Unusual Temperature-Induced Retention Behavior of Constrained β-Amino Acid Enantiomers on the Zwitterionic Chiral Stationary Phases ZWIX(+) and ZWIX(-)

The effects of temperature on the chiral recognition of cyclic beta-amino acid enantiomers on zwitterionic [Chiralpak ZWIX(+) and ZWIX(-)] chiral stationary phases were investigated. Experiments were performed at different mobile phase compositions and under 10 degrees C column temperature increments in the temperature range 10-50 degrees C. Apparent thermodynamic parameters and T-iso values were calculated from plots of ln k and ln a versus 1/T, respectively. Unusual temperature behavior was observed, especially on the ZWIX(-) column, where the application of MeOH/MeCN (50/50 v/v) containing 25 mM triethylamine and 50 mM formic acid as mobile phase led to nonlinear van't Hoff plots and increasing retention time with increasing temperature. On both columns, both enthalpically and entropically driven separations were observed. (C) 2014 Wiley Periodicals, Inc.