Page 265 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

APPARATUS 9.2

inorganic compounds which possess the requisite properties. If gas chromatography
is to be used for metal separation and quantitative analysis, the types of
compounds which can be used are limited to those that can be readily formed
in virtually quantitative and easily reproducible yield. This feature, together
with the requirements of sufficient volatility and thermal stability necessary for
successful gas chromatography, make neutral metal chelates the most favourable
compounds for use in metal analysis. 8-Diketone ligands, e.g. acetylacetone and
the fluorinated derivatives, trifluoroacetylacetone (TFA) and hexafluoroacetyl-
acetone (HFA) form stable, volatile chelates with aluminium, beryllium,
chromium(111) and a number of other metal ions; it is thus possible to
chromatograph a wide range of metals as their 8-diketone chelates.

TFA anion

HFA anion

The number of reported applications to analytical determinations at the trace
level appear to be few, probably the best known being the determination of
beryllium in various samples. The method generally involves the formation of
the volatile beryllium trifluoroacetylacetonate chelate, its solvent extraction into
benzene with subsequent separation and analysis by gas ~hromatography..~'
Various types of derivatisation have now been developed for both gas and
liquid chromatography. For more detailed information regarding the choice of
a suitable derivative for a particular analytical problem, the appropriate works
of reference should be cons~lted.~~.~~
For compounds of high molecular mass, however, the formation of derivatives
does not help to solve the problem ofinvolatility. This difficulty may be overcome
by breaking the large molecules up into smaller and more volatile fragments
which may then be analysed by gas-liquid chromatography, i.e. by using the
technique known as pyrolysis gas chromatography (PCC).
Pyrolysis GC is a technique in which a non-volatile sample is pyrolysed
under rigidly controlled conditions, usually in the absence of oxygen, and the
decomposition products separated in the gas chromatographic column. The
resulting chromatogram (pyrogram) is used for both qualitative and quantitative
analysis of the sample. If the latter is very complex, complete identification of
the pyrolysis fragments may not be possible, but in such cases the pyrogram
may be used to 'fingerprint' the sample. PGC has been applied to a wide variety
of samples, but its major use has been in polymer analysis for the investigation
of both synthetic and naturally occurring polymers. The various PGC systems
can generally be classified into two distinct types:
(a) Static-mode (furnace) reactors which typically consist of a quartz reactor
tube and a Pregl type of combustion furnace. Solid samples are placed in
the reactor tube and the system is closed. The furnace is then placed over
the combustion tube and the sample heated to the pyrolysis temperature.
In this type of pyrolysis system, the time required to reach the necessary
temperature is much longer (up to 30 seconds) than in dynamic pyrolysis,