Page 68 - Multidimensional Chromatography

Multidimensional High Resolution Gas Chromatography 59

products with the correct compositional and combustion characteristics. A typical
example of the use of gas chromatography when applied to petrochemical analysis
can be found in reference (20), while reviews of such applications are also available
in selected issues of the Journal of High Resolution Chromatography (21). What is
evident from the many reported applications of two-dimensional GC in the areas of
hydrocarbon and related analysis is the accuracy of quantitation when compared to
that of GC–MS. Since resolution is achieved by chromatographic means alone, well
characterized detectors such as the flame ionization detector (FID) may be used in
the place of the less easily quantified mass spectrometric detection (22).
Of considerable commercial interest has been the isolation of various alkyl-
substituted polycyclic aromatic compounds where these serve as indicators of the
geological history of sedimentary rock samples, highly useful in the exploration for
oil reserves. The isolation of alkyl-substituted aromatics, however, is a typical prob-
lem to which multidimensional GC may be applied. Large numbers of substituted
aromatic isomers exist, and their deconvolution by using mass spectrometry alone is
still unclear. The most accurate method to directly quantify individual isomers is
therefore to resolve each one chromatographically. Schäfer and Höltkemeir (20) in
1992 presented a two-dimensional method that had sufficient selectivity and resolu-
tion to individually isolate 1,4-dimethylnaphthalene from 2,3-dimethylnaphthalene
without the use of mass spectrometry. This method effectively used a Deans switch
to transfer fractions of crude oil between a primary low-polarity column and a 50 %
phenyl/25% cyanopropyl/25% methyl polysiloxane secondary column. Figure 3.4
shows a series of chromatograms obtained from the recombination of heart-cuts
made from this primary separation. In this case, while a single column of moderate
polarity would have enabled some resolution between the target species, to achieve
complete isolation would have required an impractical length of column, and hence
analysis time.
The analysis of combustion products presents problems of complexity similar to
that of feedstock and raw fuel analysis. A highly complex matrix of aliphatic mate-
rial often exists (as unburnt fuel in the combustion exhaust), whilst the species of
interest, for example, carcinogens or mutagens are often at very low concentrations.
A classic example of multidimensional GC is its use in the analysis of flue-cured
tobacco essential oil condensate.
The chromatograms reported by Gordon et al. (23) shown in Figure 3.5 illustrate
the huge complexity of even small heart-cuts made from the primary separation.
Once again, a Deans-type switch was used for sample transfer. For the primary chro-
matogram, each cut is seen to contain only a handful of peaks, yet when a further
secondary separation is performed (based on polarity rather than boiling point) a
large numbers of extra species can be isolated. The huge complexity in even the sec-
ond-dimension chromatogram required that the second column was temperature pro-
grammed, and a two-oven approach was therefore applied. In the case of the tobacco
condensate it becomes questionable, even with a second separation with full temper-
ature programming, whether the analytical system has sufficient capacity, and that
possibly a higher dimension was required to truly characterize the sample. In this