REVERSED-PHASE LIQUID-CHROMATOGRAPHY COUPLED ONLINE WITH CAPILLARY GAS-CHROMATOGRAPHY .2. USE OF A SOLVENT VAPOR EXIT TO INCREASE INTRODUCTION VOLUMES AND INTRODUCTION RATES INTO THE GAS-CHROMATOGRAPH

A large-volume introduction system for capillary gas chromatography (GC) is described which can be coupled on-line with reversed-phase liquid chromatography (LC) using 2 mm i.d. LC columns. An on-column interface with a special retention gap, coated with a very thin film (0.02 mum) of Carbowax was used. Insertion of a solvent vapor exit before the analytical gas chromatography column allowed the solvent evaporation rate for an azeotropic acetonitrile/water mixture (84:16, v/v) to increase from 20 to 175 muL min-1 at 91-degrees-C and 150 kPa inlet pressure. A volume of 200 muL of this mixture containing phenanthrene and the potential basic drug eltoprazine, could be injected into the retention gap at introduction rates of up to 200 muL min-1, under partially concurrent solvent evaporation conditions. Repeatability (CV = 2.4%) and linearity (at least 10-1000 ng mL-1) were satisfactory. The Carbowax-coated retention gap has a relatively large retention power for the polar solute. This means that the solvent can be completely evaporated through the vapor exit without loss of analyte. With apolar solutes, losses will occur if evaporation is unduly prolonged. In order to allow the use of a nitrogen-phosphorus detector, a special pressure-balanced carrier gas circuit was designed and tested for total solvent-free performance of the detector. With this set-up, 150 muL injections of eltoprazine in the acetonitrile/water azeotrope were made at an introduction rate of 200 muL min-1. No traces of acetonitrile were detected, while analyte detectability was 20-fold higher than with the FID. Finally, the practicability of reversed-phase LC-GC was demonstrated by on-line heart-cutting of the phenanthrene-containing fraction from a 2 mm i.d. LC column using acetonitrile/water (84:16, v/v) as eluent, and analyzing it by capillary GC-FID.