Page 285 - Multidimensional Chromatography

280 Multidimensional Chromatography

for GC analysis, i.e. SPE–GC, is a highly interesting approach for the rapid trace-
level detection and quantitation of the wide range of GC-amenable compounds that
have to be monitored. This convincingly justifies the rather special interest in this par-
ticular area of on-line LC–GC, but so far the applications have been mainly in the
environmental field and not many bioanalytical applications have been described.
On-line dialysis–SPE–GC was developed for the determination of drugs in
plasma, with benzodiazepines as model compounds (131). Clean-up was based on
dialysis of 100 l samples for 7 min by using water as the acceptor, and trapping the
diffused analytes on a PLRP-S column. After drying, the analytes were desorbed
with 375 l of ethyl acetate, which were injected on-line into the GC via a loop-type
interface. This system provides a very efficient clean-up and offers the possibility of
adding chemical agents which can help to reduce drug–protein binding. In order to
demonstrate the potential of the approach, benzodiazepines were determined in
plasma at their therapeutic levels. Flame-ionization (FID), nitrogen–phosphorus
(NPD) and MS detection were used.
The selectivity of the trace-enrichment procedure can be improved by using an
immunoaffinity precolumn: 19- -nortestosterone was used as the test compound
(132). Desorption from the antibody-loaded pre-column had to be carried out with
about 2 ml of methanol–water (95 : 5, vol/vol), which obviously could not be trans-
ferred to the retention gap. The eluate was therefore diluted with HPLC-grade water
and the mixture led through a conventional ODS pre-column. As a result of so-called
reconcentration by dilution–which means that the gain in breakthrough volume due
to increased retention caused by the decrease of the modifier percentage distinctly
outweighs the volume increase–the analyte was quantitatively trapped on this sec-
ond pre-column. Desorption and transfer to the GC system were coried out in a simi-
lar way to that described above. The method was applied to the determination of
steroid hormones in 5–25 ml human urine. The detection limit for 19- -nortestos-
terone was about 0.1 ppb with an RSD of 6 % (see Figure 11.13).
Examples of SPE–GC of biological samples are few, while the usefulness of
SPE–GC for the analysis of surface and drinking water has been demonstrated many
times (133). This might be due to the fact that biological samples are often consider-
ably more complex than environmental water samples. In addition, various
biomedically and pharmaceutically interesting analytes will not be amenable to GC.
Nevertheless, because many of the initial SPE–GC interfacing problems have now
been solved (133), it seems appropriate and worthwhile to explore its utility in the
bioanalytical field more thoroughly.

11.6 SOLID-PHASE MICROEXTRACTION COUPLED WITH
GAS OR LIQUID CHROMATOGRAPHY

Although solid-phase microextraction (SPME) has only been introduced compara-
tively recently (134), it has already generated much interest and popularity. SPME is
based on the equilibrium between an aqueous sample and a stationary phase coated
on a fibre that is mounted in a syringe-like protective holder. For extraction, the fibre