Role of the retaining precolumn in large-volume on-column injections of volatiles to gas chromatography

Abstract

In the present study the retaining precolumn, which is commonly used in a set-up for large-volume on-column injections, or when solid-phase extraction (SPE) or liquid chromatography is coupled to gas chromatography (GC), was removed after varying its length from the standard length of 3 m down to zero. A dramatic increase of the evaporation rate of the injected organic solvent was obtained from a typical value of 100 μl/min up to 300 μl/min. The increased evaporation rate allowed (i) injection of a larger volume in the same retention gap, (ii) faster injection/transfer of the organic solvent and (iii) reduction of the transfer temperature. As volatile compounds under partially concurrent solvent evaporation conditions are easily lost once the organic solvent has been removed via a solvent-vapour exit (SVE), the parameters for large-volume injection, i.e. the evaporation rate and injection speed, were optimised using accurate measurements of the real flow-rate of the carrier gas into the GC system. All these options have been evaluated over the last 4 years. In order to demonstrate that omitting the retaining precolumn had no effect on the application range of the on-column interface, analytes as volatile as benzene were injected into GC–MS using 50–200 μl of n-pentane solutions. Contaminants were extracted from river water and wastewater into n-pentane using in-vial liquid–liquid extraction. The detection limits for benzene, toluene, ethylbenzene and m-xylene were ∼10 ng/l. To obtain optimum results the SVE had to be closed 1 s before the end of evaporation. Several brands of n-pentane were analysed to check for the presence of benzene. Most of them contained interfering compounds and benzene at the low μg/l level and therefore had to be cleaned by means of column chromatography. As another example C 8–C 17 alkylphenones were extracted from wastewater with n-hexane. Detection limits were 10–40 ng/l.