Supercritical Fluid Chromatography

Supercritical Fluid Chromatography

Despite their importance, gas chromatography and liquid chromatography cannot be used to separate and analyze all types of samples. Gas chromatography, particu- larly when using capillary columns, provides for rapid separations with excellent resolution. Its application, however, is limited to volatile analytes or those analytes that can be made volatile by a suitable derivatization. Liquid chromatography can be used to separate a wider array of solutes; however, the most commonly used de- tectors (UV, fluorescence, and electrochemical) do not respond as universally as the flame ionization detector commonly used in gas chromatography.

Supercritical fluid chromatography (SFC) provides a useful alternative to gas chromatography and liquid chromatography for some samples. The mobile phase in supercritical fluid chromatography is a gas held at a temperature and pressure ex- ceeding its critical point (Figure 12.37). Under these conditions the mobile phase is neither a gas nor a liquid. Instead, the mobile phase is a supercritical fluid whose properties are intermediate between those of a gas and a liquid (Table 12.6). Specifi- cally, supercritical fluids have viscosities that are similar to those of gases, which means that they can move through either capillary or packed columns without the need for the high pressures encountered in HPLC. Analysis time and resolution, al- though not as good as in GC, are usually better than that obtainable with conven- tional HPLC. The density of a supercritical fluid, however, is much closer to that of a liquid, accounting for its ability to function as a solvent. The mobile phase in SFC, therefore, behaves more like the liquid mobile phase in HPLC than the gaseous mo- bile phase in GC.

The most common mobile phase for supercritical fluid chromatography is CO₂. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are rela- tively easy to achieve and maintain. Although supercritical CO₂ is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase’s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7.

The instrumentation necessary for supercritical fluid chromatography is essentially the same as that for a stan- dard GC or HPLC. The only important addition is the need for a pressure restrictor to maintain the critical pressure. Gradient elutions, similar to those in HPLC, are accom- plished by changing the applied pressure over time. The re- sulting change in the density of the mobile phase affects its solvent strength. Detection can be accomplished using stan- dard GC detectors or HPLC detectors.

Supercritical fluid chromatography has found many applications in the analysis of polymers, fossil fuels, waxes, drugs, and food products. Its application in the analysis of triglycerides is shown in Figure 12.38.