Optimization of large-volume on-column injection conditions in gas chromatography by monitoring the actual carrier gas flow

Abstract

The change of the evaporation rate of the solvent during injection and evaporation is the most critical aspect during optimization of large-volume on-column injection conditions in gas chromatography. The change is caused by the pressure drop along the retention gap when using an early solvent vapour exit (SVE) and can be described by a mathematical model. Four procedures for the optimization of the injection conditions were compared. It was found that different procedures often yield different evaporation rates, which may also depend on the injection speeds used during optimization. For optimization of a new set-up, i.e. if little is known about the optimal injection conditions, the evaporation rate should be determined by increasing the injection time at a fixed injection speed, injection temperature and head pressure; subsequently, an appropriate injection speed can be calculated. If a mere re-optimization is required as e.g. after the exchange of the retention gap, adjusting the evaporation rate to the injection speed by varying the injection temperature at a constant injection speed is the preferred procedure. With both methods, optimization can be achieved by means of 2–5 injections of pure solvent and monitoring the helium carrier gas flow. That is, optimization of the injection conditions takes less than 1 h. When using this strategy, analytes as volatile as monochlorobenzene can be determined in aqueous samples by in-vial liquid–liquid extraction–gas chromatography–mass spectrometry. Closing the SVE at the very end of solvent evaporation results in a considerable increase of the capacity of the retention gap compared to closing the SVE before all solvent is evaporated.