Page 49 - Multidimensional Chromatography

Coupled HPLC with HRGC 39

Figure 2.18 (a) Gas chromatogram of a standard solution of various hexachlorocyclohex-
anes (HCHs) in water, obtained after on-line isooctane extraction: 1, -HCH; 2, -HCH; 3, -
HCH; 4, -HCH. (b) Gas chromatogram obtained for a reference blank (distilled water) after
the same on-line extraction treatment. Reprinted from Journal of High Resolution
Chromatography, 13, E. C. Goosens et al., ‘Determination of hexachlorocyclohexanes in
ground water by coupled liquid–liquid extraction and capillary gas chromatography’,
pp. 438–441, 1990, with permission from Wiley-VCH.

thus leading to the loss of the more volatile components. If more-volatile compounds
have to be analysed, then transfer from the LC unit to the GC unit is best achieved by
using retention gap techniques. Due to the solvent effects explained above, this tech-
nique allows the analysis of compounds which elute immediately after the solvent
peak. The main drawback of this approach is that it is restricted to the transfer of
small fractions, due to the limited capacity of the uncoated precolumns. Larger frac-
tions can be transferred by partial concurrent evaporation, which still retains the
advantages of the retention gap technique. Indirect injection of water-containing elu-
ents seems to be the appropriate choice for the analysis of such samples (LLE, SPE
and OTT). However, direct injection of water via a vaporizer chamber/precolumn
solvent split/gas discharge interface seems to be a promising technique for transfer-
ing reversed-phase eluents.