Enantioselectivity of basic analytes in CZE enantioseparation under reversed-polarity mode using sulfated β-cyclodextrins as chiral selectors: An unusual temperature effect

Temperature effects on the enantioselectivity of basic analytes in CZE enantioseparation were studied under reversed‐polarity mode using randomly sulfate‐substituted β‐CDs (MI‐S‐β‐CD) as chiral seletors. Two catecholamines (epinephrine and isoproterenol) and two structurally related compounds (octopamine and norephedrine) were selected as test compounds in an electrophoretic system at low pH. The mobility differences between the (+)‐enantiomers and the (−)‐enantiomers of the two catecholamines and dopamine at 40°C are greater than those at 25°C with MI‐S‐β‐CD, even at a concentration as low as 0.3% w/v. Thus the enantioselectivity of these three basic analytes increases with increasing temperature. This phenomenon results from the inequality of the temperature effect on the mobility of the two enantiomers. In contrast, norephedrine behaves differently. The (+)‐enantiomers of these basic analytes were found to migrate faster than the (−)‐enantiomers. Consequently, the unusual temperature effect on the enantioselectivity can be observed when the mobility difference of the (+)‐enantiomer between 40 and 25°C is greater than that of the (−)‐enantiomer using MI‐S‐β‐CD at a concentration greater than about 0.7% w/v for enantioseparation of isoproterenol, 0.4% w/v for epinephrine, and 0.3% w/v for octopamine. This unusual temperature effect offers the advantages to enhance enantioselectivity, to improve enantioseparation, and to reduce migration times.