Modelling of the competitive isotherms and the chromatographic separation of two enantiomers

Abstract

Using frontal analysis, we measured the adsorption isotherms of each Tröger's base enantiomers on microcrystalline cellulose triacetate, with ethanol as solvent. The isotherm of the first eluted (−)- enantiomer can be described satisfactorily by the Langmuir equation. The isotherm of the (+)-enantiomer has an inflection point and can be modelled by a quadratic isotherm equation. As a consequence, both the retention time of the (+)-enantiomer and the resolution of a racemic mixture increase with increasing concentration in a certain range. Using the pure enantiomer isotherms and the equilibrium-dispersive model, we could calculate the elution profiles of racemic mixtures of the two enantiomers. At low sample sizes, the bands are well resolved, the competition between the two enentiomers can be neglected in the calculations, and satisfactory band profiles are obtained for the racemic mixture. At high concentrations, the ideal adsorbed solution (IAS) theory was applied to predict the competitive isotherms. Experimental data on the separation of racemic mixtures at high concentrations, including the individual band profiles, are well represented by the results of calculations made with these isotherms.