Liquid chromatographic separations with mobile phase additives: Influence of pressure on coupled equilibria

On the basis of equilibrium thermodynamics, pressure can cause a shift in e quilibrium for any interaction that exhibits a change in partial molar volu me. 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 with mobile-phase additives is examined from both theoretical and experimental perspectives. The theor etical development for coupled-equilibria separations shown here is general and can be applied to any separation using mobile-phase additives. Predict ions indicate that the coupled nature of these equilibria leads to pressure -induced perturbations in partitioning and complexation that can either com pete with or complement one another. Using positional isomers and enantiome rs as model solutes, experimental retention observations are fully consiste nt with these predictions, showing the diminution of individual pressure ef fects for competing cases and enhanced pressure effects for complementary c ases. 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 P-cyclodextrin as the mobile-p hase additive, solutes studied here exhibit changes in selectivity ranging from -7 to +10% for a change in average pressure of similar to 215 bar. Per haps the most dramatic change in selectivity is observed for the separation of positional isomers where pressure-induced changes in selectivity actual ly reverse solute elution order.