduces the processing time, ensures a high accuracy and reduces the cost of processing. Table 1.2 [82] conipares different methods of chromatogram processing. I t can be seen that the use of electronic integrators improves the accuracy and reduces the processing time. The amount of a substance in a chromatographic Lone is detennined both by the characteristics of the detecting systeni and by the parameters of the chromatographic peak. Therefore, in order t o detennine the content of a component in a sample, it is not sufficient to find the area (or other parameters) of the chromatographic peak of the substance, but it also necessary t o determine the coefficient of proportionality, which depends on the type of the detecting system, the experimental conditions, the nature of the test sample, and so on. Let us consider the quantitative methods for determining the content of the coniponents in the test mixture. TABLE 1.2 COMPARISON 01 SOML CIIROMATOGKAhl PROCFSSING METIIODS ~~

Characteristic

-

.~

Manual methods Use o i planimeter

Chromatogram 45- 60 processing time (min) Reproducibility 4.06 (%,)

Electronic digital integrator

____ Area Cutting o u t calculation* and weighing

45-60

50-60

100-200

15-30

5-10

4.06

2.58

1.74

1.29

0.44

*Height multiplied by width at half-height.

References p. 29

Electromechanical integrator

Triangulation method

28


Internal normalization method The modern version of this method in GC was suggested by Keulemans and co-workers 183,841. The calculation is made with eqn. 1.9 taking into account the correction coefficients for the separate compounds (see, for instance, ref. 82).

Absolute calibration method In this method, the dependence of the area (or another parameter) of the chromatographic peak on the absolute amount of the test substance is determined experimentally for each component, Le., one of the following dependences is determined: qi

= Ks = K h

= Khph,,,,kJ

(I .43)

The absolute calibration method is widely used in chromatography (see for instance refs. 85 and 86. It should be noted that absolute calibration must be checked periodically; the frequency of this checking must be determined empirically. Usually, in re-calibrating, one can restrict the check to a few points on the calibration graph.

Internal standard method This method was first used in GC by Ray [87]. A mixture of unknown composition, into which a known substance is specially introduced at a concentration R , is analyzed. The concentration of the standard is calculated with reference to the entire test mixture, which is taken as 100%. The content of the components (P)in the test mixture is calculated by the equation

P =A S i R f s t d . 'std.

or p =

fhih8

(1.44)

f h std. hstd.

where fi and f s t d . are the correction coefficients depending on the individual sensitivity of the detector to the component i and the standard. If R is constant, calibration graphs are obtained of percentage of impurity versus the ratio of the peak height of the component i to that of the standard. The use of relative calibration methods enables one greatly to improve the accuracy of measurements, because the effect of the experimental conditions on the analytical results is reduced as the change in the analytical parameters usually affects the retention time of the standard substance and that of the sample components equally (although there are exceptions). The corresponding equation can be obtained on the basis of eqns. 1.401.42 and 1.44: (1.45)

where f i = I / ( q - CUO) and fstd.= l/(astd.- ao).Note that the relative values in this equation are much less dependmt on the experimental conditions than are the absolute values of the height and area of the chromatographic peak. Another advantage of the method is that it is no longer necessary t o measure accurately the volume of the test

REFERENCES

29

sample except when calibration with a constant sample size is used in order to take into account the non-linearity of the detector. The method can also be used t o measure the content of individual components even when not all o f the compounds are recorded on the chromatogram. In choosing the standard compound, one must ensure that it is compatible with the sample being analyzed. In order t o increase the accuracy of the analysis, it is desirable that the substance used as the standard should be similar t o the test components in terms of the retention value and their content in the mixture being analyzed. The method automatically takes into account possible losses of the test con1 ponents during preliminary preparation of the sample [88] (for instance, during extraction, distillation, adsorption and other extraction and concentration procedures). Many of the above calculation and calibration methods are based on the assumption that the components of the test sample are not adsorbed and remain unchanged during the analysis, the detector readings are linear over a wide range of concentrations and are independent of the presence of other components in the sample and the values of the correction coefficients published in the literature are adequate. Unfortunately, many of these conditions are not fulfilled in practice. Therefore, accurate quantitative results can be obtained only by calibrating a given device with a mixture of known composition that contains all of the compounds present in the test mixture. The calibration of the device must be checked periodically. In conclusion, we wish t o emphasize the necessity of applying statistical methods [65,89,901 for estimating the accuracy of the chromatographic procedures used.

REFERENCES 1 J. Janik, Chromatogr. Rev., I 1 (1969) 203. 2 J. I. Walraven, Joint Symposium o n Accurate Methods of Anaiysis f o r Major Constituents, London, April 1570, Paper 1 1. 3 A. T. James and A. J. P. Martin,RiocAem. J., 50 (1952) 679. 4 M. S. Tsvet, Khromatograficheskii Adsorptsionnyi Analiz (Chronlatographic Adsorption Analysis), Academy Press, Moscow, 1946. 5 R. P. W. Scott (Editor), Gas Chromatography 1560, Butterworths, London, 1960. 6 J. S. Lewis, Coinpilafion of Gas Chromatographic Data, ASTM Special Technical Publication No. 343, American Society for Testing and Materials, Philadelphia, Pa., 1963. I W. 0. McReynolds, Gas Chrornatograpkic Retention Data, Preston Technical Abstracts Co., Evaston, Texas, 1966. 8 F. Bruner, C. P. Cartoni and M. Posanzini,Anal. Chem., 41 (1969) 1122. 9 G. N. Turkeltaub and B. M. Luskina, Zavod. I,ab., 35 (1969) 545. 10 V. G. Berezkin and N. S. Nikitina, Zh. Fiz. Khitn., 42 (1968) 2942. 1 1 V. L. Kepke and G. 1:. Sokolin, Zavod. Lab., 36 (1970) 1301. 12 S. Dal Nogare and R. S. Juvet, Gas- Liquid C/iromatograp1iy, Theory and Practice, Intersciencc, New York, 1962. 1 3 A. V. Kiselev and Y. I. Yashin, GasAdsorption Chromatography, Plenum Press, New York, 1969. 14 C. E. Doring, in E. Leibnitz and H. G . Struppe (Editors), Handbirch der Gas-C/iromatographie, Acadeniische Verlagsgesellscliaft, Leipzig, 1966, p. 737. 15 M. S. Vigdergauz and R. I. IzrnayIov, Prinieneniye Gazovos Khrornatografii dlya Opredeteniya Fiziko-khimicheskikh Svo-vsrv Veshchests (Application of Gas Chroniatograpliy for the Determination of Physicochcniical Properties of Substances). Nauka, Moscow, 1970.

30


16 S. Z. Roginsky, M. I. Yanovsky and A. D. Berman, Osnovy Primeneniya Khromatografii v Katalize (Fundamentals of Application of Chromatography in Catalysis), Nauka, Moscow, 1972. 17 A. B. Kiselev and V. P. Dreving (Editors), Experimentalnyie Methody v Adsorbtsii i Molekuliarnoy Khromatografii (ExperimentalMethods in Adsorption and Molecular Chromatography), Moscow University Press, Moscow, 1973. 18 D. A. Vyakirev, N. F. Shushunova, I. I. Chuyev, M. K. Shchennikova and G. G. Kuravskaya, Neftekhimiya, 9 (1969) 861. 19 N. E. Buyanova, G. B. Gudkova and A. P. Karnaukhov, Kinet. Katal., 6 (1965) 1085. 20 K. V. Aleksyeyva, V. G. Berezkin,S. A. Volkov and E. G. Rastyannikov, Polucheniye bChistykh Veshchestv Merodom Preparativnoy Gazovoy Khromatografii (Obtaining Pure Substances by Preparative Gas Chromatography), TSNIITNeftekhimprom, Moscow, 1968. 21 A. Zlatkis and V. Pretorius (Editors), Preparative Gas Chromatography, Wiley-Interscience, New York, London, Sydney, Toronto, 1971. 22 K. I. Sakodynsky and S. A. Volkov, Preparativnaya Gazovaya Khromatografiya (Preparative Gas Chromatography), Khimiya, Moscow, 1972. 23 V. A. Averin, V. Ya. Shnol' and B. G. Distanov, Gazov. Khromatogr., No. 9 (1969) 142. 24 M. Taramasso, Gas Chromatographia, Franco Angeli Editore, Milan, 1966. 25 J. Krugers (Editor), instrumentation in Gas Chromatography, Centrex, Eindhoven, 1968. 26 B. V. Stolyarov, I. M. Savinov and A. G. Vitenbeg, in B. V. Ioffe (Editor),Bukovodstvo k Prakticheskini Rabotam PO Gazovoy Khromatografii (Manual on Practical Work in Gas Chromatography), Leningrad University Press, Leningrad, 1973, p. 284. 27 F. van de Craats, Anal. Chem., 14 (1956) 136. 28 J. Hoomeijer, A. Kwantes and F. van de Craats, in D. H. Desty (Editor), Gas Chromatography 1958, Butterworths, London, 1958, p. 288. 29 W. R. Supina, The Packed Column in Gas Chromatography, Supelco, Bellefonte, Pa, 1974. 30 L. S. Ettre, Open Tubular Columns in Gas Chromatography, Plenum Press, New York, 1965. 31 D. M. Ottenstein,J. Gas Chromatogr., 1 (1963) 1 1 . 32 V. G. Berezkin, V. P. Pakhomov and K. 1. Sakodynsky, Tverdyie Nositeli v Gazovoy Khromatografii (Solid Supports in Gas Chromatography), Khimiya, Moscow, 1975. 33 A. A. Zhukhovitskii and N. M. Turkeltaub, Gazovaya Khromatografiya (Gas Chromatography), Gostopekhizdat, Moscow, 1962. 34 L. C. Browning and J. 0. Watts, Anal. Chem., 29 (1957) 94. 35 E. G. Hoffman,Anal. Chem., 34 (1962) 1216. 36 A. E. Messner, D. D. Rosie and P. A. Argabright,Anal. Chem., 31 (1959) 230. 37 K. L. Grob, D. Mercer, T. Gribbon and H. Wells, J. Chromatogr., 3 (1960) 545. 38 H. Veening and G. D. Dupre,J. Gas Chromatogr., 4 (1966) 153. 39 L. D. Hiushaw, J. Gas Chromatogr., 4 (1966) 300. 40 A. J. P. Martin and A. T. James, Biochem. J., 63 (1956) 138. 41 A. G. Nerheim,Anal. Chem., 35 (1963) 1640. 42 A. A. Datskevich, Gazov. Khromatogr., No. 1 (1964) 79. 43 J. Harley, W. Nel and V. Pretorius, Nature (London), 181 (1958) 177. 44 J. G. McWilliain and R. A. Dewar, Nature (London), 182 (1958) 1664. 45 E. R. Adlard, Crit. Rev. Anal. Chem., 5, May (1975) 13. 46 V. V. Brazhnikov, Differentsialnyie Detektory dIya Gazovoy Khromatografii (Differential Detectors f o r Gas Chromatography), Nauka, Moscow, 1974. 47 V. A. Botin, Radioionizatsionnoye Detektirovaniye v Gazovoy Khromatografii (Radioionization Detection in Gas Chromatography), Atomizdat, Moscow, 1974. 48 L. S. Ettre and W. H. McFadden (Editors), Ancillary Techniques of Gas Chromatography, WileyInterscience, New York, London, Sydney, Toronto, 1969. 49 J. E. Lovelock and S. R. Lipsky,J. Amer. C h e m SOC., 82 (1960) 431. 50 A. Karmen,Anal Chem., 36 (1964) 1416. 51 D. Henneberg, Z. Anal. Chem., 183 (1961) 12. 52 V. L. Talroze, V. V. Reznikov and G. D. Tantsyrev, Dokl. Akad. Nauk SSSR, 159 (1964) 182. 53 J . C. Giddings and R. L. Mallik,i,:d Eng. Chem., 59 (1967) 18.

REFERENCES

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54 A. J. P. Martin and R. L. M. Synge,Biochem. J . , 35 (1941) 1385. 55 M. Singliar, Pracrica Oomatografiei de Gaze, Trad. Dinliniba Slovaca, Buchurest, 1963. 56 J . J. van Deemter, F. .I.Zuiderweg and A. Klinkenbcrg, Cheni. Eng. S c i , S (1956) 271. 57 A. Klinkenherg and I:. Sjenitzer, Chem. Eng. Sci., 5 (1956) 258. 58 J . C. Giddings, Dynamics of Chromatography, Part 1, Marcel Dekker, New York, 1965. 59 E. Glueckauf, in D. 11. Desty (Editor), Gas Orromatography 1958, Academic I’rcss, New York, 1958, p. 33. 60 G. Schay, Theoretische Gnindlageti der Gas-Chroniatoxraphie, VEB Deutscher Verlag der Wissenschaften, Berlin, 1960. 61 IUPAC, Pure Appl. Chem., 37 (1974) 445. 62 Cs. Horvath, in 1,. Ettre and A. Zlatkis (Editors), The Practice of’ Gas Chromatography, Interscience, New York, London, 1967, p. 129. 6 3 D. A. Leathard and R. C. Shurlock, Identification Techniques in Gas Chronzatography, Wiley, London, New York, Sydney, Toronto, 1970. 64 V. ti. Berezkin, Analytical Reaction Gas Chromatography, Plenuni Press, New York, 1968. 65 R. Kaiser, Chromatoyraphie in der Gasphase, Bibliographes Institu!, Mannlieim, 1969. 66 V. G. Berezkin, L. Sojik and I. UndeovB, J. Chromatogr., 9 8 (1974) 157. 67 E. Bayer, in D. H. Desty (Editor), Gas Chromatography, Buttcrworths, London, 1958, p. 333. 68 V. G. Rerezkin, Neftekhimija, 1 (1961) 169. 69 V. G. Berezkin and V. S. Kruglikova, Neftekhiniiya, 2 (1962) 845. 70 G. Schoniburg, Advan. Chrornatogr., 6 (1968) 21 1. 71 A. J. McCormack, S. C. Tong and W. D. Cook, Anal. C%em.,3 7 (1965) 1470. 72 M. Reroza and M. N. Inscoe, in L. S. Ettre and W. H . McFadden (Editors), Ancillary Techniques of Gas Chromatography, Wiley-Interscience: New York, London, Sydney, Toronto, 1969. 73 R. L. Martin,Anal. Chem., 32 (1960) 336. 74 L. A. Kogan, Kolitscizesti~entzayaGazovaya Khromatografiya (Quan titative Gas Chroma tography), Khimiya, Moscow, 1975. 75 A. A. Zhukovitskii. B. A. Kazanskii, 0. D. Sterligov and N. M. Turkeltaub, Dokl. Akad. Nauk SSSR, 123 (1958) 1037. 76 J. Novik, Chetn. Listy, 59 (1965) 1021. 77 V. I. Kallnanovski and A. A. Zhukovitzki,J. Chromafogr., 18 (1965) 243. 78 V. G . Baranova, A. G. I’ankov and Ya. I. Turian, Ostrovy F’izikokhimicheskikh Metadov Analiza i Kontrol Proizvodstva Izoprena (Fundamentals of’f)lzysicochernicalMethods o f Analysis and Control of Isoprene ProducfionJ, NIITEKhim, Moscow, 1965. 79 J. M. Gil1,J. Chromatogr. Sci., 10 (1972) 1. 80 ti. G. Struppe (Editor), .4spects of Gas Chromatography, Deut. Akad. Wiss., Berlin, 1971, p. 144. 81 Chromatographia, 7, No. 9 (1972). 82 H. M. McNair and E. J . Bonelli, Basic Gas Chromatography, Varian Aerograph, Walnut Creek, Calif., 1967. 83 A. 1. M. Keulenians and A. Kwantes, Anal. Chim. Acfa, 13 (1955) 357. 84 A. I. M. Keulernans, A. Kwantes and G. W. Rijndcrs, Anal. Chim. Acta, 16 (1957) 29. 85 N. Brenner and L. S. Ettre,Anal. Chem., 31 (1959) 1815. 86 A. I. Dolgina, A. D. Alekseyeva and A. N. Meshcheryakova, Gazov. Khromatogr., No. 4, (1966) 120. 87 N. If. Ray, J. AppL Chetn., 4 (1954) 21. 88 11. P. Burchfield and E. E. Storrs, Biochemical Applications of Gas Chromatography, Academic Press, New York, 1962. 89 A. N. Zaydel, Elementarnyiye Otsenki Oshihok Izmerenii (Elementary Estimates o f Measurements ErrorsJ, Nauka, Leningrad, 1967. 90 K . Doerffel, Statistik in der Analytischen Chemie, Deutscher Verlag fiir Grundstoffindustrie, Leipzig, 1966.


