Page 235 - Multidimensional Chromatography

Foods, Flavours and Fragrances Applications 229

Wines and other alcoholic beverages such as distillates represent very complex
mixtures of aromatic compounds in an ethanol–water mixture. Once an extract or
concentrate of the required compounds is prepared, a suitable chromatographic
system must be used to allow separation and resolution of the species of interest.
Many applications have been developed that use MDGC.
In 1991, Askari et al. (28) carried out the direct determination of the enantiomeric
distribution of the monoterpene alcohols ( -terpineol, linalol, and the furanoid
linalol oxides) in muscat grapes, musts and wines by using MDGC analysis. This
work was carried out on trichlorofluoromethane extracts, using a SiChromat 2 chro-
matograph equipped with a Carbowax 20M pre-column and differently modified
-cyclodextrin columns. The results obtained for fresh grape juices show character-
istic enantiomeric ratios for monoterpene alcohols (stereo-controlled enzimatic bio-
genesis), while the same alcohols are present as a racemic mixture in the
corresponding wines.
Sotolon (4,5-dimethyl-3-hydroxy-2(5H)-furanone) and solerone (4-acetyl-
-butirrolactone) were claimed to be responsible for some aroma characteristic of
flor sherries wines. These compounds are present only as traces, and are chemically
unstable. A system of two gas chromatographs coupled with a four-port switching
valve was used to quantitate these components without previous fractionation. The
first chromatograph was equipped with an on-column injector, in order to avoid
thermal degradation of sotolon in the heated injector, a DB-5 column and an FID.
The second chromatograph was equipped with an on-column injector, a DB-1701
column and an FID. The method allowed quantification of solerone and sotolon at
concentrations as low as a few ppb (29).
The use of specific detectors together with the MDGC system can help in the
identification of specific compounds, for example, those containing nitrogen and
sulfur, present in very low concentrations in complex matrices. An MDGC system
that consists of a temperature-programmed cold injection system, a multicolumn
switching system (Gerstel), and two HP 5890 GC ovens connected by a cryotrap
interface, has been used for the analysis of nitrogen and sulfur components of a
whiskey extract. The second oven was equipped with a mass-selective detector, a
chemiluminescence sulfur detector and a nitrogen thermionic detector (30). Figure
10.6 shows a selected cut from the pre-column (Carbowax 20M), cryofocused and
passed into the main column (5% polydimethylsiloxane) for simultaneous sulfur
and nitrogen MS detection. Some sulfur and nitrogen compounds were easily
located in the TIC trace from their respective specific traces, and could thus be read-
ily identified.
Another way to improve the analysis of complex matrices can be the combination
of a multidimensional system with information-rich spectral detection (31). The
analysis of eucalyptus and cascarilla bark essential oils has been carried out with an
MDGC instrument, coupling a fast second chromatograph with a matrix isolation
infrared spectrometer. Eluents from the first column were heart-cut and transferred
to a cryogenically cooled trap. The trap is then heated to re-inject the components
into an analytical column of different selectivity for separation and subsequent
detection. The problem of the mismatch between the speed of fast separation and the