Page 77 - Multidimensional Chromatography

68 Multidimensional Chromatography

but also esters, alcohols and acids, all of which appear stable under two-dimensional
GC conditions (41–44).
The study of biochemical natural products has also been aided through the appli-
cation of two-dimensional GC. In many studies, it has been observed that volatile
organic compounds from plants (for example, in fruits) show species-specific distri-
butions in chiral abundances. Observations have shown that related species produce
similar compounds, but at differing ratios, and the study of such distributions yields
information on speciation and plant genetics. In particular, the determination of
hydroxyl fatty acid adducts produced from bacterial processes has been a successful
application. In the reported applications, enantiomeric determination of polyhy-
droxyl alkanoic acids extracted from intracellular regions has been enabled (45).
An obvious extension of enantiomeric two-dimensional GC of natural products is
the application of carbon isotope mass spectrometry as the detection process. While
the application of isotopic carbon abundance in pharmaceutical and food research
has been commonplace for many years, its coupling to enantiomer-selective GC is
still little explored. The ability to isotopically discriminate between the species
evolved during biological processes can provide valuable checks on the authenticity
of the analysis. More specifically, enantiomers from the same origin will have identi-
12
13
cal C/ C ratios. Since isotopic separations are generally not feasible, enantiomeric
separation in combination with isotopic mass spectrometry offers a possible analysis
route. At present, the sensitivity of 12/13 C analysers is significantly less than is
normally achieved by using benchtop quadrupole or ion-trap configurations and so
detector sensitivity is an important issue. When two-dimensional separations are
applied to such a problem, the data obtained are more reliable than those obtained by
single-column separations, since background interferences on the chromatogram are
very greatly reduced (46, 47).

3.3.2 TYPICAL ENVIRONMENTAL CONTAMINANT ANALYSES

While a small number of fuel-derived organic emissions relevant to the environment
have been described above, the main environmental application of two-dimensional
GC has been in the analysis of halogenated persistent organic pollutants (POPs).
This broad classification of pollutants contains species such as polychlorinated
biphenyls (PCBs), dioxins, furans, toxaphenes and other persistent organochlorine
molecules released via anthropogenic sources. The sources of species such as these
are too numerous to list exhaustively but include, (famously) pesticides, and trans-
former insulator oil. It is their potency to detrimentally affect human health, coupled
to long lifetimes in the environment, which has lead to such interest in their analysis.
Atmospheric lifetimes are in excess of 100 years, with the major route for degrada-
tion being via microbial action in soils. This too, however, is achieved at only very
slow rates. The analysis of PCBs is particularly challenging, given a combination of
large diversity (over 209 individual biphenyl molecules have been produced in 150
commercial products (48)) and yet a very low abundance in the environment (e.g.
pg–ng m 3 in air).