Effect of mobile-phase composition on pressure-induced shifts in solute retention for LC separations using β-cyclodextrin stationary phases

Abstract

Modest pressure (<350 bars) has a significant impact on reversed-phase liquid chromatographic retention using β-cyclodextrin stationary phase. Demonstrated here for separations of the positional isomers of nitrophenol and naphthol as well as the enantiomers of hexobarbital and mephobarbital, the effect of small pressure perturbations on solute capacity factor ranges from +11 to −14%. These changes in retention arise from pressure-induced shifts in solute equilibria and are used to estimate the change in molar volume upon solute binding with β-cyclodextrin (ΔVcomp). In this article, the effect of the mobile phase on pressure-induced shifts in solute retention is examined. Mobile-phase composition is observed to have a significant influence on the change in solute capacity factor with pressure and therefore on ΔVcomp. As the composition of water in the mobile phase increases, the change in partial molar volume of complexation is observed to become more positive for both positional isomer and chiral separations. Although these studies imply that changes in solvent composition have a general effect on the observed ΔVcomp and therefore on the change in pressure perturbation with solvent composition, it is clear that more studies are needed to establish the exact nature and breadth of this effect. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 647–652, 1998