Flow rate control in pressure‐programmed capillary supercritical fluid chromatography

A versatile and simple system is described that allows variation of the column flow rate in open‐tubular capillary supercritical fluid chromatography using both on‐column and postcolumn detection. The system is based on column‐effluent splitting in a low‐dead‐volume T piece at the column exit just before detection. The flow is split into two streams. One stream is fed to a flame ionization detector, and the second is used to control the overall mass flow through the column. Flow through linear restrictors is studied as a function of the pressure drop over the restrictor at ambient temperature. We derived a simple relationship from this study that quantifies the flow rate characteristics of the dual‐restrictor system as a function of the column pressure and the restrictor back pressure. The variable‐flow system was employed to control independently column pressure and mass flow rate in pressure‐programmed analysis. Resolution in flow‐controlled pressure‐programmed elution was shown to be superior to that achieved in fixed‐restrictor systems. Furthermore, the applicability of the system as a convenient means of measuring Van Deemter curves and diffusion coefficients is demonstrated.