Page 146 - Multidimensional Chromatography

138 Multidimensional Chromatography

approach covering both techniques (SFE and SFC) has been recently covered in sev-
eral books and reviews (17–21) and will not be discussed further in this present
chapter, whose subject is the on-line approach.

6.2 ON-LINE COUPLING OF SFE WITH
CHROMATOGRAPHIC TECHNIQUES
Hirschfeld, in his article on coupled techniques, defined, a hyphenated instrument as
‘one in which both instruments are automated together as a single integrated unit via
a hardware interface . . . whose function is to reconcile the often extremely contra-
dictory output limitations of one instrument and the input limitation of the other’
(22). Therefore, the key to combining SFE with chromatographic techniques is the
interface which should allow the optimum and independent usage of each instrument
while the couple still operate as an integrated unit (23).
Considering that in most SFE methods CO 2 is used as the extracting fluid, the on-
line coupling of such a system with a chromatographic technique became easy due to
the favourable properties of this fluid. For instance in coupling SFE with GC/flame-
ionization detection (FID) there are no detection limitations since CO 2 does not pre-
sent a signal with this detector. Therefore, the major requirement for a successful
coupling is the quantitative transfer of the SF extract to the inlet of the chromato-
graph. SFE systems have been successfully coupled to GC (24, 25) SFC (26, 27),
and LC (28, 29) systems. A review describing several applications where SFE has
been coupled on-line to various chromatographic systems is available (30).

6.3 SFE–GC

On-line SFE–GC has been so far the most investigated coupling between SFE and
chromatographic techniques. One of the reasons for this is that such a coupling is
conceptually straightforward provided that the extracted analytes are quantitatively
transferred to the GC inlet. Among the several publications on this subject, the sim-
plest approach which has gained great popularity involves the use of either an on-
column or a splitless injector as the interface. In this technique, the end of the
restrictor from the SFE instrument is inserted inside the GC inlet and the extract is
trapped inside the injector. After the extraction is accomplished, the injector is
heated and the analytes are transferred to the analytical column to start the chromato-
graphic run. The use of a cryogenic approach to trap the extracted analytes before
the GC run has also been investigated (31, 32). The use of a programmed tempera-
ture vaporizer (PTV) injector as the interface for SFE-GC has been evaluated (33).
Figure 6.3 shows a schematic drawing of a typical instrumental set-up used for on-
line SFE–GC work.
The use of more selective detectors for SFE–GC, such as a thermo-energy ana-
lyzer for the detection of explosives (34) and a two-channel optical device for the