Investigations on analyte losses in systems suited for large-volume on-column injections

Use of a large-volume injection system with a solvent vapour exit (SVE) requires optimisation. An appropriate strategy is to determine the evaporation rate by increasing the injection time at a fixed infection speed, injection temperature and head pressure. When measuring the flow rate in the carrier gas supply line to the on-column injector, optimisation can be very rapid: some five injections of pure solvent will be sufficient. When working under partially concurrent solvent evaporation conditions, loss of volatiles is often observed if no retaining precolumn is used between the retention gap and the SVE, To investigate the requirements (length and stationary phase) of the retaining precolumn, C8 - C18 n-alkanes in n-hexane were used. The minimum length of the retaining precolumn (0.32 mm diameter) needed to prevent substantial losses of volatiles was 2 m. Experiments with retaining precolumns with and without stationary phase gave identical results. This shows that there is no need to coat the capillary as it only acts as a restrictor.