Page 247 - Multidimensional Chromatography

Foods, Flavours and Fragrances Applications 241

10.5 MULTIDIMENSIONAL CHROMATOGRAPHIC METHODS
WHICH INVOLVE THE USE OF SUPERCRITICAL FLUIDS

Supercritical fluid extraction (SFE) has been extensively used for the extraction of
volatile components such as essential oils, flavours and aromas from plant materials
on an industrial as well as an analytical scale (61). The extract thus obtained is usu-
ally analysed by GC. Off-line SFE–GC is frequently employed, but on-line
SFE–GC has also been used. The direct coupling of SFE with supercritical fluid
chromatography (SFC) has also been successfully caried out. Coupling SFE with
SFC provides several advantages for the separation and detection of organic sub-
stances: low temperatures can be used for both SFE and SFC, so they are well suited
for the analysis of natural materials that contain compounds which are temperature-
sensitive, such as flavours and fragrances.
Different approaches have been developed for the direct coupling of SFE and GC.
Hawthorne et al. (62,63) analysed various flavour and fragrance compounds of natu-
ral products extracted with supercritical (SC) CO 2 by depositing the extracted ana-
lytes coming from the extraction cell outlet restrictor directly inside the GC capillary
column through the on-column injector, which was cooled to cryogenically trap the
analyte species at an appropriate oven temperature. Another approach was developed
by Hartonen et al. (64). These authors used a cryotrap
thermal desorption unit exter-
nal to the GC to collect the extracted compounds and introduce them into the GC
column. In this way, the supercritical fluid used for the extraction does not flow
through the separation column, thus increasing the lifetime of the latter. The
extracted compounds are focused not only by selecting the lowest possible trapping
temperature, but also by a low initial GC temperature. In this way, the separation of
very volatile compounds is more efficient and more reproducible.
Selected applications of coupled SFE–SFC consider the analysis of tocopherols
in plants and oil by-products (65) or the analysis of lipid-soluble vitamins (66) by
using a dynamic on-line SFE-SFC coupling, integrated in the SF chromatograph,
based on the use of micropacked columns.
A method which uses supercritical fluid/solid phase extraction/supercritical fluid
chromatography (SF
SPE
SFC) has been developed for the analysis of trace con-
stituents in complex matrices (67). By using this technique, extraction and clean-up
are accomplished in one step using unmodified SC CO 2 . This step is monitored by a
photodiode-array detector which allows fractionation. Figure 10.14 shows a
schematic representation of the SF/SPE/SFC set-up. This system allowed selective
retention of the sample matrices while eluting and depositing the analytes of interest
in the cryogenic trap. Application to the analysis of pesticides from lipid sample
matrices have been reported. In this case, the lipids were completely separated from
the pesticides.
Analogous to HPLC–HRGC, the combination of packed column SFC and capil-
lary column GC can be used for the analysis of complex samples. The advantage of
SFC/GC with respect to HPLC/GC is the absence of problems associated with the
evaporation of the HPLC mobile phase prior to the GC analysis.