Some aspects of capillary supercritical fluid chromatography

Based on dimensionless parameters, equations are given which compare speed of analysis, pressure drop and plates per bar pressure drop for capillary GC, SFC with capillary columns and SFC with packed columns. With respect to speed of analysis, contemporary capillary SFC cannot compete with packed-column SFC. A further decrease in capillary column diameter will be needed to reach this goal. Decreasing the column diameter of capillary columns at the same time decreases the sample capacity and also places extremely stringent requirements on the speed of sample introduction and on the time constants of the detection systems. If the allowable pressure drop is a serious factor, as is expected from theory, open-tubular columns are to be preferred in terms of the maximum obtainable plate number. The effects of polar organic modifiers in capillary SFC are described and compared with those in packed columns. The introduction of modifier can cause a considerable reduction of the capacity factors in capillary SFC. Generally, the effects of modifiers in capillary SFC are not as large as those observed in packed columns. The interpretation of retention data is severely hindered by the unavailability of accurate density data for binary supercritical fluids. Three methods for the calculation of critical properties of mixed fluids are compared. The coupling of SFC with FT-IR detection is discussed. For capillary SFC, a conflicting situation arises in which the cell volume that can be allowed without loss of resolution is much lower than the minimum volume required for good spectroscopy. It is shown that the on-line coupling of capillary SFC with IR spectroscopy will always be a compromise between chromatographic and spectroscopic requirements. For packed columns restrictions placed on the detector-cell volume are less stringent. Here, however, the on-line approach is severely hampered by the necessity to add modifiers to the mobile phase for the analysis of (even midly) polar solutes.