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Multidimensional Chromatography in Environmental Analysis 337

MDGC has also been used in the air analysis field. For instance, it has been
applied to the analysis of volatile organic compounds (VOCs) in air, thus enabling a
wider range of these compounds to be analysed (18).
Today, however, GC–GC coupling is seldom used to determine pesticides in envi-
ronmental samples (2), although comprehensive MDGC has been applied to deter-
mine pesticides in more complex samples, such as human serum (19). On the other
hand, new trends in the pesticide market, which is now moving towards the produc-
tion of optically active enantiomers and away from racemic mixtures, may make this
area suitable for GC–GC application. The coupling of non-chiral columns to chiral
columns appears to be a suitable solution to the separation problems that such a trend
might cause.
Multidimensional gas chromatography has also been used in the qualitative anal-
ysis of contaminated environmental extracts by using spectral detection techniques
such as infrared (IR) spectroscopy and mass spectrometry (MS) (20). These tech-
niques produce the most reliable identification only when they are dealing with pure
substances; this means that the chromatographic process should avoid overlapping
of the peaks.
Most applications in environmental analysis involve heart-cut GC–GC, while
comprehensive multidimensional gas chromatography is the most widely used tech-
nique for analysing extremely complex mixtures such as those found in the
petroleum industry (21).

13.2.2 EXAMPLES OF MULTIDIMENSIONAL GAS
CHROMATOGRAPHY APPLIED TO ENVIRONMENTAL ANALYSIS

A typical example of MDGC in environmental analysis is the determination of
PCBs. These are ubiquitous contaminants of the environment in which they occur as
complex mixtures of many of the 209 theoretically possible congeners. The compo-
sitions of environmental mixtures vary according to sample type.
Attempts to optimize the capillary GC separation conditions of 209 PCBs on a
single column of either single or mixed phases have had only limited success.
MDGC has therefore been very important. In some cases, mass spectrometry and, in
particular high-resolution mass spectrometry, may be enough to determine different
isomers which co-elute in a single column, and sensitivity may be enhanced by
selected ion monitoring (SIM) or negative chemical ionization (NCI).
In MDGC, the usual configuration normally has a non-polar phase (such as SE 54
or CPSil 8) on the first column to make the initial, well-characterized separation. The
sample is chromatographed on this column to a point just before the elution of the
unresolved peaks. The column flow is then switched into a second column of a dif-
ferent, usually more polar, phase such as CPSil 19 or CPSil88, for the duration of the
elution of these resolved peaks only. The column is again isolated and the small
group of unresolved peaks is separated on the second column (15). Other columns
which have been used include BPX5 (22), OV1(23) or Ultra 2 (11) as the first
column, and HT8 (23), OV-210 (12, 24) or FFAP (14) as the second column.