Page 224 - Multidimensional Chromatography

218 Multidimensional Chromatography

10.2 MULTIDIMENSIONAL GAS
CHROMATOGRAPHY (GC–GC OR MDGC)

A large number of the organic compounds in food and beverages are chiral
molecules. In addition, a significant number of the additives, flavours, fragrances,
pesticides and preservatives that are used in the food industry are also chiral
materials.
The enantiomeric distribution can be very useful for identifying adulterated foods
and beverages, for controlling and monitoring fermentation processes and products,
and evaluating age and storage effects (1).
The enantiomeric distribution of the components of essential oils can provide
information on the authenticity and quality of the oil, on the geographical origin and
on their biogenesis (2).
GC using chiral columns coated with derivatized cyclodextrin is the analytical
technique most frequently employed for the determination of the enantiomeric ratio
of volatile compounds. Food products, as well as flavours and fragrances, are usually
very complex matrices, so direct GC analysis of the enantiomeric ratio of certain
components is usually difficult. Often, the components of interest are present in trace
amounts and problems of peak overlap may occur. The literature reports many exam-
ples of the use of multidimensional gas chromatography with a combination of a
non-chiral pre-column and a chiral analytical column for this type of analysis.
Mosandl and his co-workers (3–17) have carried out many research studies on the
determination of the enantiomeric ratio of various components of food and bever-
ages, as well as plant materials and essential oils. Using a SiChromat 2–8 double-
oven system with two independent temperature controls, two flame ionization
detectors and a ‘live switching’ coupling piece, these workers have developed many
applications of enantioselective MDGC employing heart-cutting technique from a
non-chiral pre-separation column on to a chiral main column. In this way, direct
chiral analysis is possible without any further clean-up or derivatization procedure.
Table 10.1 summarizes some of these applications. As a typical example, Figure
10.1(a) shows the separation on a Carbowax 20M column of a dichloromethane
extract of a ‘strawberry’ tea (18). As can be seen, the GC profile is very complex.
Figure 10.1(b) shows the enantiomeric separation of 2,5-dimethyl-4-hydroxy-3[2H]-
furanone, known as ‘pineapple ketone’, from the tea extract, transferred from the
Carbowax 20M pre-column to a modified -cyclodextrin column. This analysis
allowed the detection of the synthetic racemate of ‘pineapple ketone’ that was added
to the tea to give the strawberry flavour.
For the enantioselective flavour analysis of components present in extremely low
concentrations, a MDGC–MS method has been developed (19). An example of the
application of this technique is the determination of theaspiranes and theaspirones in
fruits. These compounds are potent flavour compounds which are widely used in the
flavours industry. Figure 10.2 shows the MDGC–MS chromatogram obtained by
using multiple ion detection (MID) differentiation between the enantiomers of
theaspiranes in an aglycone fraction from purple passion fruit. In fact, using the MID
technique, interfering peaks are easily removed and the detection limit is lowered.