Page 148 - Multidimensional Chromatography

140 Multidimensional Chromatography

Figure 6.4 Schematic diagram of an on-line SPE–SFE–GC system (from ref. 40):1, car-
bon dioxide; 2, high-pressure syringe pump; 3, gas chromatograph; 4, three-port valve; 5,
oven; 6, extraction cell; 7, waste; 8, ten-port valve; 9–11 conditioning and washing solvents;
12, sample; 13, nitrogen.

of analytes trapped on to a SPE sorbent to a GC column. As CO 2 , the most widely
used solvent for SFE, is a gas at room temperature, it therefore becomes the ideal
solvent to quantitatively transport the analytes from the SPE cartridge to the GC col-
umn (5). Figure 6.4 shows a schematic drawing of a ‘home-made’ SPE–SFE–GC
system (40).
The system is built by using three independent modules (SPE, SFE and GC) in
such a way that it can be assembled to perform experiments in the on-line coupled
mode (SPE–SFE, SFE–GC, SPE–GC and SPE–SFE–GC) or as independent units
(GC, SPE, and SFE). This means that if we want to use the system for standard GC,
there will be no problems, with the same applying for both SPE and SFE.
Operation of the SPE–SFE–GC system is very easy (see Figure 6.4). The SPE
cartridge is conditioned (flasks 9 and 10) and washed (flask 11) depending upon the
sorbent used. The first and second three-port valves (items 4) are then rotated to the
SPE cartridge and waste positions, respectively. The sample contained in flask 12 is
then loaded into the SPE cartridge (item 6) with the help of nitrogen (item 13). While
the matrix and unwanted analytes goes to waste (item 7), the target compounds are
trapped in the cartridge (item 6). After loading the sample into the extraction cell, the
system is dried with nitrogen and the two valves are now switched to the SFE and
GC feed positions, respectively, thus providing the transfer of the trapped analytes to
the GC injector. A cryogenic system using liquid CO 2 is used to trap the analytes in
the first section of the capillary column. Details of the interface used for coupling the
SFE module to the GC injector is shown in Figure 6.5 (40). Figure 6.6 (b) displays
an SPE–SFE–HRGC photo ionization detector (PID) chromatogram of a water
sample contaminated with several aromatic compounds.