Chapter 29 Supercritical Fluid Chromatography and Extraction

Introduction... Supercritical fluid chromatography (SFC) is a hybrid of gas and liquid chromatography that combines some of the best features of each. Supercritical fluid chromatography is of importance because it permits the separation and determination of a group of compounds that are not conveniently handled by either gas liquid or liquid chromatography.

(1) are either nonvolatile or thermally labile so that GC procedures are inapplicable (2) contain no functional groups that make possible detection by spectroscopic or electrochemical techniques employed in liquid chromatography. These compounds:

Properties of Supercritical Fluids... The critical temperature is the temperature above which a distinct liquid phase cannot exist, regardless of pressure. The vapor pressure of a substance at its critical temperature is its critical pressure.

Instruments and Operating Variables... The pressures and temperatures required for creating supercritical fluids lie within the operating limits of ordinary HPLC equipment. Thus, as shown in the figure next, instruments for SPC are similar in most regards to the instruments for HPLC.

Effects Of Pressure... Pressure changes in supercritical chromatography have a pronounced effect on the capacity factor k’. This effect is a consequence of the increase in density of mobile phase with increase in density of the mobile phase with increases in pressure.

Stationary Phases... Both open-tubular and packed columns are used for SFC although currently the former are favored. Open-tubular columns are similar to the fused-silica columns with internal coatings of bonded and crossed- linked siloxanes of various types.

Mobile Phases... The most widely used mobile phase for SPC is carbon dioxide. It is an excellent solvent for a variety of organic molecules. In addition, it transmits in the ultraviolet and is odorless, nontoxic, readily available, and remarkably inexpensive when compared with other chromatographic mobile phases.

Detectors... A major advantage of SFC over HPLC is that the flame ionization detector of gas chromatography can be employed. Mass spectrometers are also more easily adapted as detectors for SFC than HPLC.

Applications... SFC has been applied to a wide variety of including natural products, drugs, foods, pesticides and herbicides, surfactants, polymers and polymer additives, fossil fuels and explosives and propellants.

Supercritical Fluid Extraction... Often the analysis of complex materials requires as a preliminary step separation of the analyte or analytes form a sample matrix. Ideally, an analytical separation method should be rapid, simple and inexpensive; should give quantitative recovery of analytes without loss or degradation; should yield a solution of the analyte this is sufficiently concentrated to permit the final measurement to be made without the need for concentration; and should generate little or no laboratory wastes that have to be disposed of.

Instrument components include a fluid source, commonly a tank of carbon dioxide; a syringe pump having a pressure rating of at least 400 atm; a valve to control the flow of the critical fluid into a heated extraction cell having a capacity of a few ml; an exit valve leading to a flow restrictor that depressurizes the fluid and transfers it into a collection device.

ve/wavehp.html m