EFFECT OF MOBILE-PHASE COMPOSITION ON PRESSURE-INDUCED SHIFTS IN SOLUTE RETENTION FOR LC SEPARATIONS USING BETA-CYCLODEXTRIN STATIONARY PHASES

Modest pressure (< 350 bars) has a significant impact on reversed-phas e liquid chromatographic retention using beta-cyclodextrin stationary phase. Demonstrated here for separations of the positional isomers of nitrophenol and naphthol as well as the enantiomers of hexobarbital an d mephobarbital, the effect of small pressure perturbations on solute capacity factor ranges from +11 to -14%. These changes in retention ar ise from pressure-induced shifts in solute equilibria and are used to estimate the change in molar volume upon solute binding with beta-cycl odextrin (Delta V-comp). In this article, the effect of the mobile pha se on pressure-induced shifts in solute retention is examined. Mobile- phase composition is observed to have a significant influence on the c hange in solute capacity factor with pressure and therefore on Delta V -comp. As the composition of water in the mobile phase increases, the change in partial molar volume of complexation is observed to become m ore positive for both positional isomer and chiral separations. Althou gh these studies imply that changes in solvent composition have a gene ral effect on the observed Delta V-comp 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. (C) 1998 John Wiley & Sons, Inc.