Monitoring the actual carrier gas flow during large-volume on-column injections in gas chromatography as a means to automate closure of the solvent vapour exit

Abstract

Monitoring of the helium flow into a gas chromatograph (GC) by means of an electronic flow meter has been used to optimize and control large-volume on-column injections. The nature of the observed carrier gas flow-rate profiles is discussed in some detail. A rather strong dependence of the evaporation rate on the injection speed was found for injections into a 0.32 mm I.D. retention gap, which can be attributed to the pressure drop along the retention gap when using a solvent vapour exit (SVE). The variation of the evaporation rate with the injection speed was found to be less critical for a 0.53 mm I.D. retention gap. The carrier gas flow-rate profile during the actual injection was used to effect the automated closure of the SVE precisely at the end of the evaporation process. Retention gaps of 0.53 mm I.D. were preferred over 0.32 mm I.D. retention gaps, as 0.53 mm I.D. retention gaps allowed a clearer detection of the end of the evaporation process. Compared with the conventional procedure which involves closure of the SVE at a predetermined time, the present approach is more robust and hardly any optimization is required; it did not cause losses of volatile analytes. The procedure considerably simplifies the use of large-volume on-column injections. Large-volume injections of alkanes were used to study the potential of the large-volume injection–GC system.