Chapter 2

Gas chromatographic methods for the analysis of monomers and solvents Multi-ton polymerization processes that can be realized only with very pure starting materials (monomers, solvents, protective gases, etc.) are being employed on an everincreasing scale in the petrochemical and chemical industries. As an illustration, we can cite the requirements imposed by the industry on the purity of hydrocarbon monomers. The maximum permissible concentrations are 4 lo4% for acetylene hydrocarbons, for cyclopentadiene, 1 * 6 * lo”% for piperylene, 5 * for ethers and esters, 1* for water, 2 1 0 % for sulphur as mercaptans and 5 10% for carbon monoxide [ I ] . The maximum permissible concentration of impurities usually must not exceed 10-2-104%, depending on their reactivity, i. e., under polymerization conditions the allowed concentration is from several molecules to several tens of hundreds of molecules of an impurity per million molecules of the monomer. The content of impurities in the solvents and other auxiliary compounds used in the polymerization process must not exceed the maximum permissible concentrations for monomers. Such stringent requirements are necessary because reactions of formation and transformation of polymers are very sensitive t o the presence of small amounts of impurities in the reaction medium or in the polymer chain [2-41. Numerous instances are cited in the literature where a change in leads to an increase the concentration of impurities within the range from lo-’ to in the induction period, a decrease in the rate and extent of transformation and an (undesirable) change in the molecular weight distribution of the polymers formed. In some instances, however, the presence of impurities has a favourable effect on the course of a polymerization reaction [5-71. The effect of impurities on the kinetics and mechanism of polymerization processes is complicated and has been insufficiently investigated. Therefore, in order to obtain correct information on the regularities of polymerization reactions, it is necessary to use starting materials with a high degree of purity and to control the presence of impurities in the polymerization medium during the reaction. Unfortunately, despite the importance of this problem in polymer chemistry, the control of impurities is not always given adequate attention. The analysis of impurities in “pure” compounds and the determination of impurities in commercial products is one of the principal fields of development in contemporary analytical chemistry [8]. In accordance with the definition of Dal Nogare and Juvet [9], we shall regard the components of a test mixture as impurities if their concentration is equal to or less than lo-’%. At present, the gas chromatographic (GC) analysis of impurities is an important independent field of GC that is characterized by a number of specific features of separation, special methods and equipment for analysis and, unfortunately potential sources of errors [ 10-141. In this chapter, we shall describe the specific features of impurity analysis and experimental methods that are of interest for the determination of impurity components in monomers and solvents.

-

-

References p. 52

-

34

GC METHODS FOR THE ANALYSIS OF MONOMERS AND SOLVENTS

The development of GC methods for the analysis of volatile impurities in monomers and solvents has solved most of the practical problems that might arise. The use of high-sensitivity and selective ionization detectors often make it possible to determine impurities in trace concentrations down to 10-6-10-10%. Wide opportunities for increasing the sensitivity of GC determinations of impurities arise by applying methods based on the use of the thermal parameters (temperature programming, chromathermography, etc.), which result in a considerable enrichment of impurity components in the sample, methods with preliminary concentration of impurities and methods involving analytical reaction GC. The universality of GC, the possibility of using high-sensitivity detectors for recording purposes and the rapidity of the method explain the wide use of GC for the analysis of impurities in monomers and solvents.

SPECIFIC FEATURES OF THE GC ANALYSIS OF IMPURITIES The separation of complex mixtures whose components are present in comparable concentrations has been studied comprehensively [9, 15-20]. Therefore, it was desirable to find the extent to which the previously developed concepts and known regularities retain their importance in impurity analysis. With this aim in view, Berezkin el al. [21] studied the effect of the concentrations of the components in a test mixture on their retention times and broadening of chromatographic zones for both light and heavy impurities. The investigation was carried out under conditions such that the absolute value of the concentration of the principal component and the size of the sample did not differ substantially from those used in ordinary analysis. It was found that the retention times of impurities remain virtually unchanged and therefore, in identifying impurity zones, one can use tables of retention values obtained for mixtures with approximately equal concentrations of the components. Of course, retention values are independent of the concentrations of the test substances only when it is possible to neglect the adsorption of the test impurities at the gas-liquid stationary phase interface and, particularly important, at the liquid stationary phase-solid support interface. Reversible adsorption of test compounds at the interfaces under the conditions of impurity analysis may affect two main characteristics of chromatographic separation. Firstly, adsorption causes additional spreading of the impurity zones, with a corresponding decrease in the sensitivity of the determination; secondly, it makes the retention volume dependent on the sample size and the concentration of the components of the mixture. On the other hand, if one uses for chromatographic separation a sorbent on which the retention is determined by adsorption of the liquid stationary phase ( U P ) on the interface as well, the retention times of the test components, in general, must depend on experimental conditions such as the concentration of the components in the test sample, the type of solid support, the method of deposition of the U P , the method of column ageing, the frequency of injection of the sample into the column and the nature of the other components, especially the main substance. The dependence of the retention values on the concentration of the components in a test mixture (ignoring their mutua! influence) must be similar to the dependence of the

srrcmc FJ A T U R ~ OF S THL ANALYSIS OF IMPURITIES

35

retention volunie on the sample size. The latter was studied in a number of investigations [22-341 with different supports, compounds and LSP‘s, and also in relation t o the sample composition and size. A particularly pronounced dependence of the retention volunie on the size (concentration) of the sample of the substance analyzed is usually observed for polar compounds in separation on non-polar phases deposited on a solid diatomite support. Thus, for instance, the retention value of ethanol on a non-polar LSP (6%)squalane on Spheroclironie-I) increases by 300% as the sample size decreases from 0.7 t o 0.1 pI [ 3 3 ] . For a net retention volume, VN, the following equation [ 3 S ] is valid in gas-liquid chroniatograpliy, provided that one takes into account the dissolution of the test compounds in the LSP,adsorption a t the gas -LSP interface and adsorption at the LSP-solid support interface:

where cX is the concentration of the substance in the gas phase, cI is the concentration of the substance in the liquid phase, c I x is the concentration of the substance on the surface of the liquid phase, cS is the concentration of the substance on the surface of the solid support, Vl is the volume of the UP in the column and Sl is the area of the solid support, coated with a layer of U P in the colunin. If the sainple adsorption and absorption isotherms are linear, then the following equation can be written for cI = li!cx, c I g = kl,cg, cs = k s c l , the relative retention volume and the retention index [36] :

(2.2)

where I is the Kovits retention index, PI is the percentage of the LSP on the solid support (the weight ofthe latter is lOO.l,) and X I , Xz and h3 are constants. Using coluinns with different contents of the liquid phase, one can detemiine the invariant values

KI/Klstd.and lo= 100 log

, which depend exclusively on the distribution

of tlie substance in the systen; UP-solid support. The determination of the indicated values was considered by BereLkin 1361. If the adsorption of test compounds at the interfaces is small conipared with adsorption in tlie LSP, then eqn. 2.3 becomes the known eqn. 1.6. As follows froni eqn. 2.2, if the contribution from adsorption to the retention volume is substantial, then in order to obtain reproducible values of the retention voluine it is necessary t o standardiLe not only the LSP, but also tlie solid support. In this instance, however, gas-liquid chromatography loses one of its main advantages, namely simplicity of preparation of the column with reproducible relative retention volumes. For this reason, investigatois naturally tend t o carry out chromatographic separations by References p. 52


36

means of ‘pure’ gas-liquid chromatography, when the retention volume is determined only by the dissolution in the U P . Berezkin and Pakhomov [32] derived an equation describing the dependence of the retention volume on the sample size, assuming non-linear adsorption of the compound on the surface of the solid support. Let us consider the dependence of the retention volume on the concentration (or on the sample size, which is proportional to it), assuming that we can neglect the adsorption of the components at the interface of the LSP with the carrier gas; adsorption in the LSP phase is described by the Henry equation, while the adsorption at the UP-solid support interface is described by the Freundlich equation (0 < 1): as = acf

With the above assumptions, eqn. 2.3 can be rearranged:

where VNstd. = Klstd, V, and Xis the coefficient of proportionality in the equation c,,,. = Xq/V, (see, for instance, ref. 19). It follows from the equation obtained that the relative retention volume increases with a decrease in sample size, and increase in the surface area of the solid support and non-linearity of the adsorption isotherm, i.e., with an increase in 1 - p. The experimental data obtained by Scholtz and Brandt [22] and Berezkin and Pakhomov [32] are fairly well described by eqn. 2.5 and the values of p obtained are in good agreement with the generally accepted concepts of the nature of the adsorption of organic compounds on the surface of solids. As in impurity analysis the real concentration (or the ‘individual’ sample size) of test impurities usually differs by an order of magnitude from the corresponding values in the analysis of ordinary mixtures, even small deviations of the sorption isotherms from linearity m3y lead to perceptible changes in retention values. Therefore, in many instances one can expect the retention values to depend on the concentration of the components in the mixtures being analyzed. Indeed, it was shown experimentally [37, 381 that the identification of polar impurities according to retention times obtained with the use of mixtures in which the same compounds were present in higher concentrations may lead to errors. These features must be taken into account when identifying unknown mixture components. Therefore, it is possible to carry out qualitative identifications of compounds in chromatograms by using either literature data on retention times obtained under conditions of impurity anaIysis or ordinary tabulated retention times; in the latter instance one must first show that the retention times of the expected (assumed) polar impurities are independent of their concentration. The efficiency of a GC column also depends on the concentration of the test compounds. An experimental study on the dependence of the peak width of test compounds on their concentration in the sample was carried out by Berezkin et al. [21] , and the relationship between the peak width and the retention time has also been investigated [34,39]. The presence of the main component in the mixture does not affect the efficiency of separation of impurities, which can be completely separated from the zone

SPECIFIC FEATURES OF THE ANALYSIS OF IMPURITIES

31

of the main substance. These results disagree with the widely accepted notion that in impurity analysis (all things being equal) a much better separation will be achieved if the impurity is eluted ahead of the main substance. One of the specific features of impurity analysis is that the zone of the main substance is usually eluted from the column with a very wide peak, with a greatly broad tail that may overlap (mask) the zones of impurities or impede their determination. Broadening of the zone of the main component in the low concentration range (Le., at concentrations that are a factor of 10-3-10-6 of the maximal concentration has been studied [40]. In Fig. 2.1 the shapes of toluene peaks obtained on injection of equal samples (2 pl) into the column are compared; in recording the chroniatograms, different detector sensitivity scales were used. It can be seen that as the detection sensitivity changes, the peak width and shape change abruptly. In effect, by recording the main peak in the same sensitivity range as for the impurities, we are investigating its shape ‘under a microscope’, the role of which is played by the sensitive detector. The broadening of a chromatographic zone in the low concentration range near the baseline is evidently due t o the same causes as the broadening of zones in ordinary chromatographic analysis, but their ratio is different in this instance. It is known [9, 191 that the broadening of chromatographic zones is usually due t o diffusion and kinetic factors. Longitudinal diffusion broadening of a chromatographic band is caused by molecular and eddy diffusion. These effects lead t o symmetrical broadening of the narrow initial sample peak. In considering this group of causes of peak broadening, it must be noted that while the initial widths of the zones of impurities and of the main substance are the same, the concentration at the maximum of the zone of the main component is 10-5-108times that of the impurities. Hence, in accordance with Fick’s first law, the zone of the main component must be broadened t o a much greater extent than the impurity zones.

