Table Of ContentGas Chromatography
Principles, Techniques, and Applications
Second Edition
A. B. LITTLEWOOD
School of Chemistry
The University
Newcastle upon Tyne
England
1970
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Preface
My aim in this book has been to produce a general textbook on gas
chromatography suitable for users of the technique and for research workers.
It does not presuppose any knowledge of the subject, which is introduced
in the first chapter.
The text has been largely rewritten for the second edition. At the time
of the first edition in 1962, gas chromatography was still developing as a
technique; its technology was only just becoming well defined, and its
range of application was still growing rapidly. Since then, however, it has
become fully established and is no longer a novelty. Thus, in this edition,
I have tended to emphasise what has now become the standard theory,
and what have now become standard items of equipment used in the great
majority of commercial instruments. The book is now addressed rather
to the routine user than to those engaged in developing the technique.
The first half of the book is primarily concerned with the ability of gas
chromatography to perform separations. The theory of the operation and
design of gas Chromatographie columns, both packed and open-tube, is
described in detail, and it is shown how columns may be designed so as to
secure any desired separation. The factors influencing separations are nu-
merous, and I have tried to plan my presentation so that each of the many
factors is described independently. Thus, the discussion of the thermo-
dynamics of solution (Chapter 5) is completely separate from the section
on the kinetics of chromatography (Chapter 6), and, within the latter,
I have tried where possible to separate each kinetic mechanism by discussing
independently the contribution of each to peak variance.
The third quarter of the book deals with detectors, which are the means
of obtaining quantitative analyses by gas chromatography. It also contains,
in Chapter 11, a description of the union of gas chromatography with other
techniques, and some indication of the use of the more sophisticated
methods of handling gas-chromatographic data, which will, in my opinion,
be used increasingly in analysis in the next few years.
The last quarter of the book is a single chapter in a series of sections,
each dealing with the chromatography of a particular class of chemical
ix
X PREFACE TO THE SECOND EDITION
compound. It is impossible in a textbook to deal comprehensively with all
of the enormous number of applications of gas chromatography published
to date. However, I have tried to choose my sections so as to cover those
applications which are most commonly encountered, and to be able to
show how these illustrate and use the general principles described in the
rest of the book.
Acknowledgments
The author would like to acknowledge the kindness of the following
organisations, journals, and individuals in allowing him to reproduce copy-
right material.
The American Chemical Society (Journal, Journal of Physical Chem-
istry, Analytical Chemistry)
The Biochemical Society (The Biochemical Journal)
The British Council (British Medical Bulletin)
The Chemical Society, London (Journal)
The Institute of Petroleum (Journal)
The Institute of Physics and the Physical Society (Journal of Scientific
Instruments)
The National Research Council of Canada (Canadian Journal of
Chemistry)
The New York Academy of Sciences (Annals)
The Society of Chemical Industry (Chemistry and Industry)
Academic Press Inc., New York (Proceedings of International Sym-
posia organised by the Instrument Society of America)
Butterworths & Co., London (Proceedings of International Symposia
organised by the Institute of Petroleum and others)
Elsevier Publishing Company, Amsterdam (Journal of Chromatogra-
phy, Anal. Chim. Acta)
The Macmillan Company, New York (Nature)
Pergamon Press Inc., New York (Chemical Engineering Science)
Reinhold Publishing Corporation, New York ("Gas Chromatography"
by A. I. M. Keulemans)
Dr. Robert L. Bowman, Mr. Nathaniel Brenner, Dr. J. E. Caller,
Mr. E. W. Ciepelinski, Dr. F. van der Craats, Mr. D. H. Desty;
Mr. R. D. Eanes, Dr. L. S. Ettre, Dr. A. Goldup, Prof. I. Halasz,
Dr. E. Heine, Dr. A. T. James, Dr. J. E. Lovelock, Dr. I. G.
McWilliam, Dr. L. Ongkiehong, Mr. C. S. G. Phillips, Professor V.
Pretorius, Mr. G. R. Primavesi, Dr. J. H. Purnell, Dr. G. Schom-
burg, Dr. F. C. Snowden, Dr. G. A. P. Tuey, Dr. J. J. Walraven.
xi
Chapter 1
Definitions and Technical Terms
1.1. GAS CHROMATOGRAPHY
Gas chromatography is a method for separating components of mixtures
of volatile compounds. In most applications, the separations are made to
identify and determine the quantity of each component of a sample of the
mixture, and analytical gas Chromatographie apparatus includes additional
devices for this purpose. In some applications, separations are made for
preparative purposes, but the scale is not generally greater than that
required for quantities of the order of 100 g. The technology and appli-
cations of gas chromatography overlap comparatively little with other
Chromatographie techniques (1-3), to which little reference will be made.
