Investigation of mass transfer phenomena in zwitterionic chiral stationary phases

Zwitterionic chiral stationary phases (CSPs) combine the applicability of both anion and cation exchangers parent chiral ions. For this reason, they can provide direct enantioseparation of a huge range of compounds including chiral acids, bases and zwitterions (such as amino acids and peptides). ZWIX(+) and ZWIX(-), based on a Cinchona alkaloid derived chiral selector, are among the most employed zwitterionic CSPs. The retention mechanism of zwitterions on these CSPs is rather complex and both the two charged sites of the solutes are simultaneously recognized by the charged sites of the chiral selector. In this work, mass transfer phenomena of these CSPs have been investigated at four different temperatures for the separation of mefloquine (an antimalarial agent) enantiomers. The two mefloquine peaks showed remarkably different shape and behavior. By means of peak parking experiments and a proper model of diffusion in porous media (in this case the parallel model was used), each contribution to band broadening was individually determined. In order to investigate the effect of the adsorption-desorption kinetics, the two peaks were analyzed by means of a microscopic-probabilistic approach to chromatography (stochastic theory). Important information about the heterogeneous adsorption-desorption process (i.e. the residence time of the second enantiomer on both enantioselective and nonselective sites, enthalpy and entropy of adsorption) were obtained starting from the peak shape.