Direct high-performance liquid chromatographic enantioseparation of secondary amino acids on Cinchona alkaloid-based chiral zwitterionic stationary phases. Unusual temperature behavior

• Separation of secondary amino acid analogs on Cinchona-alkaloid based CSPs.
• Mechanistic aspects of the chiral recognition process are discussed with respect to the structures of the analytes.
• Thermodynamic parameters were calculated at constant mobile phase compositions in the temperature range −5 to 55 °C.
• Unusual temperature dependence was observed on quinidine-based CSP: retention factor increased with increasing temperature.
• Both enthalpically and entropically controlled chiral separation were observed.

Two chiral stationary phases containing a quinine- or a quinidine-based zwitterionic ion-exchanger as chiral selector were applied for the enantioseparation of 27 unusual cyclic secondary α-amino acids. The effects of the nature and concentration of the bulk solvent, the acid and base additives, the structures of the analytes and temperature on the enantioresolution were investigated. To study the effects of temperature and to obtain thermodynamic parameters, experiments were carried out at constant mobile phase compositions in the temperature range −5 to 55 °C. The thermodynamic parameters indicated that in most cases the separations were enthalpy-driven, but some entropy-driven separations were also observed. The sequence of elution of the enantiomers was determined in most cases.