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

The change of the evaporation rate of the solvent during injection and evap oration 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 vapou r exit (SVE) and can be described by a mathematical model. Four procedures for the optimization of the injection conditions were compared. It was foun d that different procedures often yield different evaporation rates, which may also depend on the injection speeds used during optimization. For optim ization of a new set-up, i.e. if little is known about the optimal injectio n conditions, the evaporation rate should be determined by increasing the i njection time at a fixed injection speed, injection temperature and head pr essure; subsequently, an appropriate injection speed can be calculated. If a mere re-optimization is required as e.g, after the exchange of the retent ion gap, adjusting the evaporation rate to the injection speed by varying t he injection temperature at a constant injection speed is the preferred pro cedure. With both methods, optimization can be achieved by means of 2-5 inj ections 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-m ass spectrometry. Closing the SVE at the very end of solvent evaporation re sults in a considerable increase of the capacity of the retention gap compa red to closing the SVE before all solvent is evaporated. (C) 1999 Elsevier Science B.V. All rights reserved.