The central item in the apparatus for gas chromatography is the
Chromatographie Column, a long tube packed permeably with some ad-
sorbent (in the general, rather than the specifically chemical, sense). In
the commonest technique of gas chromatography, the Elution technique,
a stream of inert gas, the Carrier gas, passes continuously through the col-
umn, and the mixture to be separated is introduced instantaneously at the
beginning of the column (Fig. 1.1) as a sample either of a gas or a volatile
liquid. Let us suppose that the sample consists of one pure component.
After introduction, it is swept by the carrier gas on to the column, first
evaporating to form a Vapour if it is introduced as a liquid. When it
reaches the column, it is largely adsorbed, but an equilibrium is set up
between the column and the gas in the interstices of the column so that a
proportion of the sample always remains in the gas phase. This portion
moves a little further along the column in the carrier gas stream, where it
again equilibrates with the column. At the same time, material already
adsorbed in the column re-enters the gas phase so as to restore equilibrium
with the clean carrier gas which follows up the zone of vapour. This process
1
2 1. DEFINITIONS AND TECHNICAL TERMS
is shown by the arrows in Fig. 1.1b, which is an idealisation of the column
in which the carrier gas is assumed to flow down the left-hand half and the
adsorbent is contained in the right-hand half. The interface between the
phases is used as a length abscissa along the column, and positive ordinates
in each phase represent the concentrations of vapour therein. The process
in which carrier gas containing the vapour is stripped by the adsorbent in
front of the zone while the vapour enters the carrier gas at the rear goes
on continuously, with the result that the zone of vapour moves along the
column more or less compactly.
Carrier
gas
&
FIG. 1.1. Diagram of a permeably
packed column and its idealisation, in
which gas and adsorbed phases are rep-
resented separately.
(a) (b)
The speed at which the zone moves depends mainly on two factors, the
rate of flow of the carrier gas and the extent to which the vapour is ad-
sorbed. The faster the flow of carrier gas, the faster the zone moves; and
the more strongly the vapour is adsorbed on the column, the more slowly
the zone moves. The quantitative relations describing this are given in
Chapter 2. When two or more components are present in the sample, each
usually behaves independently of the others, so that for a given carrier
gas flow rate, the speed of the zone of each component will depend on the
extent to which it is adsorbed. Since different substances differ in their
1.1. GAS CHROMATOGRAPHY 3
adsorption, they may therefore be separated by making use of their dif-
ferent speeds of progress through the column. If they are eluted to the
far end of the column, they will appear one after the other in the gas stream,
the fastest first and the slowest last.
It remains to specify the nature of the adsorbent. We have used this
word in a general sense, rather than in the strict chemical sense of a sub-
strate which will retain a vapour on its surface. In a few cases, some of
which are important, adsorbents in the chemical sense, such as carbon,
alumina, or silica gel, are used as the packing material for columns, but
in more than 90% of applications, the column material is a liquid held in
place on the column by being absorbed on an inert solid support. Gas
chromatography with this kind of column is called Gas Liquid Chroma-
tography (G.L.C.). This method is a direct extension of liquid-liquid partition
chromatography used for separating solutes from mixed solutions, since
the vapour in the carrier gas is exactly analogous to the solute in the eluting
liquid. The distribution of solute between eluting liquid and stationary
liquid in partition chromatography is described by the same law as the
distribution of vapour between eluting gas and stationary liquid in gas-
liquid chromatography, so that G.L.C. has often been called "Gas Partition
Chromatography," though international convention no longer favours this
name. In the case of G.L.C, the solution of a vapour in a liquid is described
by Henry's law, which is:
Q = ßc, (1.1)
where q is the concentration of vapour in the stationary phase, c is the
concentration of vapour in the gas phase, and ß is a constant. The constant
ß is a measure of the adsorption of a vapour in the adsorbent, and therefore
determines the speed of the zone of vapour through the column. Though it
should strictly be called the "Henry's Law Constant," it is conventionally
called the Partition Coefficient by virtue of the analogy with liquid-liquid
chromatography. The illogicality of refusing to use the word "Partition"
in the name of the technique, but using it in the name of the principal
quantitative measure in the technique is too well established to change.
