Effects of temperature on retention of chiral compounds on a ristocetin A chiral stationary phase

The isocratic retention of enantiomers of chiral analytes, i.e. tryptophan, 1,2,3,4-tetrahydroisoquinoline and γ-butyrolactone analogs, was studied on a ristocetin A chiral stationary phase at different temperatures and with different mobile phase compositions, using the reversed-phase, polar-organic and normal-phase modes. By variation of the both mobile phase composition and the temperature, baseline separations could be achieved for these enantiomers. The retention factors and selectivity factors for the enantiomers of all investigated compounds decreased with increasing temperature. The natural logarithms of the retention factors (ln k) of the investigated compounds depended linearly on the inverse of temperature (1/T). van’t Hoff plots afforded thermodynamic parameters, such as the apparent change in enthalpy (ΔHo), the apparent change in entropy (ΔSo) and the apparent change in Gibbs free energy (ΔGo) for the transfer of analyte from the mobile to the stationary phase. The thermodynamic parameters (ΔHo, ΔSo and ΔGo) were calculated in order to promote an understanding of the thermodynamic driving forces for retention in this chromatographic system.