Pressure-Induced Changes in Chiral Separations in Liquid Chromatography

Abstract

In this study, pressures less than 350 bar (<5100 psi) are shown to have a significant impact on enantioselective separations using β-cyclodextrin bonded phase. Although modest by the standards of pressure studies, this pressure range has been chosen to include those conditions that are important for the practice of liquid chromatography. Demonstrated here for both aqueous reversed-phase and polar−organic chiral separations, pressure affects all important figures of merit including capacity factor, chiral selectivity, and efficiency for a variety of enantiomeric solutes. Although the bulk properties of polar mobile phases are not significantly altered by these modest pressures, equilibrium complexation shifts with pressure may induce changes in retention and band broadening. As a result, equilibrium pressure perturbations in chromatographic separations can be directly related to changes in the partial molar volume during complexation (ΔVcomp). For the separations assessed here, enantiomeric complexation is shown to exhibit a wide range of volumetric behavior based on pressure-induced capacity factor changes from −20 to +12% for an increase of ∼300 bar. In addition, differences in ΔVcomp between enantiomers give rise to pressure-induced selectivity perturbations for three out of the eight compounds examined. Pressure is shown to have the greatest impact on chromatographic efficiency, with changes in theoretical plate heights of up to +240%. These pressure perturbations combined to yield a diminution in chiral resolution for elevated-pressure conditions. Utilizing this approach, pressure is clearly demonstrated to have important practical implications for chiral separations while simultaneously providing a valuable means for the fundamental assessment of enantioselective complexation.