Effects of Hexane in Supercritical Fluid Chromatography for the Separation of Enantiomers.

Abstract

Supercritical fluid chromatography (SFC), operated in conventional mode, is normally recognized as normal phase chromatography, and uses a solvent combination of supercritical CO2 and alcohols to separate compounds. Hexane, a commonly used solvent in normal phase liquid chromatography (NP-LC), is rarely used in SFC and, in some cases, is added to the organic modifiers to increase liquid content in order to achieve better efficiency in preparative SFC for poorly retained compounds. Although hexane is believed to have similar solvent strength to that of supercritical CO2, its effects on the enantioseparation in SFC is largely unknown. To understand the chromatographic effects of an apolar solvent, such as hexane in SFC, we compared the chromatographic behaviors of 35 chiral compounds using a parallel SFC method under traditional SFC mode of only “pure” alcohol-CO2 to that of hexane-assisted SFC (HA-SFC), which uses mixtures of alcohol and hexane (as cosolvents) and CO2. We observed that, in some cases, hexane behaves just like supercritical CO2, where replacement of a portion of CO2 with hexane does not significantly change retention times or resolution of the peaks. In many cases, however, addition of hexane in mobile phases does affect chromatographic behavior of one or both enantiomers. Such effects might provide opportunities for separation of some enantiomers.