CO2 Supercritical Fluid Chromatography with Chiral Stationary Phases: A Promising Coupling for the Resolution of Various Racemates

The use of chiral stationary phases (CSPs) in supercritical fluid chromatography (SFC) with carbon dioxide is reviewed. The state-of-the-art for capillary and packed columns is discussed. However, because most of the initial reports are about packed columns, we more thoroughly investigate the resolution of optical isomers on various Pirkle-type CSPs (DNBPG, thio-DNBTyr-A or -E, DACH-DNB, etc.), cyclodextrin CSPs (Cyclobond), and polymer CSPs (ChiralCel OB and ChiralPak OT(+)). Several discrepancies in the LC and SFC behavior of these CSPs show that carbon dioxide-alcohol and hexane-alcohol mixtures are not interchangeable as mobile phases: solubilities of apolar compounds are lower in carbon dioxide than in hexane and the solvation state of both solutes and CSPs differs. For several applications, carbon dioxide is found to act like methylene chloride in regard to stereoselectivity. As a rule, the SFC-CSP coupling is a promising technique for the resolution of racemates: It usually leads to higher resolutions per unit of time than LC and sometimes allows new types of chiral separations.