Preparative Supercritical Fluid Chromatography

Supercritical Suid chromatography (SFC) on a preparative scale is of interest because of its advantages over high performance liquid chromatography (HPLC). The high diffusivity and low viscosity of supercritical Suids allow rapid separation. The solvent strength can easily be controlled by changing pressure and/or temperature, so that mobile phases with a Rxed composition can be used to separate many types of solute. Furthermore, many supercritical Suids are volatile or gaseous under normal ambient conditions, so that solutes can easily be recovered from the collected fractions by depressurization. Preparative-scale SFC is now being used to separate high-value materials where preparative HPLC can be difRcult to use. Peaks are narrower in SFC and for these difRcult separations, very little overloading can be done to take advantage of this. Consequently, the maximum amount of material obtained in a run is of the order of 100 mg in preparative SFC, compared with the much larger amounts which are sometimes obtainable in preparative HPLC. Large-scale SFC systems can either be built in the laboratory or purchased commercially. The unique physicochemical properties of supercritical Suids have convinced many workers that preparative SFC might be useful and relatively simple compared with preparative gas chromatography (GC), which is unsuitable for involatile and thermolabile compounds, and preparative liquid chromatography (LC), in which fraction-eluent separation can be problematic. Preparative SFC is based on the following steps: periodic injection of the feed into a continuous Sow of eluent, chromatographic separation due to selective interactions of the components of the sample with both the eluent and the stationary phase, detection at the column outlet, fraction collection, separation of the fractionated compounds from the eluent, further puriRcation of the compounds and, optionally, recycling the eluent (Figure 1). Preparative-scale SFC can be divided into two types: small-scale and large-scale. Small-scale can be deRned as the isolation of milligrams to a gram of pure product for, say, structure analysis. Bench-scale equipment derived from common analytical apparatus can be used with the adoption of nondestructive detection, fraction collection and eluent removal. However, for industrial-scale production (1 g h 1 to 1 kg h ), large-scale equipment has to be used, posing different problems, even if the same concept is applied. In preparative SFC, most studies have been devoted to the former aspect.