x

2

P lJ

i a,

1

-

~-

~

8

~~~~

Time

Fig. 2.1. Change in shape of peak of toluene (sample size 2 PI) with different detection sensitivities: 1 = scale 10” A; 2 = 10- A; 3 = 10” A; 4 = 10- A. References p. 52

38

GC METHODS FOR THE ANALYSIS O F MONOMERS AND SOLVENTS

A quantitative estimation of the dependence of the broadening of a chromatographic zone of the main component on the carrier gas velocity and other experimental parameters was carried out by Berezkin and Tatarinskii [40]. The resistance to mass transfer and longitudinal diffusion are the principal causes of broadening of the peak of the main component in the low concentration range. The important role of the kinetic factors in the formation of the wide band of the main substance was also confirmed by the linear dependence of its width on the particle diameter of the solvent and the decrease in zone width after modification of the support with a small amount of a polar substance (1% of triethanolamine) in chromatographing polar ethanol. Note that the increase in longitudinal diffusion may explain the increase in the width of the chromatographic peak, but it cannot explain its symmetry (see Fig. 2.1). The asymmetry of a chromatographic zone is often due to the non-linearity of the distribution isotherm. This effect may indeed occur, especially in the chromatography of polar main components on LSPs deposited on adsorption-active supports. Thus, if the adsorption isothenn of the main component from the U P on a solid support has a convex shape (for instance, for the Langmuir isotherm), we can write for the retention volume [35] :

where c, = zpcJ( 1 + pc,) and z and p are the parameters of the Langmuir equation. It follows from eqn. 2.6 that the retention volume increases with decreasing concentration of the test component, ie., the chromatographic zone of the main substance must have an extended rear edge. An asymmetrical peak of the main component, however, is also observed with a linear diffusion isotherm. Thus, for instance, the toluene peak (see Fig. 2.1) is clearly asymmetrical, although the retention times of the peak maximum of toluene coincide when chromatographing different-sized samples with the use of different sensitivities (from 10” to A), i.e., over a wide concentration range, indicating the linearity of the distribution isotherm. As the zone of the test substance moves along the column under the influence of the flow of gas carrier in the tail part of the chromatographic zone, the concentration of the substance in the gas phase is reduced with respect to the equilibrium concentration in the stationary phase [41], which leads to progressive broadening of the rear edge of the zone of the main component. Asymmetrical broadening of the peak of the main substance may be caused by various kinetic factors, such as slow kinetics of desorption of the substance from the sorbent grains or slow kinetics of desorption of the substance from stagnant zones. A theoretical explanation of the asymmetry of chromatographic zones due to kinetic effects in preparative separations was given by Giddings [42], and experimental verification was carried out in Takizawa et al. [43]. In selecting the optimal conditions for the separation of impurities and the main component, it is important to estimate the separation quantitatively in relation to the experimental parameters. The separation of the main component and the impurity has been studied [19,44, 451, and it was shown that with an increase in the ratio of the concentration on the main component to the impurity concentration at the zone maximum, the number of

srt:cmc FLATURLS OF THE ANALYSIS OF IMPURITIES

39

theoretical plates of the chromatographic column necessary t o obtain a desired separation greatly increases. Thus, for instance, from the results obtained by Genkin [45] it follows that if tlie difference between the retention times of two components is equal to three standard deviations, one can detect 0.6%'of impurity in the chrornatogram, and when the difference between the retention times increase to four standard deviations, lo-% of impurity. Note that these investigations were carried out with the assumption that the shape of the peak of the main component corresponds t o the Gaussian curve. In this connection, it is advisable t o use for separation high-efficiency columns made of inert materials (glass, stainless steel, etc.). The analysis of impurities on classical and packed capillary columns has been considered [46-481. In order to characterize the separation of the chroniatographic zones of the main component and impurity, several workers [40, 49-52] proposed special, senii-empirical criteria and established their relationship with the experimental parameters by using the concepts of the theory of theoretical plates [53]. hi the course of a chroinatographic separation, the surface of the adsorbent in gas adsorption chromatography, or the surface of the solid support in gas-liquid chromatography, may also exhibit irreversible adsorption of the compounds being separated, or catalytic transformations of the components of the test mixture and the LSP [9, 54-62]. These phenomena may lead t o serious errors in interpreting the analytical results, especially in qualitative impurity analysis. Thus, for instance, if the degree o f conversion of the test compounds in the course of a chromatographic separation is negligible, being a factor of only 104-10-6, then in ordinary analysis, when the components are contained in the mixture in comparable concentrations, the course of such reactions will not affect the results obtained. In the analysis of impurities, however, even such a small conversion of the main coinponent may result in the formation of ghost peaks corresponding t o the conversion products. Therefore, special attention must be paid t o the catalytic activity of the sorbents with respect to the main coniponent. Interfering peaks may also be caused by desorption from the surface of the equipnient or the solid support of compounds that are relatively strongly adsorbed froin previous samples during the analysis of more polar substances (displacement effect) 163-661, and the release of volatile substances from tlie rubber seals used in the sample injectors of gas chromatographs [67-691 and from other parts of the chromatographic equipnient [70] . Irreversible adsorption of the components of test mixtures on the surface of the solid support in the course of a GC separation may also cause large errors. For the analysis of mixtures with comparable concentrations of the components, this phenomenon was studied by Kusy [ % I , although it has been noted earlier [ 5 9 , 6 0 ] . Direct proof that adsorption of test compounds on the support occurred under the conditions of a chromatographic method was obtained by tracer studies [61], and the effect of adsorption on tlie surface of a solid support 1621 on the quantitative results in impurity analysis was studied by Berezkin et al. [7 11. The best results can be obtained by using solid supports modified by small additions of polar non-volatile or volatile compounds [72]. It is also necessary t o check the correctness of the procedures developed for the analysis of impurities by analyzing standard mixtures the composition of which is known and is similar t o that of the samples being analyzed, because in analyzing impurities the separation of all of the components is necessary but does not ensure the correctness of the results. References p. S Z

40


In recent years, some use has been made of the method of reducing the adsorption effects based on saturation of the carrier gas with the vapour of volatile (usually polar) substances. The choice of the carrier gas depends on the particular problem and primarily on the composition of the impurities to be determined. Thus, for instance, the use of water vapour [72-741, formic acid [63], ammonia [64] and other substances has been described. In some instances it is advisable to use, as the stationary phase, the vapour of a substance that is not recorded by high-sensitivity detectors (for instance, water or ammonia vapour when using a flame-ionization detector, or hexane vapour when using an electron-capture detector). A survey of the use of vapour mobile phases in elution GC has been published by Rudenko et al. [7S] . TABLE 2.1 SOME PROCESSES THAT IMPEDE THE ANALYSIS OF IMPURITIES BY GC Process

Analytical consequences

Recommended techniques

Reversible adsorption at the liquid stationary phase-solid support interface, and also on the surface of the solid support not covered by liquid phase, and on the surface of chromatographic equipment.

Non-reproducibility of retention time; difficulties in identification. Spreading and asymmetricity of chromatographic zones, reduced accuracy and sensitivity of determination.

Modification of solid supports. Inert materials in chromatographic equipment. High-efficiency columns, equipment ‘training’.

Irreversible adsorption of components on the surface of the solid support and on the surface of chromatographic equipment.

Complete or partial loss of impurity components; erroneous, low results.

Modification of the surface of the solid support and the surface of equipment. Equipment ‘training’.

Displacement of impurities in the course of analysis.

False peaks; erroneous, high results.

Inert carrier gases and materials in chromatographic equipment. Use of equipment for analysis of impurities of the same type. Pure carrier gases and thermally stable liquid stationary phases.

Chemical conversions of main component and impurities.

Loss of some components. False peaks.

Inert sorbents and materials in chromatographic equipment.

Spreading of zone of main Masking of impurity zones by component in low concentration main component zone. range.

High-efficiency columns. Methods of reaction chromatography and selective detectors. Intermediate isolation of main component fraction with masked impurities for the purpose of concentration.

IMPROVING THIS SENSITIVITY OF T H E DETERMINATION OF IMPURITIES

41

In analyzing aggressive polar compounds, when during the analysis they may be adsorbed on the support or the equipment, or a direct reaction with components of the liquid stationary phase may occur, a useful method is 'training' of the column, i.e., the introduction of the test compound into the chromatograph at regular intervals for a considerable length of time [76]. Adsorption phenomena should also be taken into account when preparing standard mixtures for calibration of the chromatograph. In this connection. we wish t o point out the expediency of using dynamic methods, among which the most reliable are, in our opinion, the diffusion [77-85] and electrochemical [86, 871 methods. A survey ofniethods for the preparation of reference vapour-gas mixtures in the ultramicro concentration range has been published b y Popov and Pechennikova [88]. Table 2.1 lists some processes that impede the GC analysis of impurities.These features must be taken into account in developing effective procedures for the analysis of impurities. In developing procedures for the analysis of impurities in practical systems, one usually has t o overcome specific difficulties associated with insufficient detection sensitivity and poor resolution of the impurity and the main component. In order t o solve these problems in GC, a number of methods have been developed, the application of which permits the sensitivity of the determination of impurities to be increased and their separation from the main substance t o be improved.

METHODS FOR IMPROVING THE SENSITIVITY OF THE DETERMINATION OF IMPURITIES

In order to improve the sensitivity of the methods for GC separation of impurity components, i.e., t o reduce the threshold concentration o f the impurities, which can still be reliably determined, the following methods have been developed and applied: (1) increasing the size of the sample being analyzed; (2) utilization of high-sensitivity detectors; (3) non-isothernial chromatographic methods; (4) concentration techniques. We shall now consider these methods in more detail. Increasing the size of the sample being analyzed Under the conditions of elution analysis, continuous broadening of the chromatographic zones takes place in the column, and this decreases the concentration at the maximum of the impurity zone. This fact limits the sensitivity of the GC determination of the test compounds, as the impurity component can be recorded by the concentration detector only when the concentration (or flow) of the substance at the zone maximum exceeds the minimal determinable value for the detector used in a given device. In accordance with the theory of theoretical plates [53], the maximal concentration in a zone increases with an increase in the number of theoretical plates, n(4), and the sample size, 4 , and with a decrease in the retention volume, V, :

References p. 5 2

42


It follows from this equation that one of the simplest methods for improving the sensitivity of a procedure is to increase the size of the test sample. When large samples are used for analysis, however, the width of the chromatographic zones increase, thereby affecting the efficiency of separation. This effect limits the application of the method of large samples to impurity analysis. In practice, when developing a satisfactory procedure, the task reduces to the selection of,the optimal sample size, which is a ‘compromise’ with respect to two factors that change in opposite directions with an increase in the test sample, namely sensitivity and com ponen t separation. Theoretical consideration of the shape of the chromatographic peak in relation to the sample size has been carried out [20, 89-91]. Ingenious calculation methods, which are of great practical and theoretical interest for the selection of the optimal experimental conditions, were developed by Kalmanovskii and Zhukhovitskii [92]. The change in column efficiency with increasing sample volume has also been studied [44,45,93-951 . If the concentration dependence of the test sample has a rectangular shape (the ‘piston rod‘ method), then, with an increase in the size of the sample (i. e. , of the width of the ‘rod’, the width of the chromatographic peak will also increase and so will its maximal concentration. As demonstrated [92], the maximal concentration of the substance in the chromatographic zone increases approximately linearly with sample size up to samples for which the ratio of the sample width to the peak width with a vanishingly small sample is 0.8. For samples of this size, the relative peak width at half-height increases by only 15%compared with the peak width with a vanishingly small sample, i.e., this unfavourable effect on separation can often be neglected. The concentration at the peak maximum in this instance is 65%of that in the sample. The analysis of impurities in monomers with the use of large samples and a chromatograph with a katharometer has been described [93-1001. With large samples, the broadening of the initial chromatographic zone is incomplete; only the edges of the zone broaden, and the elution curves have a step-like shape. This version of chromatography was proposed independently by Zhukhovitskii and Turkeltaub [IOl-1031 and Reilley et al. [lo41 and was termed stepwise chromatography [101-1031. Analysis under the conditions of stepwise chromatography has a number of advantages over the ordinary version: (1) a higher signal stability; (2) directly proportional dependence of the height of the zone (step) on the concentration of the substance in the test sample, which simplifies the calculations and the quantitative interpretation of chromatograms; (3) errors caused by small irreversible adsorption of the test substance on the sorbent layer are partially eliminated; and (4) the sensitivity slightly increases (the concentration of the substance in the chromatographic zone is equal to that in the initial sample). A drawback of stepwise chromatography compared with the ordinary variant of elution chromatography is a lower efficiency associated with the greater width of the step-like zones. Therefore, stepwise chromatography can conveniently be used for the analysis of impurities in systems whose final components can be separated very well. Stepwise chromatography can be applied to monomer analysis; for instance, Palamarchuk [ 1051 used it for determining trimethylchlorosilane and methyltrichlorosilane impurities in dimethyldichlorosilane. A stepwise method for determining water as an impurity in the butane-butylene fraction has also been described [106].

IMPROVING THL SI NSITIVITY OE THI DLTLKMINATION or IMPURITICS

43

In order t o reduce the broadening of an initial zone during sample injection and to decrease tlie size of the sampling volume, the use in stepwise chromatography has been suggested [lo71 of a therniostated sampling volume filled with a sorbent, which is blowed with the test mixture t o saturation. The use of this method for sample injection enables one to reduce the spread when introducing large samples and also t o carry out relative concentration of heavy impurities. If the sample size is not limited, it may be useful in impurity analysis to apply the method of vacancy chromatography, which was developed by Zhukhovitskii and co-workers [ 108-1 101 and also Reilley et d.[ 1041. In vacancy chromatography, the test mixture, and not the carrier gas. passes through the column. If, for instance, we introduce a certain volume of pure carrier gas into the flow of this mixture upstream of the chromatographic column, vacancies (regions of reduced concentration) are formed that move along the column at different speeds characteristic of the impurity components. Zhukhovitskii and Turkeltaub [ 1031 pointed out the following advantages of vacancy chromatography: (1) the test mixture is passed continuously and there is n o need t o use a carrier gas; (2) the sampling process is simplified; (3) the total concentration of the mixture components is measured continuously; and (4) the concentration being detennined is not measured instantaneously, but is averaged over a certain period of time. An interesting version of vacancy chromatography, which may also find application in impurity analysis, is differential chromatography [ 1 101 . In differential chromatography, the test mixture is continuously passed through the column and a mixture of pre-assigned composition is sampled periodically. The peaks on the chromatograni characteriLe the deviation of tlie current concentrations from the pre-assigned concentrations. If the concentrations of the components in the test mixture do not differ from those of the corresponding conipo~indsin the pre-assigned mixture, no vacancies (or peaks) are formed. Vacancy chromatography is currently used in the analysis of impurities in pure gases [Ill].

Use of high-sensitivity detectors An impurity can be recorded only if the following conditions are met: for a concentration detector L'max.'>

Clim.

and for a continuous flow detector

c,,x.F

jiim.

(2.9)

