Page 41 - Multidimensional Chromatography

Coupled HPLC with HRGC 31

Figure 2.10 Schematic representation of a vaporizing chamber/precolumn solvent split/gas
discharge interface, where the vaporizer is packed and heated at a suitable temperature for
solvent evaporation. The vapour exit can be positioned at the end of the retention gap.

2.5 INDIRECT INTRODUCTION OF WATER

2.5.1 SOLID PHASE EXTRACTION

A method for coupling reversed phase LC to GC without introducing water into the
GC unit uses precolumns or solid-phase extraction (SPE) cartridges for the solutes
enrichment, followed by drying and extraction with organic solvent. Normally, a
large volume of water (1 ml) containing the organic compounds is passed through
the precolumn. The organic compounds are then eluted by an organic solvent into the
gas chromatograph, after flushing of the precolumn with nitrogen to remove the
residual water. This method, mostly used by Brinkman and co-workers was first
developed (27) by using an SPE cartridge (4 mm 1 mm id) inserted in a six-port
valve and packed with C-18 bonded silica.
Subsequently, Vreuls and co-workers (28–31) used a small LC column installed
on a 6- or 10-port valve. These columns usually had a length of 2–10 mm and an id of
1–4.6 mm (32). A large volume (1–10 ml) of aqueous sample containing the organic
compounds is then passed and concentrated on a polymer-packed column. After sev-
eral washings with water, the column must be dryed with nitrogen. The organic com-
pounds are then eluted by using an organic solvent such as hexane, or ethyl acetate
(e.g. 50 l) which is introduced on-line into the GC unit. Due to the large amount of
organic solvent introduced into the gas chromatograph, large-volume introduction
techniques are required, such as those discussed above. A typical on-line SPE–GC
set-up is shown in Figure 2.12 (32).
Drying carried out with purging of a precolumn is time consuming, and an inter-
esting alternative is the use of a drying cartridge positioned between the SPE and GC
units. This small cartridge should be reconditioned during the GC run by heating. In