Chapter 33Supercritical-Fluid Chromatography

Supercritical-fluid chromatography (SFC), in

which the mobile phase is a supercritical fluid, is

a hybrid of gas and liquid chromatography that

combines some of the best features of each. For

certain applications, it appears to be clearly

superior to both gas-liquid and high-

performance liquid chromatography.

Important Properties of Supercritical Fluids

A supercritical fluid is formed whenever a substance is heated above its critical temperature. At the critical temperature, a substance can no longer be condensed into its liquid state through the application of pressure.

The physical properties of a substance in the supercritical-fluid state can be remarkably different from the same properties in either the liquid or the gaseous state.

Instrumentation and Operating VariablesInstruments for supercritical-fluid chromatography are similar in design to high-performance liquid chromatography.

ColumnsBoth packed columns and open tubular columns are used in supercritical fluid chromatography. Packed columns have the advantages of greater efficiency per unit time and the capability of handling larger sample volumes. Packed columns for SFC can be much longer than those for HPLC, providing well over 100,000 plates. Because of the low viscosity of supercritical media, columns can be much longer than those used in liquid chromatography, and column lengths of 10 to 20 m and inside diameters of 50 or 100 m are common.

Mobile PhasesThe most widely used mobile phase for supercritical-fluid chromatography is carbon dioxide. It is an excellent solvent for a variety of nonpolar organic molecules. In addition, it transmits in the ultraviolet and is odorless, nontoxic, readily available, and remarkably inexpensive relative to other chromatographic solvents. Its critical temperature of 31oC and its pressure of 73 atm at the critical temperature permit a wide selection of temperatures and pressures without exceeding the operating limits of modern high-performance liquid chromatographic equipment.

Detectors

A major advantage of supercritical-fluid chromatography is that the sensitive and universal detectors of gas-liquid chromatography are applicable to this technique as well. For example, the convenient flame ionization detector of gas-liquid chromatography can be applied by simply allowing the supercritical carrier to expand through a restrictor and into a hydrogen flame, where ions formed from the analytes are collected at biased electrodes, giving rise to an electrical current.

Supercritical-Fluid Chromatography versus Other Column Methods

Several physical properties of supercritical fluids are intermediate between the properties of gases and liquids. As a consequence, this new type of chromatography combines some of the characteristics of both gas and liquid chromatography. Thus, like gas chromatography, supercritical-fluid chromatography is inherently faster than liquid chromatography because of the lower viscosity and higher diffusion rates in the mobile phase.

Applications

It is applicable to a class of compounds that is not readily amenable to either gas-liquid or liquid chromatography. These compounds include species that are nonvolatile or thermally unstable and, in addition, contain no chromophoric groups that can be used for photometric detection. Separation of these compounds is possible with supercritical-fluid chromatography at temperatures below 100 oC; furthermore, detection is readily carried out by means of the highly sensitive flame ionization detector.