I n analydng impurities, the most important characteristic of the detector is the sensitivity threshold, although for practical use of a detector other characteristics are also important. When using chromatographs with ordinary katharonieters, it is possible t o determine [ 112-1 141. If special conditions impurities at concentrations of about (thermostating, flow stability, etc.) are observed, a high-sensitivity katharometer makes and when the it possible t o determine impurities at concentrations as low as katharonieter is equipped with themiisters, even 5 * 10% [ 115, 1161 . The sensitivity of the katharometer can be increased by one or two orders of magnitude compared with References p. 5 2

44


the detector with filament sensitive elements by using fim-type sensitive elements (for instance, a thin layer of platinum plated on a quartz filament) [ 117, 1181 . The sensitivity of the katharometer can be increased 10-fold by using an a.c. power supply with subsequent strengthening of the signal and phase detection, also by using a reference signal [119]. In determining gaseous compounds, the sensitivity of the katharometer can also be improved by reducing the temperature of the detector block (walls) [ 1201 . For organic compounds, the sensitivity of detection with a katharometer can be increased by first converting the test compounds into carbon dioxide [121] or hydrogen [ 1221. It has been shown [123] that the last-mentioned method permits the sensitivity of the katharometer to be increased by a factor of 20-30. The katharometer is a simple, reliable and universal detector, but its application in impurity analysis is limited by its relatively low sensitivity. The katharometer is usually employed in determining impurities of inorganic gases in procedures involving a concentration stage. Ionization detectors, the sensitivity of which is 102-107 times that of the katharometer, are used much more frequently in methods for determining impurities. In order to determine organic impurities in monomers, use is generally made of the flame-ionization detector. This method was instrumental, for instance, in developing the procedure for the analysis of impurities in ethylene [67, 124-1261, propylene [127, 1281 and styrene [129, 1301. The flame-ionization detector is also used in industrial analysis for determining impurities in inonomers and inert gases [ 13, 1 31, 1321 . Improvement in detection sensitivity is one of the main aims in the development of chromatographic equipment. The importance of this problem in chromatography was emphasized by Martin [133]. Detailed surveys on chromatographic detectors have been made by Brazhnikov [134] and Adlard [ 1351. The advances achieved in recent years in the field of impurity analysis due to the use of high-sensitivity detectors with batch-produced devices are demonstrated in Fig. 2.2 [136]. It can be seen that the widely used ionization detectors permit the determination of impurities present in monomers and solvents at very small concentrations. Note that the application of concentration methods usually makes it possible to increase the sensitivity, i.e., to reduce the concentration being determined for any detector used a further 10-1000-fold. Concentration can be achieved either by employing special methods or by applying non-isothermal methods. Application of non-isothermal methods Compared with separation under isothermal conditions, the use of the thermal effects in impurity analysis enables one to increase, in the course of a chromatographic separation, the concentration of the test substance at the maximum of a chromatographic zone and hence to improve the sensitivity of determination. Also temperature programming makes it possible to reduce the analysis time and to analyze impurities that differ widely in their boiling points. The widest use in chromatographic practice is made of the temperature programming method [I371 in which the temperature is increased along the entire length of the

IMPROVING THE SENSITIVITY OF THE DETERMINATION OF IMPURITIES

45

U

C

cn

0’

6

10-8

c (%I

Fig. 2.2. Determination of impurities with high-sensitivity chrolnatographic detectors. 1 = Katharometer; 2 = flame-ionization detector; 3 = electron-capture detector; 4 = thermionic detector; 5 = helium detector; C, concentration (being determined) of component in sample.

column during the separation of a mixture. Harris and Habgood [138] noted that when the principal task is the separation of two closely spaced peaks, the best separation can probably be obtained under an isothermal regime, but for widely differing substances tern perature programming niay improve the degree of separation. The temperatureprogramming method is particularly useful when the difference in the boiling points of the sample components exceeds 50- 100°C, and in some instances (for example, when using selective sorbents and in rapid analysis) with a smaller temperature difference. Zhukhovitskii and Turkeltaub [ 139, 1401 were among the first investigators t o point out the possibility of improving the sensitivity of impurity analysis in GC by using temperature programming. Questions of the enrichment of both light and heavy impurities in temperature prograniming were specially considered by Datskevich ef al. [141], who found that in GC with temperature programming the concentration of the test compounds at the zone maximum on the initial curve niay considerably exceed (25- t o 30-fold) the concentration in the initial sample. References p. 5 2

46


An important contribution to chromatographic practice was made by the work of Janlk on analysis with temperature programming [ 142- 1441 . The use of time-programmed column heating permits the time required for the analysis of impurities t o be reduced and impurities that boil over a wide temperature range to be analyzed, on a single column [128, 145-1471. A disadvantage of the thermal methods is that in some instances a reduced separation efficiency compared with the isothemial regime is obtained. Berezkin and Tatarinskii [ 1481 proposed a version of temperature programming (the ‘thermal shock’ method) that over comes this disadvantage. Zhukhovitskii and co-workers [ 149-1 531 proposed and developed a new version of GC utilizing thermal effects, namely chromatography, the application of which is particularly promising for the analysis of impurities. l o achieve a separation, chromatography uses a temperature field moving along the column; the change in the column temperature along the length of the column does not occur instantaneously, as in temperature-programmed chromatography, but over a certain period. In stationary chromatography, the direction of the movement of the carrier gas and the motion of the oven coincide; the temperature gradient in the oven has a negative value, that is, the temperature increases in the direction of movement of the oven. Molecules that ‘outrun’ the chromatographic zone for any reason enter the region of the cold sorbent, where the speed of their motion along the column decreases. Molecules that on the contrary, ‘lag behind’ the chromatographic zone enter a zone that contains hotter sorbent compared with the sorbent in the centre of the zone. The speed of molecular motion along the column in the hot zone is higher than the average speed of the molecules of a given component, and therefore the ‘lagging’ molecules quickly ‘overtake’ their chromatographic zone. Compared with chromatography with temperature programming stationary chromathermography makes it possible to obtain: (1) symmetrical peaks, even with a non-linear sorption isotherm, which does not, in chromathermography, lead to zone asymmetry, as in this instance the rear (usually broadened) edge of the chromatographic zone is at a higher temperature than its front edge; and (2) a considerable enrichment of concentration of the impurity components. In order to obtain narrow zones, it is advisable to use columns of small diameter and a fine-grained sorbent. A considerable contribution t o the development of stationary chromatography for impurity analysis was made by Kaiser. He demonstrated the possibility of the determination, in’gases, of heavy impurities at concentrations of lo-’’% when using very large samples [154-1581 . A modification of this method for determining heavy impurities in solvents and volatile liquids was proposed by Berezkin and Starostina [ 1591 ; In chromatographic practice, despite its above advantages in impurity analysis, chromathermography is used much more rarely than chromatography with temperature programming; in our opinion, this is associated with the need for more sophisticated equipment. Berezkin and co-workers [160, 1611 and later Fatscher and Vergnand [162] proposed a new version of chromathermography in which separation is effected simultaneously along the entire length of the column under a negative temperature gradient. This version of chromathermography consists in using, for separation, a constant temperature gradient dong the column together with temperature programming.

IMPROVING THE SI NSITIVITY OF THE DCTrRMINATlON O € IMPURITIES

41

The variation of temperature with time under a negative temperature gradient along the colunm is equivalent to the motion of a thennal field under conditions of chromathemiography [ 161 ] . This method perniits chr(~iiiatliennograp1iicseparation with the use of a column of any length and shape. The method was checked experimentally for the detemiination of inipurities in toluene; the concentrations at the peak maximum hicrease 10- t o 15-fold conipared with the isothennal method. An ingenious pulse-thernial method of gas analysis was developed by Dantsig [ 1631. One of the advantages o f the method tein perature prograniming is the (usually) weak dependence of the chroniatographic separation efficiency on the size of the sample being analyzed [164, 1651. The use of different versions of temperature programming considerably increases the sensitivity of the detemiination of impurities and the role and importance of nonisothemial methods in such analyses will increase in the future. Concentration methods If the conventional methods of chromatographic separation are not adequate (for instance, because of the low detector sensitivity or poor separation of the impurity and the main coinponent), special methods of sample preparation can be used (in particular, pre-concentration arid separation of the impurity from the main component). Concentration is also used when non-chromatographic niethods are employed for identification of impurities (special optical methods, mass spectrometry, nuclear magnetic resonance spectroscopy, etc.). Therefore, impurity concentration is often a necessary stage in the analytical investigation of nionomers and solvents.

Prepura t iw d u t ion c I i ro ma tography Preparative elution chromatography is widely used for concentration in impurity analysis [9, 19, 166-1701 owing to high separation efficiency, the possibility of separating comparatively large samples, repeated separation cycles and complete automation of the separation process. One of the principal sources of errors in quantitative analysis when using preparative elution GC for concentration is incomplete trapping of the separated fractions. In order to trap the fractions isolated on a preparative column, cold traps are used, the design of which depends on the size of the preparative column, the properties of the impurities isolated and the methods used for their subsequent analysis [171-1771. Roggus and Adains [ 1781 were among the first researchers who used comparative elution chromatography for the analysis of impurities. The applications of the method to impurity analysis are described elsewhere [ 179- 1851 . CIi 1’0rnatography

w i t h i i t a carrier gas (high-concen tru tion GC)

The main drawback of concentration methods based on elution chromatography is the low concentration of the impurities eluted from the chromatographic column and the need for a special operation to trap impurity components from the flow of carrier gas. References p. 5 2

48


The methods of chromatography without a carrier gas (which Zhukhovitskii et al. [186] also call high-concentration chromatography) permit one to obtain, under isothermal conditions, a concentrated zone of both light and heavy impurities at the column outlet. In chromatography without a carrier gas, the procedure (the absence of a carrier gas, large samples, etc.) and the separation mechansim differ from those adopted in elution chromatography. In chromatography without a carrier gas, the zones are not separated from each other by a layer of carrier gas but adjoin each other. The different velocities of the gas flow in the zones formed and the different adsorbabilities of the components being separated are the cause of the formation of zones with clearly defined boundaries, whose broadening is limited. In contrast to isothermal elution chromatography, chromatography without a carrier gas permits enrichment such that virtually pure components are obtained at the column outlet, and the difference in the flow-rates in the chromatographic zones leads to a sharp decrease in edge broadening [186] . A concentrated impurity zone can be directed straight to the analytical column for detailed separation into individual components, or to some other analytical device for their identification. Important contributions to the development of some versions of chromatography without a carrier gas have been made by Dubinin and co-workers [187,188], Schay [41], Zhukhovitskii and co-workers [186, 189,1901, Claesson [191], Altshuller and co-workers [192, 1931 and Guiochon and Jacob [194]. A survey of new versions of chromatography without a carrier gas has been published by Sazonov [ 1951. Below we consider the principal versions of chromatography without a carrier gas that have been or may be used for impurity concentration. Frontal chromatography As a general method of concentration, frontal chromatography was developed by Mirzayanov et al. [ 1961 ;individual applications of frontal chromatography for concentration were described previously [197, 1981. Frontal concentration of poorly adsorbed impurities can be effected in two ways: (1) by filling the column before the experiment with an inert gas that is less sorbable than the impurity [196, 1991 ; or (2) in the absence of an inert gas filler, when a vacuum is set up in the column [200]. The first method is simpler as regards the equipment, while the second is useful when concentrating very light gases if it is difficult to select a filler gas that meets the condition impurity distribution constant > filler gas distribution constant. In the simplest system for the analysis of poorly adsorbed impurities, the concentration and separation columns are connected in tandem, i.e.. they are actually combined in a single column [196,201]. The method of frontal concentration has been used successfully for analyzing poorly adsorbed impurities in ethylene [196,201,202] and propylene [203], and also for determining the content of light (C4-C,) hydrocarbons in dimethylformide and acetonitrile [204]. Some time ago, a Soviet design office developed a laboratory-type gas chromatograph ('Luch'), which is also based on frontal-adsorption enrichment of light impurities in a pre-evacuated column [205].

IMPROVING THE SENSITIVITY OF T H E DETERMINATION OF IMPURITIES

49

The chromatograph can be used, in particular, for determining helium, neon and hydrogen in micro-impurities in atmospheric air, hydrogen in argon, etc. The minimal determinable concentration of light gas impurities is 10-5-10-6% (the volume of the sample analyzed being up t o 1000 ml). For concentrating heavy impurities, wide use is made of the frontal method coupled with subsequent thermal desorption [206-2131. In this method, a definite amount (up to tens of litres or more) o f the sample being analyzed is passed through a trap (sometimes a cold trap) filled with sorbent. It should be noted that the use of high temperatures for the desorption of impurities from the concentration column may lead t o various undesirable side reactions, especially when determining unstable compounds. Therefore, in many instances it is better t o use the method of displacement (washing out) of impurities with a suitable solvent [ 2 1 4 , 2 1 5 ] . All the above methods for adsorbing impurities in a concentration trap are based on complete absorption of heavy impurities by the trap packing from the entire volume of the gas being analyzed. A radically different method was proposed by Novik et al. [216], in which the gas sample is passed through a small concentration column (with the corresponding stationary phase) that is at the tem perature of the surrounding medium, until the test impurities begin t o emerge, i.e., the concentration of the impurities in the sorbent is in equilibrium with that in the initial mixture along the entire length of the column. This method has the following advantages: (1) there is no need for accurate measurement of the volume of the gas passed through the trap, it being sufficient t o determine its excess and to know the precise temperature of the concentrator; (2) the method permits a selective increase in the sensitivity of impurity determination or elimination of components that interfere with determination by selecting the appropriate packing (for instance, the use of a non-polar liquid phase can eliminate the effect of water vapour, which is sometimes the cause of difficulties in sorption methods of concentration); and (3) there is a possibility of ‘smoothing out’ the amount of individual components in the trap, as the concentration effect usually increases proportionally with the increase in molecular weight. A disadvantage of the method is the need to maintain a constant temperature during sorption (concentration). A technique for the application of this method in analyzing monomers (ethylene and propylene) was successfully developed by Lulova and co-workers [2 17-2 191 . When analyzing heavy impurities, Zhukhovitskii and Turkeltaub developed a modification of the frontal method, the thermodynamic method [ 1 9 , 2 2 0 , 2 2 1 ] , which has been widely applied by both Soviet and other workers [ 154-1 58, 222-2241. In the thermodynamic method, a heated oven with a negative temperature gradient is displaced periodically along the column while the mixture is fed continuously into the column. The heavy components (impurities) adsorbed at the head of the column start t o move along the column in the oven zone under the effect of the heat field of the test mixture and are periodically eluted from the column. The role of the carrier gas in this instance is played by the main component of the test mixture. In order t o increase the working length of the layer, it is advisable t o use a circulation diagram [225] and a moving sorbent [226]. For the purpose of concentrating impurities, use has been made successfully (in addition t o the frontal method) of other modifications of chromatography without a carrier gas: References p. 52

v1

TABLE 2.2

0

METHODS OF ANALYTICAL REACTION GC FOR THE ANALYSIS OF IMPURITIES Reacting compound

Main substance

Impurity

Carrier gas

Change in characteristics of test compounds as a result of chemical reactions

