Programmed two-stage flow controller for supercritical fluid chromatography

The flow rate in a capillary supercritical fluid chromatographic (SFC) column can be maintained near the optimum value of a Van Deemter plot throughout the entire pressure or density programme by use of a programmed two-stage flow controller. The second stage is a conventional linear restrictor, which is designed to provide less restriction than is customary in capillary SFC. The first stage consists of a length of deactivated fused silica of 7 µm i.d. held in a temperature-controlled environment. As the pressure in the column is increased during a pressure or density programme, the temperature of the 7 µm fused-silica tube can be varied to maintain a constant flow rate.