Effect of the feed injection method on band broadening in analytical supercritical fluid chromatography

The behavior of a novel type of SFC injector, the feed injector, was investigated. In SFC, the sample compounds are usually diluted in a solvent which has a higher elution strength than the mobile phase, which leads to solvent mismatch upon injection and evidently band broadening. The feed injector differs from standard injectors as the sample, contained in the sample needle or loop, is not switched in line with the mobile phase flow, but directly injected/added to the mobile phase flow (F). The subsequent mixing of sample and mobile phase flows inherently results in a dilution of the sample, thus reducing the solvent mismatch. However, for a given injection/feed flow rate F_feed, the total volume in which the sample is contained increases with a factor (F_feed + F)/F_feed. In addition, to ensure that all of the loaded sample is injected on the column, an additional overfeed volume (V_ov) needs to be injected after the sample plug. To better understand the effect of these operating parameters, a wide range of injection conditions was investigated by varying the F_feed/F-ratio, V_ov, overfeed solvent etc. under SFC conditions. It was found that an optimal F_feed/F exists which is independent of F and decreases with increasing solvent strength dependency of the sample compound. Decreasing V_ov has a beneficial effect on peak dispersion but can only be varied over a certain range to ensure the full injection of the loaded sample. On the other hand, it was found that a much larger gain could be made by switching the overfeed solvent to one more compatible with the CO₂-based mobile phase. Further reduction of the band broadening could be achieved by applying partial sample injections.