Change in characteristics of detection of test compounds as a result of chemical reactions

Retention time increases

Detection sensitivity increases

Retention time decreases

Detection of impurities against background of main substance forming non-detectable compound [2611

Separation of impurities and main component forming low-volatility compound with reagent [249] Frontal-chemical concentration [251-2531 Impurity concentration by using chemical absorbents forming low-volatility compound with impurity [254,255]

Detection sensitivity decreases

4:

3a

Conversion of nondetectable Separation of inipurity and compounds into compounds main component by recordable by high-sensitivity converting impurities into volatile compound with the detectors, by means o f use of: (a) b u b h h g liquid (a) single-stage conversion reactor [256] or (b) tubular [258] or (b) two-stage reactor [257] conversion of compounds that do not contain carbon [259] Non-selective concentration of zones of impurities and main substance as a result of chemical binding of part of carrier gas [260]

n

z>

z U m

Z

DEMASKINC IMPURITIES

51

thermal displacement [227-2301, displacement [231-2331 and elution-thermal displacement chromatography [234-2361. Wide use has also been made of non-chromatographic methods of concentration, which are combined with subsequent GC analysis. Of course, in order t o solve a practical problem by the optimal concentration method, any selective separation method can be used, such as distillation [237], recrystallization [237], extraction [238], liquid column chromatography [239, 2401 and thin-layer chromatography [241] . The phase equilibrium method is particularly promising as a method of concentration for subsequent detemiination of trace impurities by gas chromatography [242]. A detailed survey of the method has been published by Vitenberg et al. [243] .

METHODS FOR DEMASKING IMPURITIES AGAINST THE BACKGROUND OF THE MAIN COMPONENT When analyzing impurities in specially pure substances, apart from the usual problems of separation of all impurity components of a complex mixture, which often have similar properties, other problems arise that are associated with the masking of the zones of the impurity components by the broad zone of the main substance. Utilization of selective sorbents and selective detectors

As the final result of a chromatographic analysis is determined by characteristics of both the sorbent and the detector, in the analysis of impurities the use of selective sorbents and detectors is of special importance. To determine the test impurities that are masked by the zone of the main component, one can use a selective sorbent on which the retention times of the impurity and the main components differ widely. Another solution t o this problem consists in the use of a selective detector whose sensitivity t o the impurity component is much higher than that t o the main substance. In this version, it is also possible t o detemiine the content of the impurity component, even if the test impurity is eluted in the zone of the main substance. The sorbent selectivity (a) for determining impurities must meet the condition

a > 1 +%

(2.10)

tm

where (Y = r i / r , , ri and tm are the retention times of the impurity and the main component, respectively; and p m is the width of the zone of the main component (the region of the chromatographic zone in which impurities cannot be determined). For a preliminary choice of a selective LSP it is advisable to use published tables of relative retention times. In gas-liquid chromatography, selective stationary sorbents of various types are known (complexing phases, liquid crystals, volatile solvents, etc.), which have been surveyed in detail [15, 16, 18, 2441. The selectivity of solid sorbents has been comprehensively discussed by Kiselev and Yashin [62]. As the column efficiency with respect t o the main component increases, i e . , when p, decreases, the requirements imposed on the selectivity o f the sorbent are relaxed. Therefore, particular care must be taken t o reduce the zone width of the main component. References p. 52

52


In recent years, selective detectors have found wide application in GC; they show high sensitivity only to certain groups of compounds. Comprehensive surveys of selective detectors has been published by KrejEi and Dressler [245] and Adlard [246]. The mass spectrometer is the most selective detector. The use of combined chromatography-mass spectrometry for the analysis of impurities is a particularly promising method [247-2481. Analytical reaction GC

In recent years, a new aspect of GC has been developed, namely analytical reaction GC, which has been applied successfully in the analysis of impurities. It is interesting to note that even in the first investigations on analytical reaction GC, problems of determining impurities masked by the main component [249] and of improving detection sensitivity [250] were solved. The combination of the chromatographic and chemico-analytical methods resulted in a new method, the potential of which is much greater than those of either of the two original methods. The combined methods of analytical reaction gas chromatography that have been developed for analyzing impurities, assuming different reactivities of the impurity and main components, are listed in Table 2.2. Methods of analytical reaction chromatography are currently widely used in analytical practice [262] . REFERENCES 1 L. S. Kofman and V. S. Vinogradova, Izv. Akad. Nauk SSSR, Ser. Khim, (1965) 375. 2 V. A. Kargin, Osnovnyie Problemy Khimii Polimerov (Principal Problems of Polymer Chemistry), Plenarnyi Doklad na VIII Mendeleyevskom Syezde PO Obshchey i Prikladnoy Khimii, Academy Press, Moscow, 1958. 3 S. E. Bresler and B. L. Erusalimsky, Fizika i Khimya Makromolekul (Macromolecule Physics and Chemistry), Nauka, Moscow, Leningrad, 1965. 4 Kh. S . Bagdasarian, Teoriya Radikal'noy Polimerizatsii (Radical Polymerization Theory), Nauka, Moscow, 1966. 5 E. B. Lyudvig, A. R. Gantmakher and S. S. Medvedev, Dokl. Akad. Nauk SSSR, 156 (1964) 1163. 6 R. H. Biddulph, P. H. Plesch and P. P. Rutherford, J. Chem. Soc., (1965) 275. 7 K. Ueno, K. Hayashi and S. Okamura, Polymer, 7 (1966) 431. 8 P. Auger, Current Trends in Scientific Research, UNESCO, Geneva, 1963. 9 S. Dal Nogare and R. S. Juvet, Gas-Liquid Chromatography, Interscience, New York, London, 1962. 10 V. Svojanovsk?, M. KrejEi, K. Tesa'fik and J. Janak, Chromatogr. Rev., 8 (1966) 90. 11 V. G. Berezkin and V. S. Tatarinskii, Gas Chromatographic Analysis of Trace Impurities, Consultants ' Bureau, New York, London, 1973. 12 R. Kaiser, in K. V. Chmutov and K. I. Sakodynsky (Editors), Uspekhi Khromatografii, Nauka, Moscow, 1972, p. 193. 1 3 E. V. Vagin, in K. V. Chmutov and K. I. Sakodynsky (Editors), Uspekhi Khromatografii, Nauka, Moscow, 1972, p. 262. 14 V. G. Baranova, A. G. Pankov and N. K. Loginova, Metody Analiza v Proizvodstve Monomerov dlya Sinteticheskikh Kauchukov (Methods of Analysis in Production of Monomers for Synthetic Rubbers), Khimiya, Moscow, 1975. 15 E. Leibnitz and H. G. Struppe (Editors), Handbuch der Gas-Chromatographie, Akademische Verlagsgesellschaft, Leipzig, 1966. 16 R . Kaiser, Chromatographie in der Gasphase, Bibliographisches Institut, Mannheim, 1969. 17 J. C. Giddings, Dynamics of Chromatography, Part I, Marcel Dekker, New York, 1965. 18 W. R. Supina, The Packed Column in Gas Chromatography, Supelco, Bellefonte, Pa., 1974.

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54

GC METHODS FOR THE ANALYSIS O F MONOMERS AND SOLVENTS

63 D.M. Geddes and M . N. Gilmour,J. Chromatogr. S c i , 8 (1970)394. 64 I. I. Kan, D. Kh. Sembayev and B. V. Suvorov, Zh. Anal. Khim., 25 (1970)377. 65 R. C. Dressman,J. Chromatogr., Sci., 8 (1970)265. 66 R. A. Simonaities and G. A. Guvemoter, J. Gas Chromatogr., 5 (1967)49. 67 R. H. Kolloff,Anal. Chem., 34 (1962)1840. 68 I. M. Calery, J. Chromatogr. Sci, 8 (1970)408. 69 A. V. Alekseyeva, E. E. Kugucheva, A. V. Kovalev, A. N. Kostyutsenko and B. 1. Turbina, Khim. Prom., No. 5 (1968)351. 70 G. Mizuno, D. Jarvis and J. R. Chipaulf, J. Gas Chromatogr., 6 (1968)179. 71 V. G. Berezkin, V. P. Pakhomov and V. R. Alishoyev, Khim. Teckhnol. Topl. Masel, No. 10 (1967) 56. 72 L. H.Phifer and H. K. Plummer, Anal. Chem, 38 (1966)1652. 73 H. A. Saroff, A. Karmen and J. W. Healy, J. Chromatogr., 9 (1962) 122. 74 B. A. Rudenko, M. A. Baydarovtseva, V. A. Kuzovkin and V. F. Kucherov, J. Chromatogr., 104 (1975)271. 75 B. A. Rudenko, M. A. Baydarovtseva and M. A. Agayeva,Zh. Anal. Khim., 30 (1975)1191. 76 Yu. S. Drugov, Candidate’s Thesis, Institut Gigieny Truda i Professionalnykh Zabolevaniy Akademii Meditsinskikh Nauk SSSR, Moscow, 1969. 77 A. E. O’Keefe and G. C. Ortman, Anal. Chem., 38 (1966)760. 78 L. Demaio, Instrum. Technol., 19 (1972) 37. 79 G. S. Beskova, L. P. Russo and Z. L. Baskin, IV Vsesoyuznaya Nauchno-tekhnicheskaya Konferentsiya ‘Sostoyaniye i Perspekitivy Razvitiya Cazovoy Khromatografii: Baku, Tsentr. Pravleniye NTO ‘Priborprom’, Moscow, 1973,p. 59. 80 N. Sh. Vol’berg, Tr. Glav. Geofiz, Obs., No. 238 (1969)229. 81 A. P. Altschuller and I. R. Cohen, Anal. Chem., 32 (1960)802. 82 D. K. Kollerov and K. D. Mgaloblishvili, Izmer. Tekh., No. 7 (1968)50. 83 A. E. O’Keefe and G. C. Ortman, Anal. Chem., 38 (1966)766. 84 P. P. Lucero, Anal. C h e m , 43 (1971)1744. 85 N. Sh. Vol’berg and E. D. Egorova, Tr. Glav. Geofiz. Obs., No. 293 (1973)93. 86 P. A. Hersch,J. AirPollut. Contr. Ass., 19 (1969)164. 87 M. Pribyl, Z. Anal. Chem., 217 (1966)7. 88 V. A. Popov and E. V. Pechennikova, Zavod. Lab., 40 (1974) 1. 89 E. Glueckauf, Trans. Faraday SOC.,51 (1955)34. 90 1;. Gliieckauf, Trans. Faraday SOC.,60 (1964)729. 91 J. J. van Deemter, F. J. Zuiderweg and A. Klinkenberg, Chem. Eng. Sci., 5 (1956)271. 92 V. I. Kalmanovski and A. A. Zhukhovitski, J. Chromatogr., 18 (1965)243. 93 H. Purnell and D. T. Sawyer, Anal. Chem., 36 (1964)668. 94 M. S. Vigdergauz and M. I. Afanasiev, Neftekhimiya, 3 (1963)911. 95 V. G. Berezkin and 0. L. Gorshunov, Zh. Fiz. Khim., 42 (1968)2587. 96 G. Nodop, Z.Anal. Chem., 164 (1958)120. 97 L. M. Kontorovich, A. V. Iogansen, G. T. Levchenko, G. N. Semina, V. P. Bobrova and V. A. Stepanova, Zavod. Lab., 28 (1962)146. 98 E. Bua, P. Monaresi and L. Motta, Anal. Chem., 31 (1959) 1910. 99 L. M. Kontorovich and V. P. Bobrova, Gazovaya Khromatografya, Tnrsy II Vsesoyuznoy Konferentsii, Nauka, Moscow, 1964,p. 150. 100 V. A. Balandina and M. S . Kleshcheyeva, Gazov. Khromatogr., No. 2 (1964)61. 101 A. A. Zhukhovitskii and N. M. Turkeltaub, Dokl. Akad. Nauk SSSR, 144 (1962)829. 102 A. A. Zhukhovitskii and N. M. Turkeltaub, Gazovaya Khromatografiya, Trudy II Vsesoyuznoy Konferentsii, Nauka, Moscow, 1964,p. 12. 103 A. A. Zhukhovitskii and N. M. Turkeltaub, Neftekhimiya, 2 (1962)818. 104 C. N. Reilley, G. P. Hildebrand and J. W. Ashley, Anal. Chem., 34 (1962)1198.‘ 105 N. A. Palamarchuk, Gazov. Khromatogr., No. 1 (1964)125. 106 V. G. Berezkin, T. A. Dyakova, A. K. Zhomov, E. I. Semenenko, V. S. Tatarinsky and I. I. Kholdyakov, Neftepererab. Neftekhim. (Moscow), No. 7 (1969)32. 107 L. G. Berezkina and N. A. Elefterova, Author’s Certificate, 281,886 (1970);Byull. Izobr., No. 29 (1970);J.Chromatogr., 105 (1975)65.

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108 A. A. Zhukliovitskii and N. M. Turkeltaub, Dokl. Akad. Nauk SSSR, 1 4 3 (1962) 646. 109 A. A. Zhukhovitskii, N. M. Turkeltaub, G. Gayer, M. N. Lagashkina, L. A. Malyasova and G. P. Shlepuzhnikova, Zavod. Lab., 29 (1963) 8; A. A. Zhukhovitskii, in A. Goldup (Editor), Gas Chronzatography 1964, Institute of Petroleum, London, 1965, p. 161. 110 A. A. Zhukhovitskii, N. M. Turkeltaub, L. A. Malyasova, M. S. Selenkina, M. M. Lapkin and A. V. Somov, in A. A. Zliukhovitskii (Editor), Gazovaya Khromatografiya, Trudy 111 Vsesoyuznoy Konferentsii, Izdaniye Dzerzhinskogo Filiala OKBA, Dzerzhinsk, 1966, p. 5. 11 1 D. C. Myers and F. A. Schiiiidt-Bleek, Talanta, 1 3 (1 966) 1695. 112 A. V. Alekseyeva and K . A. Golbert, Zavod. Lab., 27 (1961) 972. 11 3 C. E. Bennet, S. D. Nogare, L. W. Safranski and C. D. Lewis,Anal. Chem., 30 (1958) 898. 114 L. M. Kontorovich and A. V. logansen, Zavod. Lab., 28 (1962) 146. 115 W. 1:. Wilhite,J. Gas Chromatogr., 4 (1966) 47. 116 E. Bua, P. Manaresi and L. Mota,Anal. Chem., 31 (1959) 1918. 117 V. V. Buvaylo, E. M. Kazinnykh and I. M. Plekhov, Zavod. Lab., 33 (1967) 526. 118 V. V. Buvyalo, V. G. Berezkin, V. N. Anokhin and P. I. Stal’nov, Zavod. Lab., 4 0 (1974) 1188. 119 M. N. Chumachenko and S. S. Goncharov, XIMendeleyesskii S’ezd po Ohshcliey i Prikladnoy Khimii, Analiticheskava Khirriiya (A hstracts of papers and communications), Nauka, Moscow, 1975, p. 52. 120 A. E. Lawson and J. M. Miller,J. Gas Chrornatogr., 4 (1966) 273. 121 A. Martin and J. Smart, Nature /London), 175 (1955) 422. 122 G. E. Green,Natztre (London), 180 (1957) 295. 1 2 3 N. M. Turkeltaub, N. T. Ivanova, V. P. Shvartsman, A. G. Latukhova and M. L. Sazsonov, Gazovaya Khromatografijza, Trudj8Vsesoyuznogo Nauclzno-issledovateI‘skoRoInstitu la Geologorazvedochnogo Neftyanogo Instituta, No. 64, Nedra, Moscow, 1970, p. 11 1. 124 A. V. Alekseyeva, I vgAst& and M % uBM,, we find that the denominator in eqn. 8.10 is close to unity. Because the volatile standard compound interacts specifically with the terminal fragments of the polar molecule, it can be assumed that VgB/VgA std. % VgA /&A std. Therefore (8.1 1) Thus, the relative retention time of a standard compound that interacts specifically with the terminal groups of polymers must also be a linear function of 1/M [30]. A dependence of the type in eqn. 8.1 1 was established experimentally by Berezkin et al. [31]. The LSP consisted of samples of poly(diethy1ene glycol maleinate adipate) of different molecular weight, while the test mixture contained ethanol, propanol and benzene (internal standard). In order to reduce the adsorptive effect of the support on the retention time of the polar alcohols, samples of the polyester under investigation were deposited on

DETERMINATION OF THE MOLECULAR WEIGHT OF OLIGOMERS

20 3

t

Fig. 8.3. Dependence of specific retention volume of polar volatile compounds for poly(propy1ene glycols) of different molecular weight. Column, 200 X 0.4 cm; temperature, 76.8"C; sorbent, 15% poly(propy1ene glycol) o n porous PTFE. 1 = Methanol; 2 = diethylamine; 3 = ethanol; 4 = water.

sodium chloride (0.25-0.5 mm fraction) in an amount of 1% of the support weight (column, 200 X 0.4 cm; experimental temperature, 30°C). The results obtained for three series of experiments in the form of the dependence of the relative retention time of ethanol and propanol on the molecular weight (M) and its reciprocal are presented in Fig. 8.4. It can be seen that the relative retention time of the volatile compounds interacting specifically with the terminal groups of the polymer depends linearly on the reciprocal of its molecular weight. Naturally, in orde,r to improve the sensitivity of the method, one should use standard compounds that have a strong specific interaction with the polar terminal groups of the polymer, l e . , for which VgB is substantially greater than with VgA. Eqn. 8.12 can be obtained on the basis of the known relationship (see, for instance, ref. 32) of the partition coefficient of a test substance on the binary stationary phase on the concentration of that solution component which interacts specifically with the volatile substance:

K , = K I (1

+ K , xB)

(8.12)

where K , is the equilibrium constant of the complexing reaction (A + R =+AB) and xB is the molar fraction of substance B in the stationary phase. In the case under review, the functional groups play the part of the complexing agent B, and their molar fraction is 2miM, where m is the 'molecular weight' of the segment. Hence (8.13) References p. 221

204

INVERSE GC

Fig. 8.4. Dependence of relative retention time of ethanol (1, 2) and propanol (3,4) (internal standard benzene) on the molecular weight (M) and its reciprocal (1/M) of poly(ethy1ene glycol maleinate adipate) 0 , and a, series of experiments.

Thus, the value of K, (and consequently the retention volume) is proportional to the reciprocal of the molecular weight of the polymer. On converting into the relative retention values, we obtain (8.14) Eqns. 8.13 and 8.14 are similar to the previously obtained eqns. 8.9 and 8.11. It was interesting to consider the effect of temperature on the parameters of eqn. 8.11. It would be expected that the first term on the right-hand side of this equation would not be perceptibly temperature dependent, as it is related to the non-specific interaction of the molecules of volatile substances with the middle fragments of the polymer molecule. The second term, which reflects the specific interaction of the polar compound with the terminal groups of the polymer, on the contrary, must be temperature dependent. In order to verify these qualitative considerations, we calculated, from the data of McReynolds [ 171, the relative retention volumes of methanol (internal standard decane) on PEGS in the range of molecular weights from 300 to 6000 at 100 and 120°C. The dependence of the relative retention volume on the reciprocal of the molecular weight is shown in Fig. 8.5. In accordance with eqn. 8.1 1, the dependences are linear, and the first terms are practically the same for both temperatures, while the second term decreases with increasing temperature. The established dependences of the retention values on the reciprocal of the molecular weight of polymers with terminal functional groups that differ considerably from the main unit of the macromolecule make it possible to determine the average molecular weight of the polymer (up to 30,000-50,000). Also, the dependence of the retention values of the reciprocal of the molecular weight is steeper for standard compounds that interact selectively with the termha1 groups of the polymer (see eqns. 8.9 and 8.1 1) than for non-specifically interacting c o m p u n d s (see eqns. 8.3 and 8.4).

DETERMINATION OF THE MOLECULAR WEIGHT OF OLIGOMERS

.,

60

1

10

- - I

6

205

I

4

M.102

Fig. 8.5. Dependence of relative retention volume (Vrel.) of methanol (internal standard decane) o n the molecular weight (M)of poly(ethy1ene glycols). 0 , 100°C; A, 120°C.

