Stationary phase-related investigations of quinine-based zwitterionic chiral stationary phases operated in anion-, cation-, and zwitterion-exchange modes

The concept of recently introduced Cinchona alkaloid-type zwitterionic chiral stationary phases (CSPs) is based on fusing key cation- and anion-exchange (CX, AX) moieties in one single low-molecular mass chiral selector (SO) with the resulting CSPs allowing enantiomer separations of a wide range of chiral ionizable analytes comprising acids, bases, and zwitterionic compounds. Herein, we report principal, systematic investigations of the ion-exchange-type retention mechanisms available with the novel zwitterionic CSPs in nonaqueous polar organic mode. Typical CX and AX processes, corresponding to the parent single ion exchangers, are confirmed also for zwitterionic CSPs. Also the mechanism leading to recognition and retention of zwitterions was found to be ion exchange mediated in a zwitterion-exchange (ZX) mode. In both AX and CX modes the additional ionizable group within the SO besides the site responsible for the respective ion-exchange process could be characterized as an intramolecular counterion (IMCI) that effectively participates in the ion-exchange equilibria and thus, contributes to solute elution. In the ZX mode both oppositely charged groups of the zwitterionic SO were found not only to be the sites for simultaneous ion pairing with the analyte but also functioned as IMCIs at the same time. The main practical consequences of the IMCI feature were significant reduction of the amounts and even elimination of acidic and basic additives required in the eluent systems to afford analyte elution while still providing faster analysis than the parent single ion-exchanger-type CSPs. The set of ten structurally different zwitterionic CSPs employed in this study facilitated the establishment of correlations between chromatographic behavior of the CSPs with particular SO elements, thereby supporting the understanding of the working principles of these novel packing materials on a molecular level.