Pressure-regulating fluid interface and phase behavior considerations in the coupling of packed-column supercritical fluid chromatography with low-pressure detectors

Packed-column supercritical fluid chromatography is usually performed with flow control on the upstream side of the column and pressure control on the downstream side. This arrangement is compatible with detectors that can be operated at column-outlet pressure and placed between the column outlet and a pressure-regulating device. However, mass transfer problems may arise when low-pressure detectors like a mass spectrometer or evaporative-light-scattering detector are used with downstream pressure regulators or programmable nozzles. These problems can be avoided by replacing the regulator or nozzle with a tee delivering a pressure-regulating make-up fluid from a pressure-controlled pump. Parameters are chosen to avoid the liquid-vapor two-phase regions of the phase diagram for the resulting mixture. We demonstrate this approach and show how to use phase diagrams and their pressure-temperature projections for setting operating conditions. (C) 1998 Elsevier Science B.V. All rights reserved.