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62 Multidimensional Chromatography

example, it was essential that further chemical information was available to produce
even group-type classifications in such a mixture, with the addition of a mass
spectrometer generally offering the required extra analytical dimension.
A number of environmental applications of two-dimensional GC have arisen as a
direct result of the impact of combustion emissions on the environment. The analysis
of aromatic species in air may be achieved in a similar manner to petrochemical
analysis, although sample pre-concentration becomes essential as the abundances
are proportionally lower. Similarly, the measurement of carcinogenic polycyclic aro-
matic compounds, in particular nitrogen-and oxygen-containing derivatives, is of
particular epidemiological interest. The isolation of species in the atmosphere pre-
sents similar problems to analysis in fuels, in that a high-concentration aliphatic
matrix often masks the target analytes. Multidimensional methods using both
GC–GC and LC–GC offer excellent resolution with simple quantitation when using
flame ionization detectors, with a number of such applications being covered in
Chapter 13. An application worth mentioning from a technical viewpoint, however,
is where two-dimensional GC has been used to reduce the analysis time and utilizes
isothermal conditions. In particular, a valve-transfer system has been reported for the
determination of isoprene and dimethyl sulfide in air, which involves the combina-
tion of carrier gas pressure programming with non-polar and PLOT columns (24).
Although limited applications have been reported in environmental analysis,
GC–GC offers great potential for deconvoluting much of the complexity in these
samples, and as well as enabling the use of simple analytical systems which allow
high-speed repetitive measurements.

3.3.2 FLAVOURS, FRAGRANCE AND FOOD
ANALYSIS METHODOLGIES

In common with the previous application category, this grouping is of great commer-
cial importance to a number of global industries. The determination of species central
to the flavour and fragrance industries is highly complex in both chemical complexity
and human response. A recurring theme, however, in this type of analysis is that very
low concentrations of specific compounds must be isolated from a potentially com-
plex, high-concentration matrix to enable both quantitation and assessment. In this
field, assessment may be structural or functional, through the use of a mass spectrom-
eter, or by impact on the human senses through organoleptic assessment, respec-
tively. The use of organoleptic methods (that is, the use of the human nose to ‘detect’
components in a mixture) often highlights problems of resolution in GC separations
of complex mixtures. While ideally each strongly detected peak on a chromatogram
would have a correlated peak on the detector, this is rarely the case. Often multiple
elutions at a given retention time mean that organoleptic assessment merely narrows
the field of possible compounds, and very commonly that the nose recognizes com-
ponents that are not apparent on the detector. Overcoming problems of detector sen-
sitivity inevitably results in preconcentration, elevations in matrix concentrations and
a required increase in peak capacity for the separation system.