Page 264 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

9 CAS CHROMATOCRAPHV

1. A supply of carrier gas from a high-pressure cylinder. The carrier gas used
is either helium, nitrogen, hydrogen or argon, the choice of gas depending on
factors such as availability, purity required, consumption and the type of detector
employed. Thus helium is preferred when thermal conductivity detectors are
employed because of its high thermal conductivity relative to that of the vapours
of most organic compounds. Associated with this high pressure supply of carrier
gas are the attendant pressure regulators and flow meters to control and monitor
the carrier gas flow; the operating efficiency of the apparatus is very dependent
on the maintenance of a constant flow of carrier gas.
It is appropriate to emphasise here two important safety considerations:
(a) free-standing gas cylinders must always be supported by means of clamps
or chains;
(b) waste gases, especially hydrogen, must be vented through an extraction
hood.
2. Sample injection system and derivatisation. Numerous devices have been
developed for introducing the sample, but the major applications involve liquid
samples that are introduced using a microsyringe with hypodermic needle. The
latter is inserted through a self-sealing silicone rubber septum and the sample
injected smoothly into a heated metal block at the head of the column.
Manipulation of the syringe may be regarded as an art developed with practice
and the aim must be to introduce the sample in a reproducible manner. The
temperature of the sample port should be such that the liquid is rapidly vaporised
but without either decomposing or fractionating the sample; a useful rule of
thumb is to set the sample port temperature approximately to the boiling point
of the least volatile component. For greatest efficiency, the smallest possible
sample size (1-10 PL) consistent with detector sensitivity should be used.
It should be noted here that the difficulty of accurately injecting small
quantities of liquids imposes a significant limitation on quantitative gas
chromatography. For this reason, it is essential in quantitative GLC to use a
procedure, such as the use of an interna1 standard, which allows for any variation
in size of the sample and the effectiveness with which it is applied to the column
(see Sections 9.4(5) and 9.7).
Many samples are, however, unsuitable for direct injection into a gas
chromatograph because, for example, of their high polarity, low volatility or
thermal instability. In this respect the versatility and application of gas
chromatography has been greatly extended by the formation of volatile
derivatives, especially by the use of silylation reagents. The term 'silylation' is
normally taken to mean the introduction of the trimethylsilyl, -Si(CH,),, or
similar group in place of active hydrogen atoms in the substance under
investigation. A considerable number of such reagents is now a~ailable,~'
including some special silylating agents which give improved detector response,
usually by incorporating a functional group suitable for a selective detector
system. Reagents containing chlorine and bromine atoms, for example, in the
silyl group are used particularly for preparing derivatives injected on to gas
chromatographs fitted with electron-capture detectors. Derivatisation can also
give enhanced resoluti~n from other components in a mixture and improved
peak shape for quantitative analysis.
Although inorganic compounds are generally not so volatile as are organic
compounds, gas chromatography has been applied in the study of certain