Page 74 - Multidimensional Chromatography

Multidimensional High Resolution Gas Chromatography 65

It is the determination of volatile organic compounds produced from natural prod-
ucts that requires separation techniques that allow isolation of stereoisomers. The
most commonly determined groups are the terpene and sesquiterpene species pre-
sent in essential oils, which are used as key indicators of biological factors such as
the growth season, geographic location, climate, etc. These species are also released
directly into the atmosphere by very many plants and trees, and make a substantial
contribution to global biogeochemical cycles.
The determination of enantiomers is possible with GC–GC by combination of a
nonpolar, basic fractionating primary column used in conjunction with a chiral selec-
tive secondary columns. The sophistication of chiral columns has increased signifi-
cantly since their introduction in the late 1970s (33), with enantiomer-selective
two-dimensional separations first being performed in 1983 on amino acids by using
Deans switch technology (34). More recent examples related to the food and flavour
industry include the work of Mondello et al. (35, 36)and Mosandl et al. (37). Recent
analyses reported by Mondello operate without midpoint preconcentration and
demonstrates that a high speed of transfer, without band broadening, may be
achieved with modern valve interfacing.
The introduction of synthetic materials into natural products, often described as
‘adulteration’, is a common occurrence in food processing. The types of compounds
introduced, however, are often chiral in nature, e.g. the addition of terpenes into fruit
juices. The degree to which a synthetic terpene has been added to a natural product
may be subsequently determined if chiral quantitation of the target species is
enabled, since synthetic terpenes are manufactured as racemates. Two-dimensional
GC has a long history as the methodology of choice for this particular aspect of
organic analysis (38).
Figure 3.7 shows some early examples of this type of analysis (39), illustrating
the GC determination of the stereoisomeric composition of lactones in (a) a fruit
drink (where the ratio is racemic, and the lactone is added artificially) and (b) a
yoghurt, where the non-racemic ratio indicates no adulteration. Technically, this sep-
aration was enabled on a short 10 m slightly polar primary column coupled to a chi-
ral selective cyclodextrin secondary column. Both columns were independently
temperature controlled and the transfer cut performed by using a Deans switch, with
a backflush of the primary column following the heart-cut.
In any form of analysis it is important to determine the integrity of the system and
confirm that artefacts are not produced as a by-product of the analytical procedure.
This is particularly important in enantiomeric analysis, where problems such as the
degradation of lactone and furanon species in transfer lines has been reported (40).
As chromatography unions, injectors, splitters, etc. become more stable and greater
degrees of deactivation are possible, problems of this kind will hopefully be reduced.
Some species, however, such as methyl butenol generated from natural emissions,
still remain a problem, undergoing dehydration to yield isoprene on some GC
columns.
Isolated problems of racemization, rearrangement or dehydration should not
overshadow the fact, however, that the range of species amenable to enantiomeric
two-dimensional GC is very wide indeed, including not only terpenes and lactones,