Supercritical fluids can be used as the mobile phase to separate analytes with chromatographic columns. As in supercritical-fluid extraction (SFE), supercritical fluids can have solvating powers similar to organic solvents, but with higher diffusivities, lower viscosity, and lower surface tension. The lower viscosity allows higher flow rates compared to liquid chromatography, and the solvating power can be adjusting by changing the pressure. A major advantage of supercritical-fluid chromatography (SFC) is that it offers the advantage of liquid-like solubility, with the capability to use a non-selective gas-phase detector such as flame ionization detector. Analytes that can not be vaporized for analysis by gas chromatography, yet have no functional groups for sensitive detection with the usual liquid chromatography detectors, can be separated and detected using SFC.
A supercritical fluid chromatograph consists of a gas supply, usually CO2, a pump, the column in a thermostat-controlled oven, a restrictor to maintain the high pressure in the column, and a detector. The column is usually a capillary GC column, but packed LC columns can also be used. The FID is the most common detector, but other GC or LC detectors can also be used.