Chiral stationary phase optimized selectivity supercritical fluid chromatography: A strategy for the separation of mixtures of chiral isomers

Abstract

The possibilities of chiral Stationary-Phase Optimized Selectivity Supercritical Fluid Chromatography (SOS-SFC) are studied for improving separations of mixtures of enantiomers on coupled chiral polysaccharide columns. This new chiral methodology in SFC requires a limited number of preliminary analyses, while it allows for the prediction of the separation profiles on all possible combinations of the available chiral columns. This is followed by a facile selection of the optimal column combination, which is then assembled and tested. The approach was investigated with a carbon dioxide/ethanol based mobile phases with and without additives on mixtures of pharmaceuticals comprising 4 and 5 racemates. Adjustment of the applied back pressure in SFC, leading to improved correspondence in mobile phase density between the individual and coupled column experiments, allowed for minor improvements in predictive strength except for acidic solutes in non-buffered media. Changing the order of the columns proved not to affect the quality of the separations, but did lead to some overall shifts in retention times. The best correspondence between the predictions and the experiments were obtained when commonly used additives such as isopropylamine and trifluoroacetic acid were used. Predictive errors ranged from 0.36 and 19.43% and from 1.15 and 5.03% in the experiments without and with additives, respectively. Elution profiles were sufficiently well predicted to allow implementation of coupled column selection tool in chiral SFC.