Page 180 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GQT Final
Encyclopedia of Physical Science and Technology EN006F-275 June 29, 2001 21:12

Gas Chromatography 471

have been developed over the years to identify the sources
of water and air pollution, to study biodegradation and
transformation of various pollutants in the environment,
and to monitor their levels on a continual basis. A
great majority of such methods involve GC measurement
principles: both packed and capillary columns, selec-
tive detectors, the gas chromatography/mass spectrometry
combination, and so on. For example, in film-processing
laboratories air analysis must periodically be carried out
to measure the levels of toxic aromatic amines. With a cap-
illary column (Fig. 14) and the nitrogen-sensitive flame-
based detector, five different aromatic amines can be quan-
3
titated at the airborne levels of 3 to 13 µg/m .
Prior to the GC analysis, the air sample is first con-
centrated by passing it through a small adsorbent column.
Such a preconcentration step is common if trace organics
are to be analyzed in dilute media (air, water, soil, etc.).
Gas chromatography has been applied to analyze nu-
merous biologically important substances such as fatty
acids, amino acids and peptides, steroids, carbohydrates,
and prostaglandins. Since these compounds are mostly
polar and nonvolatile, chemical modifications (sample
derivatization) are necessary to block the polar groups
and thus enhance volatility of such compounds. To en-
sure the necessary reliability of GC analyses, such chemi-
cal modifications must have highly reproducible yields.
For compounds with diverse functional groups, multi-
ple derivatizations (through more than one reaction) are
FIGURE 15 Urinary steroid profiles (after sample derivation)
needed. Examples of these are the various steroid hormone
from a normal and a diabetic patient, as recorded by capillary
metabolites that feature ketonic and hydroxy functional chromatography. [From Alasandro, M., Wiesler, D., Rhodes, G.,
groups in their molecules. Prior to their GC analysis, these and Novotny, M. (1982). Clin. Chim. Acta 126, 243.]
compounds are first subjected to treatment with methoxy-
lamine hydrochloride (to form methoximes from ketones)
and then to reaction with a trimethylsilyl donor reagent (to closely related substances can be monitored under differ-
form trimethylsilylethers from alcohols). An example of ent circumstances of health and disease (Fig. 15). While
a fully derivatized steroid is a methoxime-trimethylsilyl this demonstrated case has been related to an effort to
product of the glucocorticoid hormone, cortisol. improve our understanding of hormonal alterations in hu-
man diabetes, similar analytical GC techniques have been
employed to detect abnormalities in adrenal function and
OSiMe 3
reproductive processes.
MeON
Gas-chromatographic methods are widely used to an-
OSiMe 3 alyze amino acids in the hydrolyzates of small pro-
Me 3 SiO
tein samples. The method’s sensitivity is the major
reason for these applications. In addition, GC-based tech-
niques provide opportunities to distinguish and quanti-
tate amino acids (and several other compound types) as
MeON different optical isomers. The most popular procedure to
Me = methyl separate R and S isomers employs an optically active (chi-
ral) stationary phase. Because of the zwitterionic nature of
Other steroids (i.e., compounds structurally related to amino acids, a two-step derivatization is necessary prior to
cortisol) can also be derivatized in a similar manner and GC.Asthefirststep,theacid(carboxy)functionisblocked
subjected to GC analysis. If a high-resolution (capillary) through esterification. During the second treatment, the
column is employed for their separation, entire profiles of amino groups are acylated. Figure 16 demonstrates