Supercritical-fluid chromatography

(analytical chemistry) Any chemical separation technique using chromatography in which a supercritical fluid is used as the mobile phase.

Any separation technique in which a supercritical fluid is used as the mobile phase. For any fluid, a phase diagram can be constructed to show the regions of temperature and pressure at which gases and liquids, gases and solids, and liquids and solids can coexist. For the gas-liquid equilibrium, there is a certain temperature and pressure, known as the critical temperature and pressure, below which a gas and a liquid can coexist but above which only a single phase (known as a supercritical fluid) can form.

For example, the density of supercritical fluids is usually between 0.25 and 1.2 g/ml and is strongly pressure-dependent. Their solvent strength increases with density, so that molecules that are retained on the column can often be eluted simply by increasing the pressure under which the fluid is compressed.

Diffusion coefficients of solutes in supercritical fluids are tenfold greater than the corresponding values in liquid solvents (although about three orders of magnitude less than the corresponding values in gases). The high diffusivity of solutes in supercritical fluids decreases their resistance to mass transfer in a chromatographic column and hence allows separations to be made either very quickly or at high resolution. Molecules can be separated in a few minutes or less by using packed columns of the type used for high-performance liquid chromatography (HPLC).

Despite the relatively large number of separations made using packed-column supercritical-fluid chromatography that have been described since 1962, supercritical-fluid chromatography became popular only in the late 1980s with the commercial introduction of capillary (open-tubular) supercritical-fluid chromatographs. While supercritical-fluid chromatography may never be as widely used as gas chromatography or high-performance liquid chromatography, it provides a useful complement to these chromatographies. See also Chemical separation techniques; Chromatography; Gas chromatography; Liquid chromatography.