For the practical application of chromatography, it is necessary to use substances with a known molecular weight in order to carry out preliminary calibration. We should, however, also iuention some advantages of this method. The fact that the experiment is conducted in the flow of the carrier gas, tlie possibility of pie-heating of the sample and the increased experimental temperature sharply reduce the amount of possible lowmolecular-weight impurities, which usually interfere in the determination of compounds of average molecular weight. In addition, the method is sensitive mainly to impurities that are capable of specific interactions with the volatile substances of the standard mixture. The possibility of determining the molecular weight of a number of oligomers with polar functional groups [PPGs, poly(diethy1ene glycol adipates) and thiocols] was confirmed by Kogan and Fedotova [27].Qualitatively similar results were obtained by Yaniamoto et al. [33]in studies of the dependence of the retention of benzene and b e n ~ y l b e n ~ e none the molecular weight of poly [2,2-propane-bis(4-phenylcarbonate)] ; The second practical application of the dependence of the retention of volatile substances on the molecular weight of oligoiners was indicated by Kogan arid Fedotova [27]. They demonstrated the possibility of utiliLing this dependence in selecting effective solvents for the fractionation of oligomers, as the solubility of polymers and the retention volumes of the solvents are related agreeably. For tlie fractionation of oligomers, one should use solvents in which the solubility of the members of the homologous series of polymers undei study depends most prominently on the molecular weight. The use of solvents (and precipitants) selected on the basis of the above considerations for the fractionation of oligomers has yielded good results [27]. We wish t o mention a simple and accurate method for determining the molecular weight of polynieric compounds, which was proposed by Burova et al. [34].This method is similar in principle t o inverse chromatography and uses chromatographic equipment. The device iiicludes a katharometer, the chambers of which receive gas flows saturated with a light solvent. The saturation of the gas flows with the solvent is achieved in thermostated vessels. The addition of a small amount of the test compound t o one of the vessels containing the solvent reduces the vapour piessure ofthe solvent and therefore disturbs the balance of the katliarometer bridge because of the different solvent vapour concentrations in the flow. As the reduction in vapour pressure is associated with the niolal fraction of the involatile test compound, its molecular weight can be determined. References p. 221

206

INVERSE GC

INVESTIGATION OF THE THERMODYNAMICSOF INTERACTION OF VOLATILE COMPOUNDS WITH POLYMERS Even in the earliest days of gas-liquid chromatography, one of its proponents, Martin [ l ] , emphasized the wide possibilities for its utilization in determining thermodynamic characteristics: ‘The method provided perhaps the easiest of all means of studying the thermodynamics of the interaction of a volatile solute with a non-volatile solvent, and its potential value for providing this type of data should be very great’. This statement is also true for polymer compounds used as UPS. In determining the thermodynamic parameters of the interaction of low-molecularweight compounds with polymers by using GC, one must take into account a number of specific features of high-molecular-weight compounds. The complexity of the molecular structure of polymeric materials leads to a diversity of states in which polymer phases can exist. As is well known, polymers can be in two states that differ in the degree of orderliness in the disposition of the units and chains, namely the crystalline and the amorphous states. Solid polymers can also be partially crystalline, and then crystalline and amorphous regions exist in the polymer. Such a polymer can sometimes be regarded as an ordinary mixture of impermeable crystals and a quasi-liquid amorphous medium [35]. Depending on the temperature, amorphous polymers can be in three physical states [36] : glassy (oscillatory atom motion), highly elastic (oscillatory motion of units that leads to flexibility of the polymer chain), and viscous fluid (mobility of the entire macromolecule). The mechanism of the retention (sorption) of low-molecular-weight compounds by polymer U P S is largely determined by the phase and physical state of the polymer. If the polymer phase is at a temperature below the glass transition point, the molecules of the test compound are unable to penetrate through the entire mass of the polymer phase, and in this instance the retention is determined by adsorption on the polymer surface. At temperatures above the glass transition point, amorphous polymers can both dissolve low-molecular-weight test compounds and adsorb them at the interfaces of the polymer LSP. These features must be taken into account when investigating polymers by GC. As is well known [32], a knowledge of the retention volume due to the dissolution of a substance makes it possible to calculate the important thermodynamic characteristics of the solution process, namely the partition coefficient, the activity coefficient and the change in the excess partial molar thermodynamic functions of the solute in the given stationary phase. Even in the first investigations on solution thermodynamics, satisfactory convergence of the partition coefficients for polymer U P S obtained by static and chromatographic methods was demonstrated. Table 8.2 gives the results obtained by Anderson and Napier [37] for the partition coefficients of benzene and cyclohexane using the cresyl ether of PEG measured by the static and GC methods. Most of the literature data refer to the use of non-polymeric compounds [32,38] as LSPS in analytical Chromatography. As an illustration, the thermodynamic functions of the solution of volatile organic compounds have been determined by the following polymer phases: Reoplex 400, Carbowax 600, fluorosilicone FS-16, silicone DC-550 [39] , polyglycols [40,41], silicones [42] a i d solutions ofmanganese chloride in PEG 4000 [43] .

207

INTERACTION OF VOLATILE COMPOUNDS WITH POLYMERS TABLE 8.2 COMPARISON 01: PARTITION COEFFICIENTS OBTAINED BY STATIC AND GC METHODS FOR THE CRESYL ETHER OF PEG [ 3 7 ] .-____

Compound

Benzene Cyclohexane

80°C

100°C

131°C

GLC Static method

GLC Static method

GLC Static method

71.6

43.4 13.5

23.4 9.3

18.9

73.0 19.9

44.1 15.0

20.1 7.8

The application of GC in studies o f the thermodynamics o f the interaction o f a volatile substance with the polymer phase was specially considered by Smidsrod and Guillet [ 101. For the description of the interactions of acetic acid, butanol, o-chloronaphthalene, naphthalene and hexadecane with poly(N-isopropylacrylamide), they used excess thermodynamic functions of mixing at temperatures above the flow-point of the polymer:

Ac, = RTlny'

(8.15) (8.16) (8.17) (8.18)

where Ac,, M,,, and A$,, are the excess free energy, enthalpy and entropy, respectively, and yo is the activity coefficient. The excess partial molar functions characterize the deviation (positive or negative) of the thermodynamic properties of a given solution from the corresponding properties of the ideal solutions. In calculating the activity coefficient from chromatographic data by eqn. 8.2, Smidsrod and Guillet [lo] used, as the characteristic of the molecular weight, the average molecular weight, although they noted that the nature and physical meaning of the quantity M in this equation remain unclear and that possibly a certain parameter should be introduced that is determined by the effective value of the segment of the polymer chain. Investigations into the temperature dependence of the specific retention volume for hexadecane, o-chloronaphthalene and naphthalene showed that the curve of the logarithm of the retention volume versus the reciprocal of the temperature has a minimum at the glass transition temperature of the polymer (130°C) and a maximum in the melting point range (160- 170°C). The graph of the logarithm of the retention volume for butanol and acetic acid, which are capable of forming strong hydrogen bonds, versus the reciprocal of the temperature is a striaght line, owing t o the ability of the molecules of these compounds t o diffuse into the polymer b y a different mechanism. The excess thermodynamic functions of mixing were determined in a temperature range above the flow-point of the polymer (Table 8.3). Hence inverse GC can be used successfully for determining the thermodynamics of interaction of volatile standard compounds with a polymer stationary phase. However, because the References p. 22 1

208

INVERSE GC

TABLE 8.3 EXCESS THERMODYNAMIC FUNCTIONS OF MIXING VOLATILE COMPOUNDS O F VARIOUS TYPES IN POLY(N-ISOPROPY L ACRY LAMIDE) [ 101 Temperature, 200°C. ~

Compound

AfiM(cal/mole)

AH,

(cal/mole)

Acetic acid Butanol Naphthalane o-Chloronaphthalene n-Hexadecane

-6740 -5390 -4360 -4010 -1760

-2400 +1100 +3900 +3606 +2200

AS,

(cal/"C * mole)

+9.2 +13.7 +17.5 +11.3 +8.4

thermodynamic interpretation of chromatographic data is not clear for polymer phases at present [ 111, it is more expedient to use, for characterizing the interaction of the volatile compound with the polymer phase, relative rather than absolute values, in particular relative activity coefficients [44] . Patterson et al. [44] considered the possibility of estimating the interaction parameter x (which is an important thermodynamic characteristic of polymer solutions) directly from the values of the retention volumes, rather than from the values of the activity coefficient $,for the determination of which it is necessary to know the molecular weight of the polymer LSP (MH). The interaction parameter is calculated by the equation (8.19)

where Pp is the vapour pressure of the pure sorbent at the column temperature T , Bii is the second virial coefficient of the ith pure dissolved substance, Vmi is the molal volume of the sorbate at the column temperature, v p is the unit volume of the polymer and V i is the specific retention volume. When M, is sufficiently large, the term Vi/Mnvp can be neglected, and in this instance the value of x can be calculated without using the data on the molecular weight of the polymer and its dispersity. The determination of the interaction parameter , x, is of great interest in investigations of polymer solutions. At a later date, the same workers [45] considered the application of gas-liquid chromatography in studies of the thermodynamics of the interaction of polymers with volatile substances. The interaction of linear and branched polyethylenes with volatile hydrocarbons was investigated. The determination of the equilibrium values and thermodynamic functions of lowmolecular-weight compounds for polymers is of great interest not only for GC, but also for the theory of polymer solutions [ 2 5 ] and the technical characteristics of polymer materials. It is also necessary to consider certain limitations of inverse GC when using polymers as LSPs. Firstly, the retention volume being measured is, in general, an additive function of at least three partial retention volumes associated with the dissolution of the volatile standard in the polymer phase and with absorption of the standard on its surface and on the surface of the solid support. Therefore, one must first determine the partial value of I

THE STUDY OF PHASE TRANSITIONS

209

the retention volume due to dissolution. It is this value that is used in calculations with eqns. 8.2 and 8.4. Methods for calculating the retention volume due to dissolution have been suggested [46,47]. Secondly, the viscosity of the LSP under the experimental conditions must not be too high because, for phases that are too viscous, the gas-solid, and not the gas-liquid, version of chromatography is realized. Thirdly, the results obtained may not be in agreement with the simple chromatographic dissolution model considered above, because the properties of a substance in a thin layer may differ considerably from those in a thick layer and because the solid (support) on to which the layer of the liquid phase under investigation is applied also affects the properties of the test substance [48,49]. In this connection, it is worth noting that GC can be used for characterizing the interaction forces of the binder with the solvent [SO] and for studying the role of the filler [Sl, 521. At temperatures below the glass transition point, the penetration of the molecules of the standard volatile substance (‘molecular probes’) into the polymer is impeded [l 11 . It should be noted, however, that in order to effect diffusion of small molecules in the polymer medium, it will suffice to obtain even relatively uncoordinated displacements of small segments of the polymer chain. The diffusion coefficients of low-molecularweight compounds in polymers are determined by the mobility of the segments and the packing density of the chains, and then the molecular weight of the polymer often affects the diffusion rate and the permeability through the polymer film only slightly [ 3 5 ] . Therefore?in spite of the high viscosity of polymers, the coefficients of diffusion of small molecules in them are only one or two orders of magnitude less than in ordinary liquids. Thus, for instance, the coefficient of diffusion of benzene in natural rubber cm2/sec, and for carbon dioxide in poly(dimethy1siloxane) rubber it is is 3.8 * 3.8 lo-’ cm2/sec [36]. The practice of GC in which highly efficient columns containing polymer stationary phases (for instance, silicone rubbers) are used indiclites that equilibrium of the molecules of the test substances between the gas and polymer phases is established sufficiently rapidly. However, in each specific experimental thermodynamic study, the question of the establishment of polymer equilibriuni must be treated separately. It should be noted that the determination of the kinetic parameters from the data on the broadening of the chromatographic Lones is also of interest for the characterization of polymeric compounds. The study of the dependence of the retention values on the composition of a binary (multicomponent) LSP is one of the highly sensitive methods of physicochemical analysis. In a number of studies [53,54], this method served to show the presence of extremes in the dependence of properties on composition.


As a result of a phase transition, the original stationary phase is replaced by a new stationary phase (new sorbent), which must, in general, have different physicochemical properties; this change must be reflected in unusual changes in the chromatographic characteristics of the volatile standard substances in the region of the phase transition References p. 221

210

INVERSE GC

of the stationaty phase. Therefore, inverse GC can be applied in the study of phase transitions in polymers. When investigating phase transitions, it is expedient t o use, as chromatographic characteristics, not only the retention volume (retention time) but also the peak width of the volatile standard compound [7] . The retention volume is proportional to the partition coefficient of the test compound in the system gasstationary phase, while the peak width is related to the diffusion coefficient of the volatile compound in the stationary phase [4,551. Some time ago, sharp changes in retention values with variation of temperature in the melting region of the LSP were noted [56-591. The attention of most workers, however, has usually been focused not on the study of the phase transition phenomenon, but on the investigation of its effect or utilization in analytical chromatography [56-591. Thus, for instance, the drastic decrease in the retention values of test compounds after the column has been cooled with eicosane to a temperature below its melting point was used [56] as a method for the rapid elution of heavy hydrocarbons from the column following the analysis of the lighter components. The first investigations in which the chromatographic properties of stationary phases were specially investigated in the region of phase transitions were carried out during the study of liquid crystals as U P S [60-621. LSPs that are in the liquid crystal state are very selective for the separation of isomers of organic compounds (for instance, 0-,m-, and p-xylene). At the temperatures of transition from the crystalline to the mesomorphic liquid crystal state, and also during the transition from the mesomorphic to the isotropic liquid state, sharp anomalous changes in the specific retention volume are observed. According to Guillet and Stein [63], Chromatography was first applied specially for the study of phase transitions in polymers by Alishoyev e f al. [7], who investigated stereoregular polymers with a high degree of crystallinity (polyethylene and polypropylene). A mechanical mixture of the powder of the polymer under investigation with glass beads (I%, w/w) was placed in a column (100 X 0.4 cm), which was connected to a chromatograph and heated at the rate of 0.2-0.5"C/min. A sample of the standard substance was introduced into the column at regular intervals and its chromatogram was recorded. The results were plotted as the retention time and the peak width of the standard substance (for polyethylene, tetradecane; for polypropylene, n-hexadecane) versus the colunin temperature (Fig. 8.6 [7]). In the cases investigated, we observed a maximum on the curves of the retention time and the peak width versus column temperature, and a plateau in the temperature range of the phase transition on the curve of the retention time of n-hexadecane versus column temperature for polypropylene. The appearance of the maximum is due to the change in the partition coefficients and diffusion coefficients of the standard volatile compound as a result of the phase transition. For polypropylene, we detected a sensitivity of the method to the 'history' of the sample. When polypropylene was deposited on glass beads from its solution in decalin at 100°C or from a melt in an inert atmosphere, the plateau on the curve of retention time versus temperature associated with the phase transition almost disappeared. A similar phenomenon was noted when studying phase transformations of crystals of thalliuni(1) nitrite [64] . As shown by Alishoyev ef al. [7] inverse GC can also be used for studying phase transformations in the transition from one crystal modification to another in the case of


21 1

2001

L

1 150

I

160

170

TPC)

Fig. 8.6. Dependence of (1) retention time and (2) peak width on column temperature. (a), Stationary phase, mechanical mixture of polyethylene (1%) with glass beads; standard volatile compound, n-tetradecane; (b), stationary phase, mechanical mixture of polypropylene (1%) with glass beads; standard volatile compound, n-hexadecane.

carbon tetrabromide. Later, this method was used successfully by Guran and Rogers for investigations of the phase transitions of crystals of thallium(1) nitrate [64] , Cu2Hg14 and AgzHgI4 [65]. In all of these instances, the GC results agreed fairly well with those obtained by other methods. The corresponding processes in polymers can also be studied by inverse GC. An important contribution t o the development of inverse GC for the study of phase transitions in high-molecular-weight compounds and their qualitative interpretation was made by Guillet and co-workers [ 1 1, 631 . They proposed a method for determining the crystallinity of poIymers on the basis of GC data found from the dependence o f the logarithm of the specific retention volume of the reciprocal of the absolute temperature. Guillet and co-workers [9-11,63,66] considered in detail the dependence of the logarithm of the unit retention volume on the reciprbcal of temperature and its interpretation for polymers used as stationary phases. The general pattern of this dependence is illustrated in Fig. 8.7. Similar dependences of the characteristics, for instance for polypropylene and crystalline polystyrene, are observed when using n-alkanes as volatile standards. In the range AB, the polymer temperature is below the glass transition temperature, and hence the retention of the volatile standards depends exclusively on adsorption on the surface of the glassy polymer, as the molecules of the standard cannot penetrate into thc polymer fiim. In the range AB, one observes a linear dependence of the logarithm 01 retention on tne reciprocal or the telnperature up t o the glass transition point, Tg (point B). The increase in retention volume in the range BC with a further increase in temperature (above the glass transition point) is due t o the radical change in the structure of the polynier LSP. Guillet [9] explained this increase as follows. The range BC corresponds t o the non-equilibrium adsorption of the vapour of the polymer phase. The molecules of the standard begin t o penetrate into the U P , but the diffusion rate is very low and therefore the equilibrium sorption of the LSP by the whole film is acheved only References p. 22 1

