Page 176 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN006F-275 June 29, 2001 21:12

Gas Chromatography 467

TABLE II Properties of Some Gas Chromatography tral thermal fragments. Most typically, the samples under
Selective Detectors investigation are large and nonvolatile compounds, such
Approximate as synthetic or natural polymers. A reproducible pyroly-
Detector Selectivity mode sensitivity (g) sis/GC process results in the formation of pyrograms that
are often highly indicative of some structural details of
Electron-capture Affinity to low-energy 10 −13 –10 −14 the original substance; both the presence of certain chro-
electrons
Thermionic Nitrogen 10 −12 matographic peaks and their areas are judged. As small as
Phosphorus 10 −13 submicrogram samples have been successfully analyzed
by this combination.
Flame-photometric Sulfur 10 −9
Design of a precolumn pyrolysis unit and the method
Phosphorus 10 −11
of thermal degradation are crucial to the acquisition of
Electrolytic-conductivity Halogen compounds 10 −10
diagnostically useful pyrograms. Sample size and the py-
Ultraviolet Aromatics 10 −9
rolysis temperature must also be carefully controlled. The
Photoionization Partially enhanced 10 −11 –10 −12
three most common pyrolysis techniques use (a) filament
response to certain
organic molecules as (ohmical) heating, (b) rapid warp-up of a ferromagnetic
compared with flame conductor in a high-frequency field (Curie-point pyroly-
ionization (not sis), and (c) direct thermal degradation in a heated quartz
truly selective)
tube. In each case, the sample is deposited from its so-
lution onto a suitable matrix, and the solvent is dried off
prior to pyrolysis. Alternatively, small pieces of solids are
The most common GC selective detectors are listed in
directly pyrolyzed.
Table II together with their analytically important features. Pyrolysis/GC is used extensively in the analysis of poly-
These selective detectors have been finding an increasing mers, paints, textile fibers, and even whole microorgan-
utilization in the analysis of environmental and biological isms. Certain materials of forensic interest have been char-
mixtures. As seen from Table II, sensitivities at the low acterized by this approach. A unique pyrolysis/GC system
nanogram level are very common, while some detectors was aboard the Viking 1975 Mission spacecraft to investi-
reach levels even below picogram amounts. Parallel uses gate the possible occurrence of organic compounds in the
of a nonselective and a selective detector are quite popular martian soil.
in chemical identification efforts.

2. GC/Mass Spectrometry
B. Ancillary Techniques
Mass spectrometers are sophisticated instruments that
While GC is a powerful separation method, it provides
work on the principles of compound ionization and frag-
only limited information on the chemical nature of the sub-
mentation (typically through the bombardment by elec-
stances it so effectively separates. Consequently, it has to
trons or selected ions), the physical separation of the
be combined with ancillary techniques. These are certain
charged fragments, and their detection. The information
sample manipulative techniques that are coupled in either
obtained by mass spectrometry is a mass spectrum (ion
a precolumn or a postcolumn arrangement to GC. Their
intensity versus mass) that is highly indicative of the
purpose is to enhance qualitative information about the
sample’s original structure, virtually a fingerprint of a
sample, to characterize it chemically, or ideally, to deter-
molecule. Consequently, the method provides a power-
mine unequivocally its structure. Some of these ancillary
ful means to identify various organic compounds but
techniques chemically alter the sample during the process;
works more effectively with pure substances than with
others measure only its physical parameters, such as op-
substance mixtures. The combination of GC with mass
tical spectra. On occasion, ancillary tools may represent
spectrometry provides an ideal analytical system, in which
instruments that are considerably more sophisticated and
the complex mixtures are first separated, and the mass
expensive than the GC instrumentation itself. The three
spectrometer is permitted to analyze the substances, one
GC ancillary techniques discussed below are among the
at a time.
most powerful and illustrative of this direction.
Commercial instruments that combine the two tech-
niques vary in several respects. The low-resolution in-
1. Pyrolysis/GC
struments provide the designation of nominal molecular
This combination is an example of the precolumn arrange- weights, while the high-resolution instruments can work
ment. Pyrolysis/GC combines a controlled thermal degra- up to the precision of a small fraction of such nomi-
dation of a sample with the subsequent separation of neu- nal masses. For example, a low-resolution masses. mass