Influence of Temperature on the Enantioselectivity of Koga Tetraamines on Amylose Chiral Stationary Phases

Enantioseparation is largely based on the formation of transitional complexes, the solvation species, the stationary phase configurations or the diastereomeric complexes formed by analytes and the chiral stationary phase. Temperature and the chemical nature and composition of the eluent play significant roles during that process. In this study; unique temperature-induced behaviors were observed during the enantioseparation of Koga tetraamines, also known as Koga bases, on polysaccharide chiral stationary phases, in which van’t Hoff plots were acquired over a temperature range of 10 °C to 40 °C with 5 °C increments. Koga bases were eluted by a mixture of methanol and 2-propanol with 0.03% triethylamine as a modifier. The van’t Hoff plots are linear in the case of eluent containing equal volumes of methanol and 2-propanol. Increasing 2-propanol concentration from 50% to 85% in volume led to non-linear van’t Hoff plots over the entire temperature range studied. Examination of the individual non-linear plots revealed two linear regions of 10 °C–20 °C and 20 °C–40 °C. Transition from one linear region to the other at 20 °C indicates alterations of chiral stationary phase conformation and/or enantioseparation mechanism as a result of temperature changes.