INVERSE CC

212

I/%

Fig. 8.7. Dependence of unit retention volume on reciprocal of absolute temperature of column filled with semicrystalline polymer.

at point C. We believe that it would be advisable to confirm this explanation by measuring, in this range, the dependence of retention on the gas flow-rate and the dependence of retention on the thickness of the film of the polymer UP. Therefore, another explanation of the processes that occur in the range BC is possible at present [ 6 7 ] . It is well known that the possibilities of the flexibility of the chain (forward motion of the segments) in the amorphous regions of polymers increase with increasing temperature. Also, a temperature increase leads to the disintegration of ordered supermolecular structures (packs), and the molecules acquire shapes of randomly wound and entangled balls of 'threads' [ 6 8 ] . The processes indicated are most pronounced in the range BC, and they can explain the increase in retention, for instance, as a result of the increased fraction of the polymer that sorbs molecules of the standard. The resolution and depth of the minimum at point B depend on the nature and size of the molecules of the volatile dissolved substances. Thus, it was shown [ 101 that for butanol and acetic acid the plot of the logarithm of the retention volume versus the reciprocal of the temperature is a straight line throughout the entire temperature range. This result may be attributed to the fact that the molecules of the alcohol and acid form strong hydrogen bonds with the amide group of the polymer, which results in a qualitatively different mechanism of their retention compared with hydrocarbons. At point C, a state of the amorphous polymer or the amorphous portion of the semi-crystalline polymer which is similar to the state of ordinary liquids evidently occurs. Thus, for poly(N-isopropylacrylamide), point C is close to its flow-point. Therefore, in the range CD, together with the possible adsorption on the surface and at the boundaries of the amorphous and crystalline regions, one observes the dissolution of the vapour of the volatile compounds throughout the entire volume of the amorphous portion of the polymer. In this range, it is also possible t o determine the thermodynamic functions of dissolution. In the range DF, the polymer melts and in the vicinity of point F, the crystalline portion of the polymer is completely melted. In this region, one observes the characteristic maximum on the curve under consideration. The range FG corresponds to the state

213

THE: STUDY OF PHASE TRANSITIONS

where the whole polymer mass takes part in the dissolution of the sample molecules. As it is assumed that in the range CD the crystalline regions of the polymer do not

participate in the retention of the molecules of the dissolved substances, it is possible to estimate the degree of crystallinity of the sample in the range CD. Indeed, the extrapolation of the line FG into the temperature range below the melting point makes it possible to estimate the retention volume that would correspond to the hypothetical amorphous state of the entire polymer in this temperature range. Therefore, the degree of crystallinity (PC)can be calculated from GC data by means of the equation of amorphous phase of polymer p c = weight __total weight of polymer

*

100% =

‘gextr.-

Vg extr.

g‘

-

100%

(8.20)

where VRextr.and Vgare the extrapolated and measured values of the unit retention volume at a given temperature, respectively. Note that in calculating the degree of crystallinity, it is possible to use, in place of the values of the unit retention volume, the values of the retention time corrected for the column dead time, which are proportional to them. Fig. 8.8 [63] shows the dependence of the logarithm of the retention volume of dodecane on the reciprocal of the absolute temperature for linear polyethylene. The maximum on the curve corresponds to the temperature of complete disappearance of polymer crystallinity; this temperature was regarded as the melting point. It is assumed that below the melting point, volatile standard substances dissolve and interact only with

Fig. 8.8. Dependence of logarithm of specific retention volume of dodecane on reciprocal of absolute temperature for linear polyethylene. 0 , first experiment; A , experiment after 24 h. References p. 221

214

INVERSE GC

unordered ('amorphous') regions of the polymer under investigation, and the enthalpy of this interaction is equal to that of dissolution in a completely melted polymer. Adsorption at the interface of the crystalline and amorphous regions, as well as at the gas-polymer, polymer-solid support interface is not taken into account in this model. Therefore, extrapolation of this linear dependence for the region above the melting point into the low-temperature range makes it possible to obtain a hypothetical dependence for this region with respect to a completely amorphous polymer. Eqn. 8.20 was used for determining the degree of crystallinity of a high-density polyethylene as a function of the temperature. Fig. 8.9 depicts the dependence of the degree of crystallinity on the temperature, the results being obtained by GC and by differential thermal analysis. It can be seen that the agreement between the two methods is satisfactory. Inverse GC can also be used, as shown by Guillet and co-workers [lo, 111, for determining the glass transition point, Tg. The minimum on the curve of the dependence of the logarithm of the retention volume against the reciprocal of the absolute temperature is usually located near the glass transition point. Table 8.4 [l 11 gives data on glass transition points obtained by differential thermal analysis and inverse GC. The agreement between the two methods is satisfactory; in determining the glass transition point of a polymer by GC, it is expedient to use, as the characteristic value, the temperature of the first deviation from linearity on the curve of the dependence of the logarithm of the retention volume on the reciprocal of the absolute temperature. Inverse GC has also been used successfully in studies of the kinetics of crystallization of organic substances from a melt. The application of this method to polymeric compounds was described by Stein et nl. [66] and to monomeric compounds by Andreyev et al. [69]. An interesting application of inverse GC is the determination of the thermodynamic compatibility of oligomers and polymers [ 151 . As is well known, mixing of polymers

T("C)

Fig. 8.9. Dependence of the degree of crystallinity ( 7 ) of linear polyethylene on temperature. 0 , data from differential thermal analysis (cooling at the rate of 1.25"C/min, heating at the rate of 1.25"C/min); 0 , gas chromatographic data (cooling at the rate of l.O"C/min, heating at the rate of 0.5" C/min); A, gas chromatographic data (abrupt cooling, heating at the rate of O.S"C/min).

INVESTIGATION OF THE KINETICS AND EQUILIBRIA OF CHEMICAL REACTIONS

215

TABLE 8.4 DETERMINATION OF GLASS TRANSITION POINT BY INVERSE GC [ 11] Characteristic

Poly(viny1 chloride)

Polystyrene

Poly(methy1 methacry late)

Molecular weight Standard volatile compound Glass transition point ("C) (DTA) Temperature of first deviation of linear dependence, T , ("C) Temperature of minimum, T , ("C)

68,000 Dodecane

5 1,000 Dodecane

48,000 Hexadecane

*2

35,500 Dodecane 15 i 2

95 r 2

81

81

88

91

91

91

100

105

15

100 It 2

is one o f the methods for obtaining polymeric materials with new properties, and the properties of the mixture are largely determined by the compatibility of the constituent polymers. By the 'compatibility' of polymers is usually meant the formation of a thermodynamically stable polymer-polymer system, Le., the formation of a true solution of one polynier in the other. Here, the free energy of the system decreases as a result of the formation of the thermodynamic system. A qualitative and accurate solution of the problem of the thermodynamic compatibility of polymer systems can be achieved even on the basis of the experimental determination of the dependence of the logarithm of the unit retention volume of the standards of a polymer mixture on the reciprocal of the absolute temperature. For instance, in the cases studied, if the polymers are compatible, the melting point of the mixture lies between those of the individual components, the degree of crystallinity of the mixture is higher than those of the components and the curve for the investigated dependence for the mixture lies between those for the individual components [ 151. The advantageous features of inverse GC when studying phase transitions are the relative simplicity in setting up and carrying out the experiments and the possibility of effecting rapid measurements of chromatographic characteristics for volatile standard substances of various types with the use of small amounts of the polymers under investigation. Another advantage over the widely used methods based on the measurement of the heat effect of the phase transition is the possibility of studying phase transitions characterized by slow kinetics.

INVESTIGATION OF THE KINETICS AND EQUILIBRIA OF CHEMICAL REACTIONS In investigations into the kinetics of chemical reactions by inverse GC, the reaction proceeds in a chromatographic column, the reaction mixture being used as the stationary phase for the standard mixture of volatile non-reacting compounds. The variation in the composition of the reaction mixture with time is checked periodically by measuring the elution characteristics of the volatile standards. The determination of the composition of the reacting system with time is based on the use of a dependence of the type (8.2 1) ci = f ( Vi) References p. 2 2 1

216

INVERSE GC

where cj is the concentration of one of the components in the reacting mixture and V, is the retention volume of one of the standards. Note that the term ‘component’ also includes the functional groups of the test polymer, and eqn. 8.21 may be complex. This method has the following advantages: the thin film of the phase enables one easily to realize equilibrium kinetic regimes and to conduct investigations of the reaction kinetics without collecting a sample; the method requires small amounts of test materials; and it can be used for investigations into the kinetics of reactions of systems that cannot be analyzed chromatographically. The method was proposed by Berezkin et al. [70] as a result of investigations into the kinetics of the oxidation of benzaldehyde to benzoic acid [83. It was shown that the oxidation of benzaldehyde by atmospheric oxygen (carrier gas) proceeds according to a first-order equation, and the rate constant obtained in a chromatographic experiment differs by only 3% from that obtained in a bubble reactor. Later, this method was used by Davis and Petersen to study the oxidation of asphaltenes [ 7 1 ] , Fig. 8.10 demonstrates the variation of the specific interaction coefficient (see eqn. 8.1) for a number of volatile standards against the time of oxidation of asphaltenes by air at 130°C. Similar to inverse GC are the impulse methods for investigating the kinetics and equilibria of chemical reactions. In these methods, a non-volatile reacting compound in a reactor column is used as the stationary phase and simultaneously as one of its components while the volatile reacting compound passes in the carrier gas flow through the column in the form of a pulse, the variation of which can be used for the quantitative characterization of the reaction occurring in the reactor column. In the impulse methods, the investigation of the chemical reaction and the analysis are usually combined as regards the procedure and equipment. Although we have no knowledge of the direct application of this method to the investigation of polymerization or other chemical reactions of polymers, we do not doubt its expediency and efficiency in polymer chemistry. Therefore, 6

T

I l F F

140 140

I

/

3

2

Q

I

6

18

30 Time (h)

I

I

I

42

Fig. 8.10. Variations of the specific interaction coefficient (0 with oxidation time for asphaltenes. 1 = Butanol; 2 = propionic acid; 3 = p;rrrole; 4 = phenol; 5 = forniamide; 6 = rnethylpyrrolidone.

INVESTIGATION OF THE KINETICS AND EQUILIBRIA OF CHEMICAL REACTIONS

217

it is considered appropriate t o give a brief description of the fundamentals of the impulse method, using examples from other branches of chemistry. Kokes et al. [72] were the first t o propose the impulse method for studying the kinetics of catalytic reactions. A considerable contribution to the development of chromatographic methods for studying catalytic processes was made by Roginsky and co-workers [73, 741, who noted some characteristic features of chemical reactions in chromatographic reactors. The direct impulse method for studying liquid-phase reactions kinetics was used by Gil-Av and Herzberg-Minzly [75] when investigating reactions of dienes with conjugated bonds with chloronialeic anhydride. As a result of the further development of this method, it became possible t o determine the order of the reaction with respect t o the second non-volatile component, and also the rate constant o f the bimolecular reaction [76, 771. A critical survey containing a number of procedural and technical suggestions has been published [78]. In studying the kinetics of chemical reactions, one can use either a simple [77] or a circulation scheme. The idea of circulating elution GC was first suggested by Martin [79, 801, and has been further developed by other workers [81-831. The use of circulation diagrams in GC for studying the kinetics of chemical reactions makes it possible t o use columns with a small amount of the liquid phase under study or the catalyst, t o measure the kinetic value at lower temperatures and to determine the degree of conversion for different contact times in a single experiment, which speeds up the investigations and increases the reliability of the results obtained [84] . Fig. 8.1 1 shows schematically the

t t

cQ

I

f?

I-

cL

Fig. 8. l l . Tinie variation of reagent concentration in a Chromatographic reactor. Cg, concentration of volatile substance in gas phase; C,, concentration of volatile substance in liquid phase; I , reactor length. References p. 221

218

INVERSE GC

variation of the concentration of the volatile substance in a chromatographic reactor for three times, t l , tz and t 3 , when studying the kinetics of liquid-phase irreversible reactions by the impulse method. The impulse of substance A enters the chromatographic reactor, in which a solution of the second non-volatile reagent, B, is used as the U P ; its concentration exceeds by 1-2 orders that of the volatile substance A (C, S C,). This enables one to achieve conditions under which the observed kinetic regularities of reactions with respect t o the volatile components are described by a first-order equation. In the course of the experiment, it is possible t o determine accurately and easily the amount of substance A before and after the reaction, the temperature of the experiment (reaction), the concentration of substance B and the reaction time. These data are sufficient for determining the firstorder constant by the equation 2.3 logr?)

K=

(8.22)

c; t

where K is the reaction rate constant, CoAis the initial concentration of component A, C, is the final concentration of component A , C, is the concentration of component B, t is the contact time and n is the reaction order with respect to component B. It should be noted that in contrast t o heterogeneous catalytic reactions, the interpretation of the results obtained in liquid-phase reactions is usually simpler and the kinetic characteristics usually agree with the data obtained under static conditions. As an example of application of this method in investigations of the kinetics of chemical reactions, a study of the kinetics of the diene synthesis reaction of isoprene with maleic anhydride was conducted. Under the conditions used, this reaction is irreversible, its product is non-volatile and the reaction occurs in the kinetic region. In running the reaction, use was made of two columns (reactors) connected according t o the circulation scheme (Fig. 8.12). One of the reagents, maleic anhydride as a solution in tricresyl phosphate saturated at 35"C, was applied on t o Chromosorb P in an amount of 15%. The sorbent thus obt5ned was placed in two thermostated reactors, each consisting of a copper column of 210 X 0.2 cni I.D. Tricresyl phosphate was used as the solvent in order t o extend the temperature range of the reaction under investigation (because the melting point of maleic anyhdride is 55°C) and t o increase the contact time of the reagents. The concentration of the inaleic anhydride in tricresyl phosphate at 35°C was 0.358 g/ml. It was verified experimentally that the concentration of the maleic anhydride in the liquid phase remained virtually unchanged during the time necessary for conducting a series of experiments at different temperatures. Another reagent (isoprene) was fed pulsewise at the reactor input. If a chromatographic separation coluniti is placed in front of the reactor, the impulse of the reagent can be isolated from the impurities, which eliminates the need for preliminary purification of the initial reagents. With the aid of the sanip!e injector ( l ) , the sample of the substance is delivered t o the separation column (2) and the3 enters the reactor (4) through the four-way tap (3).

INVESTIGATION OF THE KINETICS AND EQUILIBRIA O F CHEMICAL REACTIONS

4

219

1

t Fig. 8.12. Circulation scheme for studying the kinetics of chemical reactions by the impulse method. 1 = Sample injector; 2 = separation column; 3 = four-way tap; 4 = first reactor; 5 = detector; 6 = second reactor.

