Reversal of elution order for profen acid enantiomers in normal phase LC on Chiralpak AD

Enantiomeric separations of four 2-substituted propionic acid drugs and two related acids have been studied using normal phase liquid chromatography with amylose (tris 3,5-dimethylphenylcarbamate) coated on silica as support (Chiralpak AD). At standard conditions (i.e. flow-rate, 1.0 ml/min; column temperature, 30 °C) the elution order can be reversed when the polar alcohol modifier in isohexane, 2-propanol, is replaced by methanol/ethanol 2:1. This is the case for ibuprofen with 2.5% (v/v) alcohol and for mandelic acid with 10% (v/v) alcohol using synthetic mixtures with unequal proportions of the respective enantiomer. Thermodynamic studies in the range 10–45 °C on retention and selectivity of ibuprofen and mandelic acid gave both linear and curved plots. These results stress the importance of investigating enantiomer elution order during the development of enantioselective methods when both old and new CSPs are evaluated. One should also keep in mind that reversal can take place for rather common analytes in well established enantioselective chromatographic systems.