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.