After the sample has left the first reactor, it enters the detector chamber ( 5 ) via the tap ( 3 ) . When the sample has been completely transferred from the first reactor t o the second, the transfer is ascertained by the detector iesponse and the tap is turned t o the position shown by the broken line. Having passed through the second reactor (6), the sample again enters the detector chamber (5) and proceeds to the first reactor (4). The tap is then turned t o the previous position and the cycle is repeated. The circulation can continue until the diene is completely absorbed. The chromatogram o f isoprene obtained after five cycles is depicted in Fig. 8.13. In this method, the degree of conversion was determined from the change in the areas of the chromatographic peaks obtained after the first reactor (this value was taken t o be the initial one) and subsequent switchings of the tap. The contact time was determined as the difference between the retention times of the diene after the first reactor (t = 0) and the subsequent switchings, minus the dead time of the reactors. Fig. 8.14 displays the dependence of log ( S l / S j )on the contact time for the reaction of isoprene with nialeic anhydride. The data obtained were used t o calculate the rate constants of the reaction of isoprene with maleic anhydride for different temperatures. The value of the activation energy obtained by the proposed method is in good agreement with the literature data [85].

Refercnces p. 22 1

220

INVERSE GC

I

51.6.

1

/

0.6 -

0.5

-

0.4

-

45.4.

'll

s

y1 R

0.3 -

L Ql L

"

c Ql

c

n

Time

0

100

200

T i me b e d

Fig. 8.1 3. Chromatogram of isoprene after five successive cycles through the reactors. Peaks I-V, chromatographic zones of isoprene in the course of circulation. Fig. 8.14. Variation of the logarithm of peak area ratios (S,/Sj)a t various temperatures with time.

Some workers have also described the successful application of inverse GC for investigating, by the impulse method, reversible reactions of addition in which the volatile reagent forms a complex that is capable of dissociation [86-901. Genkin and Petrova [86] developed the impulse method, demonstrating its applicability for determining the equilibrium constant of an exchange chemical'reaction of the type A + BC AB C, which occurs in the liquid phase between the eluted substance A and the non-volatile substance BC dissolved in the LSP; the products AB and C formed are virtually involatile. This method was used successfully for determining the equilibrium constant of the reaction of 1,3-butadiene and 1-butene with rhodium acetylacetonatecarbonylcyclooctene in a solution of dodecane, with the formation of cyclooctene. The above methods can undoubtedly be applied successfully in the study of chemical reactions of polymeric compounds. We also wish to indicate the possibility of carrying out, by means of inverse GC, measurements of the surface properties of polymers and the diffusion coefficients of volatile compounds in polymers. The experimental methods for measuring the surface areas, the adsorption isotherms, the heats of adsorption and the diffusion coefficients have been described in detail elsewhtie [74,91-951. These methods have practical

2 +

REFERENCES

221

applications in polymer investigations. Thus, for instance, Mohlin and Gray [96] determined the adsorption of volatile organic compounds (n-octane, n-decane, toluene, butanol, dioxan, butanol-2) on cellulose fibres, which were used as the stationary phase in the column. The method is particularly suitable for measuring small surface areas and for investigating weak adsorbate-adsorbent interactions. Chabert and Soulier [97] studied the sorption of water vapour on nylon, poly(ethy1ene glycol terephthalate) and other polymers. As has already been noted, an important feature of inverse GC is the possibility of using small polymer samples for analytical and physicochemical investigations. A minimal amount of polymer is required when using a capillary column. In addition, the interpretation of experimental results is most reliable in capillary chromatography. Therefore, the use of capillary columns [98] in inverse GC is especially promising. The successful implementation of this technique has been described [ 1001. In conclusion, we wish t o emphasize that although the development of inverse GC is far from completed, the experience accumulated so far seems t o be sufficient for solving various problems in polymer chemistry.

REFERENCES 1 A. J. P. Martin, Analyst (London), 81 (1956) 52. 2 T. C. Davis, J. C. Petersen and W. E. I-laines, Anal. Chem., 38 (1966) 241. 3 V. G. Berezkin, Analytical Reaction Gas Chromatography, Plenum Press, New York, 1968. 4 A. A. Zhukhovitskii and N. M. Turkeltaub, Gazovaya Khromatografiya (Gas ChrornatographyJ, Gostoptekhizdat, Moscow, 1962, p. 240. 5 V. G . Berezkin, A. A. Zhukhovitskii, V. P. Pakhomov, L. L. Starobinets and Z. P. Markovich, Gazovaya Khromatografiya, Trudy 111 Vsesoyuznoy Konferentsii, Izdaniye Dzerzhinskogo Filiala OKRA, Dzerzhinsk, 1966, p. 247. 6 11. Kelker, Z. Anal. Chern., 198 (1963) 254. 7 V. R. Alishoyev, V. G. Rerezkin and Yu. V. Melnikova, Zh. Fiz. Khim., 39 (1965) 200. 8 V. G. Berezkin, V. S. Kruglikova and V. E. Shiryayeva, Kinet. Katal., 6 (1965) 758. 9 3. E. Guillet, J. Macromol. Sci., A4 (1970) 1669. 10 0. Smidsrod and J. E. Guillet, Macromolecules, 2 (1969) 272. 11 A. Lavoie and J. E. Guillet, Macromolecules, 2 (1969) 443. 12 J. E. Smith, Polvm. Prepr. Amer. Chern. Soc. Div. Polym. Chem., 11 (1970) 1176. 13 A. L. Buchachenko, Stabil’nye Radikaly (Stable Radicals), Izvestiya Akademii Nauk SSSR, Moscow, 1963. 14 E. C. Maysya, in Yu. S. Lipatov (Editor), Novyie Metody Issledovariiya Polimerov (New Methods o f Polymer Investigation), Naukova Dumka, Kiev, 1975, p. 66. 15 A. E. Nesterov and Yu. S. Lipatov, in Yu. S . Lipatov (Editor), Novyie Metody Issledovaniya Polimerov (New Methods of Polymer Investigation), Naukova Duinka, Kiev, 1975, pp. 66-93. 16 A. A . Zhukhovitskii, A . 1. Kolyubyakina, M. L. Sazonov and A. F. Shlyakhov, in A. Korol’ (Editor), Teoriya i Prirneneniye Nepodvizhnoy Fazy v Gasoshidkostnoy Khromatografii (Theory and Application of Stationary Phase in Gas-Liquid Chromatography), Znaniye, Kiev, 1971, p. 47. 17 W. 0. McReynolds, Gas Chromatographic Retention Data, Preston Technical Abstracts, Evanston, Texas, 1966. 18 J. S. Lewis, Compilation of Gas Chromatographic Data, ASTM Special Technical Publication No. 343, American Society for Testing and Materials, Philadclphia, Pa., 1963. 19 G. C . Devyatykh, A. D. Zorin, S. B. Lyakhn~anovand A. E. Ezheleva, Dokl. Akad. Nauk SSSR, 156 (1964) 1105. 20 F. A. Barbour, S. M. Dorrence and J . C. Petersen, Anal. Chem., 42 (1970) 668.

222

INVERSE GC

2 1 G . P. Kurbsky, G. M. Usachova and V. G . Khasanov, Neftekhimiya, 15 (1975) 615. 22 Broniarz, M. Wisniewski and 1. Szymanowski, Tenside, 10, No. 2 (1973) 75. 23 V. S. Krivich and M. Kh. Gluzman, Kolloid. Zh., 35 (1973) 364. 24 S. M. Dorrence and J. C. Petersen, Anal. Chem., 4 1 (1969) 1240. 25 P. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, N.Y., 195 3. 26 D. E. Martire and J. H. Purnell, Trans. Faraday. Soc., 62 (1966) 710. 27 V. B. Kogan and M. D. Fedotova, Vysokomol. Soedin, Ser. A, 10 (1968) 1704. 28 H. Morawetz, Macromolecules in Solution, Interscience, New York, London, Sydney, 1965. 29 A. B. Littlewood and F. W. Willmott, J. Gas Chromatogr., 5 (1967) 543. 30 V. R. Alishoyev, V. G. Berezkin and G. A. Mirzabayev, DokL Akad. Nauk SSSR, 190 (1970) 1365. 31 V. G. Berezkin, V. P. Pakhomov, V. R. Alishoyev, L. L, Starobinets, E. P. Markovich and L. N. Sedov, Vysokomol. Soedin., 7 (1965) 185. 32 S. M. Vigdergauz and R. 1. Izmailov, Prirneneniye Gazovoy Khromatografii dlya Opredeleniya

Fiziko-Khimicheskikh Svoistv Veshchestv (Application of Gas Chromatography f o r Determination o f the Physicochemical Properties of Substances), Nauka, Moscow, 1970. 33 T. Yamamoto, S. Tsuge and T. Takeuchi, Bull. Chem. SOC.Jap., 44 (1971) 1145. 34 M. 0. Burova, A. A. Zhukhovitskii, M. L. Sazonov and M. S. Selenkina, in A. A. Zhukhovitskii, K. F. Rodionova, M. L. Sazonov and A. F. Shlyakhov (Editors), Gazovaya Khromatografiya i ee Primeneniye v Geokhimicheskikh Issledovaniyakh (Gas Chromatography and its Application in Geochemical Investigations), USSR Research Institute for Geological Prospecting (VNIGNI), Trudy VNIGNI, Moscow, No. 112, 1973, p. 111. 35 K. Rogers, in D. Fox, M. M. Zabes and A. Weissberger (Editors), Physics and Chemistry o f t h e Organic Solid State, Vol. I, Interscience, New York, London, 1963, p. 229. 36 A. A. Tager, Physicochemistry of Polymers, Khimiya, Moscow, 1968, p. 131. 37 J. R. Anderson and K. H. Napier,Aust. J. Chern., 10 (1957) 250. 38 A. N. Korol', Nepodvizhnaya Faza v Gaso-Zhidkostnoy Khromatografii (Stationary Phase in Gas-Liquid Chromatography). Naukova Dumka, Kiev, 1969. 39 M. S. Vigdergauz and V. V. Pomazanov, Uspekhi Gasovoy Khromatografii (Advances in Gas Chromatography), VKhO im. D. 1. Mendeleyeva, Kazan, No. 2, 1970, p. 29. 40 A. 1. M. Keulemans, Gas Chromatography, Reinhold, New York and Chapman Hall, London, 1956. 4 1 C. Bighi, A. Betti, C. Saglietto and F. Dondi,J. Chromatogr., 35 (1968) 309. 42 C. J . Hardy,J. Chrornatogr., 2 (1959) 490. 43 E. N. Drake and W. 0. Wilson,Ind. Bng. Chem., 59 (1967) 63. 44 D. D. Patterson, G. Tewari, H. P. Schrieber and J . E. Guillet,Macromolecules, 4 (1971) 356. 45 H. P. Schreiber, Y. B. Tewari and D. Patterson,J. Polym. Sci, P o l y m Phys. Ed., 11 (1973) 15. 46 V. G . Berezkin, V. P. Pakhomov, V. M. Pateyeva and V. S. Tatarinskii, Dokl. Akad. Nauk SSSR, 180 (1968) 119. 47 J. K. Conder, D. C. Locke and J . H. Purnell, J. Pbys. Chem., 73 (1969) 700. 48 J. Serpinet, Etude par Chromatographie en Phase Gazeuse d e la Disposition et d e l'ktat Physique de Composis Gras Non- Volatils a la Surface de Matkriaitx Poreux, Th2se; Universiti Claude Bernard d e Lyon, Lyon, 1974. 49 V. G. Berezkin, V. P. Pakhomov and K. 1. Sakodynsky, ,Derdyie Nositeli v Gasovoy Khromatografii (Solid Supports in Gas Chromatographyj, Khimiya, Moscow, 1975. 50 M. Schneider and K . 4 . Rechert, Farbe Lack, 76 (1970) 1081. 5 1 A. E. Ncsterov and Yu. S. Lipatov, Vysokomol. Soedin., Ser. A, 17 (1975) 617. 52 G. Perrault, M. Tremblay, M. Bedard, G. Duchesne and R. Voyrelle, Eur. Polym. J., 10, No. 2 (1974) 143. 53 I. M. Shevchuk, V. A. Granzhan, V. M. Sakharov and N. V. Kovalenko, Zh. Obshch. Khim., 39 (1969) 2638. 54 R. N. Novitskaya and M. S. Vigdergauz, Izv. Akad. Nauk SSSR, Ser. Khim., (1970) 2798. 55 S. Dal Nogare and R. S. J tivet, Gas-Liquid Chromatography, Theory and Practice, Wiley-lnterscience, New York, 1962. 56 T. 0. Tiernan and J. If. Furrell, Anal. Chem., 34 (1962) 1838. 57 C. G. Scott, in M. van Swaay (Editor), Gas Chromatography 1962, Buttcrworths, London, 1962, p. 36.

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58 W. Fiddler and R. C. Docrr,J. Chroniatogr., 21 (1966) 481. 59 M. S. Vigdergauz and M. 1. Afanasiev, Khim. Tekhnol. Topl. Masel, No. 5 (1965) 5 5 . 60 H. Kelker, 2. Anal. Chem., 198 (1963) 254. 61 M. Dewar and J . P. Scliroeder, J. Amer. Chem. Soc., 86 (1964) 535. 62 E. M. Harrall, 11, R . S . Porter and J . F. Johnson,J. Chromatogr., 21 (1966) 392. 63 J. E. Guillct and A. N. Stein, Macromolecules, 3 (1970) 102. 64 B. T. Guran and L. B. Rogers, J. Gas Chromatogr., 3 (1965) 269. 65 B. T. Guran and L. B. Rogers, J. Gas Chromatogr., 5 (1967) 547. 66 A. N. Stein, D. G. Gray and J. E. Guillet,Macromolecules, 4 (1971) 129. 67 V. G. Rerezkin and A. N. Genkin, Usp. Khim., 41 (1972) 1136. 6X B. Rousen (Editor), Razrusheniye Tverdykh Polimerov (Disintegration of Solid Polymers), Khimiya, Moscow, 1971. 69 L. V. Andreyev, V. D. Kuvsliinnikov and V. L. Baiburskii, Zh. Prikl. K h i m , 42 (1969) 2131. 70 V. G. Rerezkin, V. S . Kruglikova and V. E. Shiryayeva,Authors Certificate, 163,008 (1964); Byull. Izobr., No. 11 (1964). 71 T. C. Davis and J . C. Petersen, Anal. Chem., 38 (1966) 1938. 72 R. T. Kokes, H. Tobin and P. H. Emmett, J. Amer. Chem. Soc., 77 (1955) 5860. 73 S. Z. Roginsky, M. 1. Yanovsky and G. A. Gaziyev, Dokl. Akad. Nauk SSSR, 140 (1961) 1125. 74 S. Z. Roginsky, M. I. Yanovsky and A. D. Berman, Osnovy Primeneniya Khromatografii v Katalize (Fundamentals of the Application o f Chromatography in Catalysis), Nauka, Moscow, 1972. 75 E. Gil-Av and J. Herzbcrg-Minzly, Proc. Chem. Soc., (1961) 316. 76 V. G. Berezkin, V. S. Krcglikovaand N. A. Belikova, Dokl. Akad. Nuuk SSSR, 158 (1964) 182. 77 V. G. Berezkin, Usp. Khim., 37 (1958) 1348. 78 P. Steingaszner, in L. S . Ettre and W. H. McFadden (Editors), Ancillary Techniques of Gas Chromatography, Wilcy-Interscience, New York, 1969, p. 13. 79 A. J . P. Martin, Gas Chromatography, Academic Press, New York, 1958, p. 237. 80 A. J . P. Martin, Gas Chromafography, Butterworths, London, 1957, p. 1. 8 1 E:. P. Skornyakov, K . I. Sakodynsky and V. P. Chizhkov, Zavod. Lab., 32 (1966) 1975. 82 V. P. Chizhkov and 1. Samogil, Zavod. Lab., 34 (1966) 1975. 83 V. P. Chizhkov, Zavod. Lab., 35 (1969) 129. 84 V. G. Berezkin and V. E. Shiryacva, J. Chromatogr., 69 (1 972) 25. 85 D. Craig, J. J. Shipman and R. B. Tawler,J. Amer. Chem. SOC.,83 (1961) 2885. 86 A. N. Gcnkin and N. A.Pctrova,J. Chromatogr., 105 (1975) 25. 87 E. Gil-Av and J. Herling, J. Phys. Chem., 66 (1962) 1208. 88 B. Fcihush, M. F. Richardson, R. E. Sievers and C. S. Springer, J. Amer. Chem. Soc., 94 (1972) 6717. 89 C. E. Doring and H. G. Hanstha1,J. Prakt. Chem., Ser. A , 22 (1963) 58. 90 J . H. Purnell and 0. P. Srivagtava,Anal. Chem., 45 (1973) 1111. 9 1 A. V. Kisclev and V. P. Dreving (Editors), Eksperinientalhyie Metod)) v Adsorptsii Molekulyarnoy

Khromatografii (Experimental Methods in Adsorption and Molecular Chromatography), Moscow University Press, Moscow, 1973. 92 N. N. Avgul, A. V. Kiselcy and D. P. Poshkus, Adsorbtsiya Gazov i Parov na Odnorodnykh Poverkhnostyakh (Adsorption of Gases and Vapours on Homogeneous Surfaces), Khimiya, Moscow, 1975. 93 A. V. Kiselev and Ya. 1. Yashin, Gas-adsorption Chromatography, Plenum Press, New York, London, 1969. 94 J . C. Giddings and K. L. Mallik, Ind. Eng. Chem., 59, No. 4 (1967) 18. 95 J. R. Conder, in J. H. Purnell (Editor), Progress in Gas Chromatography, Interscience, New York, London, Sydney, 1968, p. 209. 96 U. B. Mohlin and D. G . Gray, J. Colloid Interface Sci., 47 (1974) 147. 97 B. Chabert, J . P. Soulicr and G. Ede1,Ann. Chim. (Pans), 5 , No. 3 (1970) 193. 98 L. S. Ettre, Open Tubular Columns, Perkin-Elmer, Norwalk, Conn, 1973. 99 V. G. Rerezkin, V. R. Alishoyev and I . B. Neniirovskaya, Gazovaya Khromatografiya v Khimii Polimerov (Gas Chromatography in Polymer Chemistty), Nauka, Moscow, 1972, p. 258. 100 D. G. Gray and J . E. Guillet,J. P o l y m Sci, PartB, 12 (1974) 231.


Conclusion The first experimental investigations on gas-liquid chromatography, published in 1952 by Martin and James, demonstrated the advantages of the new method and stiniulated the development of all branches of chromatography, which was originally discovered by M. S. Tsvet at the beginning of the 20th century. The vigorous development of gas chromatographic (GC) methods still continues, and about 2000 publications in this field of physical and analytical chemistry appear annually. In spite of the extremely rapid development of the techniques, ideas and fields of application of GC during the past two decades, its possibilities are far from exhausted, and each year yields new and interesting investigations in this area. This is also true of the chemistry of polymers, where the field of application of GC appears t o be particularly extensive. The versions of the method considered in this book show that GC is an efficient method for studying polymers and the reactions of high-molecular-weight compounds, and also for controlling the relevant processes in the polymer industry. Unfortunately, the abundant possibilities of the method have so far been insufficiently utilized in the chemistry of polymers. In conclusion, we would like to mention several lines of development of GC which, in our opinion, show much promise in the chemistry of polymers. (1) Development o f methods of reaction GC for the identification and quantitative analysis of high-molecular-weight compounds. (2) Study of the stiucture of phase transitions and the thermodynamics of interactions with volatile substances in polymers by inverse GC. (3)Development of the GC of polymers at high pressures, in particular for studying molecular-weight distribution. (4) Development of specialized automatic devices for studying reaction of polymerization, degradation, etc. Developments along these lines will promote the wider utilization of GC methods in the chemistry of polymers.


GAS CHROMATOGRAPHY OF POLYMERS, Volume 10

Instrumentation of Gas Chromatography

Modern Practice of Gas Chromatography

Modern Practice of Gas Chromatography

Basic gas chromatography

capillary gas adsorption chromatography

Modern Practice of Gas Chromatography

Modern Practice of Gas Chromatography

Basic Gas Chromatography

Analytical Gas Chromatography

Analytical Gas Chromatography

Basic gas chromatography

Analytical Gas Chromatography

Basic Gas Chromatography

Analytical Gas Chromatography

Capillary Gas Adsorption Chromatography

Basic Gas Chromatography

Temperature-Programmed Gas Chromatography

Introduction to analytical gas chromatography

Stationary Phases in Gas Chromatography

Gas Chromatography and Mass Spectrometry

Analytical Gas Chromatography, Second Edition

Detectors in Gas Chromatography (Journal of chromatography library)

Chemical Derivatization in Gas Chromatography (Journal of Chromatography Library)

Comprehensive two dimensional gas chromatography, Volume 55 (Comprehensive Analytical Chemistry)

CHROMATOGRAPHY OF STEROIDS, Volume 8 (Journal of Chromatography Library)

Gas Chromatography: Analytical Chemistry by Open Learning

Gas chromatography in air pollution analysis

Gas Chromatography and Lipids: A Practical Guide

Columns for Gas Chromatography: Performance and Selection

Gas Chromatography Analytical Chemistry by Open Learning

GAS CHROMATOGRAPHY OF POLYMERS, Volume 10

Instrumentation of Gas Chromatography

Modern Practice of Gas Chromatography

Modern Practice of Gas Chromatography

Basic gas chromatography

capillary gas adsorption chromatography

Modern Practice of Gas Chromatography

Modern Practice of Gas Chromatography

Basic Gas Chromatography

Analytical Gas Chromatography

Analytical Gas Chromatography

Basic gas chromatography

Analytical Gas Chromatography

Basic Gas Chromatography

Analytical Gas Chromatography

Capillary Gas Adsorption Chromatography

Basic Gas Chromatography

Temperature-Programmed Gas Chromatography

Introduction to analytical gas chromatography

Stationary Phases in Gas Chromatography

Gas Chromatography and Mass Spectrometry

Analytical Gas Chromatography, Second Edition

Detectors in Gas Chromatography (Journal of chromatography library)

Chemical Derivatization in Gas Chromatography (Journal of Chromatography Library)

Comprehensive two dimensional gas chromatography, Volume 55 (Comprehensive Analytical Chemistry)

CHROMATOGRAPHY OF STEROIDS, Volume 8 (Journal of Chromatography Library)

Gas Chromatography: Analytical Chemistry by Open Learning

Gas chromatography in air pollution analysis

Gas Chromatography and Lipids: A Practical Guide

Columns for Gas Chromatography: Performance and Selection

Gas Chromatography Analytical Chemistry by Open Learning

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