Liquid adsorption chromatography with mixed mobile phases

Can the theory of gradient liquid chromatography be useful in solving practical problems?

Advances in the theory of gradient liquid chromatography and their practical impacts are reviewed. Theoretical models describing retention in reversed-phase, normal-phase and ion-exchange modes are compared. Main attention is focused on practically useful models described by two- or three-parameter equations fitting the experimental data in the range of mobile phase composition utilized for sample migration during gradient elution. The applications of theory for gradient method development, optimization and transfer are addressed. The origins and possibilities for overcoming possible pitfalls are discussed, including the effects of the instrumental dwell volume, uptake of mobile phase components on the column and size of the sample molecules. Special attention is focused on gradient separations of large molecules.

Effect of the organic modifier concentration on the retention in reversed-phase liquid chromatography I. General semi-thermodynamic treatment for adsorption and partition mechanisms

A semi-thermodynamic treatment is adopted to account for adsorption or partition of solute molecules from aqueous mobile phases on/in reversed-phase liquid chromatography stationary phases. The theoretical expressions of ln k' versus organic modifier content are tested against 10 data sets covering a variety of solute molecules. It is shown that the mean field approximation, adopted widely in ptevious studies, is marginally valid in aqueous mobile phases, especially in the presence of solute molecules, and the lattice model approximation, which is also used in relevant studies, is a poor approximation. Clear conclusions about the validity of either the adsorption or the partition model for the retention mechanism could not be drawn. The equations of the adsorption model describe all data sets absolutely satisfactorily and yield a physically reasonable picture about the behavior of modifier and solvent at the adsorbed layer. However, the high applicability of the adsorption model may not safely entail the validity of the adsorption mechanism at a molecular level, especially in the case of solutes with small and non-polar molecules, where our analysis gives strong indications about the validity of the partition mechanism. The next steps needed for the final elucidation of the retention mechanism in reversed-phase chromatographic columns are indicated.