It’s been over a month since my last post on Concurrent Solvent Recondensation – Large Volume Splitless Injection (CSR_LVSI), so I thought I would cover solvent focusing, a key element of the technique, today.
The CSR-LVSI solvent peak widths are minutes larger than those seen in a typical 1 µL injection. Figure 1 shows the difference in solvent peak width between a 10 µL and 50 µL injection of dichloromethane boiled off when the oven is raised to 50 °C during the solvent focusing step of the CSR-LVSI mechanism, following the solvent recondensation step which occurs at 35 °C (well below the pressure adjusted boiling point of dichloromethane). In order to optimize peak shape and column capacity, it is important to time the beginning of the optimal oven heating rate with the end of the CSR-LVSI solvent focusing step, when the narrow band of focused analytes enter the analytical column. We determined this starting time semi-experimentally, by measuring the time the solvent peak ends (shown in Figure 1) and subtracting the Total Second Dimension Void Time (sec) which is calculated by the Leco GCxGC column calculator (Figure 2). The 50 °C oven hold time is then adjusted so that the sum of the first and second ramp times is equal to this number (8.9 min in the case of the 50 µL dichloromethane injection).
This way, even very volatile components, such as THF, are well separated from the solvent peak, and every chromatographic separation is independent of the injection volume (allowing even 250 µL injections of volatile analytes). To demonstrate this, we used a 50 µL injection and 10 µL injection of nitrosamines in dichloromethane. Each injection volume put 5 ng of each analyte on column (Figure 3). Note that in Figure 1, the 50 µL dichloromethane solvent peak is 5 minutes wider than that of the 10 µL injection. This same time difference holds up for every peak in the two chromatograms in Figure 3. The first peak, THF, elutes at 5 and 10 minutes respectively on the 10 µL and 50 µL injection runs while the last peak, N-nitroso-di-n-butylamine, elutes just after 13.5 min on the 10 µL run and 18.5 min on the 50 µL run.