Liquid Chromatographic Separations with Mobile-Phase Additives: Influence of Pressure on Coupled Equilibria

摘要

On the basis of equilibrium thermodynamics, pressure can cause a shift in equilibrium for any interaction that exhibits a change in partial molar volume. This shift in equilibrium can be observed in liquid chromatography as a pressure-dependent shift in solute retention. In this paper, the impact of pressure on liquid chromatographic separations i’~ith mobile-phase additives is examined from both theoretical and experimental perspectives. The theo-retical development for coupled-equilibria separations shown here is general and can be applied to any separa-tion using mobile-phase additives. Predictions indicate that the coupled nature of these equilibria leads to pressure-induced perturbations in partitioning and com-plexation that can either compete with or complement one another. Using positional isomers and enantiomers as model solutes, experimental retention observations are fully consistent with these predictions, showing the dimi-nution of individual pressure effects for competing cases and enhanced pressure effects for complementary cases. When pressure-induced changes in capacity or retention factor differ between individual solutes, changes in solute selectivity are predicted and observed. Using a C18 stationary phase with /)-cyclodexlrin as the mobile-phase additive, solutes studied here exhibit changes in selectivity ranging from —7 to ±10% for a change in average pressure of -‘-215 bar. Perhaps the most dramatic change in selectivity is observed for the separation of positional isomers where pressure-induced changes in selectivity actually reverse solute elution order.