Though the elution technique is the most common, there are two other
techniques by which gas chromatography can be used, Displacement and
Frontal analysis. Both techniques apply only when chemical adsorbents
are used for the columns, and chemical adsorbents are more suitable for use
with these techniques than with elution. Displacement and frontal tech-
niques will be described briefly in part of Chapter 7. The rest of the book is
concerned solely with elution chromatography.
4 1. DEFINITIONS AND TECHNICAL TERMS
1.2. BASIC APPARATUS OF GAS CHROMATOGRAPHY
The instrumentation of gas chromatography is remarkably simple
compared to many other recent analytical techniques. Figure 1.2 shows a
block schematic diagram of a typical gas-liquid chromatography apparatus,
such as might be used for analysis.
Flow
1 Column Detector Collector Flowmeter
A* ^nt rol
Sample I A I
injector
Recorder
FIG. 1.2. Block schematic diagram of apparatus for analytical gas chromatography.
Carrier gas from the tank of compressed gas first passes to a controller,
the usual purpose of which is to maintain a constant flow of gas. The gas
then passes to the beginning of the column, at the inlet to which is a Sample
Injector through which the sample to be analysed can be introduced. The
carrier gas then elutes the components of the mixture through the column.
At the far end is a device, the Detector, the purpose of which is to detect
the separate components of the mixture as they emerge one by one. The
detector uses some physical or chemical property of the vapours by which
they can be indicated and, if possible, measured. A further piece of appa-
ratus not always incorporated is a Flowmeter to measure the rate of flow
of gas.
The details of the design of the apparatus for chromatography will be
considered in later chapters. For the moment it is sufficient to outline the
operation of the various sections of the apparatus simply to indicate how
the technique works and to introduce technical terms.
(1) Any gas which is easily distinguishable in the detector from any
components of the mixture can be chosen as the carrier gas. In most cases
a permanent gas such as helium, argon, nitrogen, or hydrogen is used.
(2) Flow controls can either be true flow controls, which allow a definite
rate of flow of gas, or they can be pressure controls which control the input
pressure to the column. Since the column generally acts as a constant
choke, a constant input pressure will itself ensure a constant flow. Pressure
controls are simpler than flow controls and are used more commonly.
1.2. BASIC APPARATUS OF GAS CHROMATOGRAPHY 5
(3) For analytical purposes, reasonable dimensions for a permeably
packed column might be 6 feet long by J inch diameter. In G.L.C. the
actual dissolving agent is a liquid which must remain stationary in the
column without draining. It is therefore absorbed or otherwise held upon
an inert solid support of some porous material which will allow the passage
of gas. Kieselguhr, which can absorb up to 40% by weight of a liquid
without becoming unduly moist, is commonly used. The application of this
property in making dynamite is, of course, well known. More recently other
supporting materials have been used, such as ground firebrick, glass beads,
etc., and these will be discussed in detail in Chapter 7. A further necessary
property of the supporting material is that it should not itself adsorb any
of the components of the mixture, and therefore must be inert. The support-
ing material is often simply called the Support. The liquid solvent is called
the Stationary Liquid. The two together are called the Stationary Phase,
though this phrase is often used for the stationary liquid alone.
Another kind of column consists of a long fine tube of which the inner
surface is coated with stationary liquid. Such columns, called Capillary
Columns, are particularly useful in the analysis of microscopic samples.
A monograph on their use has been written by Ettre (4).
The stationary liquid must be of such a nature as to dissolve all the
components of the mixture (with the possible exception of one) and further-
more it must be completely involatile relative to any of them. For relatively
involatile mixtures, columns are operated at elevated temperatures. The
best temperature at which to run the column for a given mixture depends
upon the design of the column and upon the mixture; the temperature of
the column may be anything between 100°C above and 100°C below the
boiling points of components in the mixture. Stationary liquids must have
boiling points which are high in comparison with the boiling points of the
components of the mixture. Almost any high-boiling liquid may be used,
for example, silicone oils, high-boiling esters such as phthalates, high-
boiling paraffins, and liquid polymers. Factors affecting the choice of station-
ary liquids and other column conditions are discussed in Chapter 3.
(4) One of the properties most commonly used for detecting vapours is
the electrical conductivity of a hydrogen flame in which the vapour is burned.
In the detector, the carrier gas (if not hydrogen already) is mixed with a
constant flow of hydrogen, and the mixed gases are burned at a small metal
jet, above which is positioned an electrode. A potential of about 300 V is set
between the electrode and the jet, and the current passing is measured by
suitable electronic equipment. In the absence of organic vapour, the current
is extremely small (^10~14 amp), but organic vapour causes a large increase
