Improving injection precision in packed-column supercritical fluid chromatography

It is often observed that peak area precision in packed column supercritical fluid chromatography (pSFC) is rather poor when compared to values obtained in HPLC. Since the valves used for sample introduction in both techniques are identical, it is reasonable to expect that injection precision should be similar, especially when full-loop injections are performed. We found that in pSFC the evaporation of the mobile phase from the loop, when decompressed just prior to sample loading, contributes strongly to injection problems. However, these problems can be avoided if the injection valve, sample loop, and associated port tubes are entirely filled with liquid before the sample solution is loaded into the sample loop. By being mindful of the sequence of events that occurs during a full-loop injection, and by taking into account both the compressible nature and volatility of pSFC mobile phases and the geometry of the injection device, injection precision in pSFC can be comparable to that observed in HPLC.