Effect of mobile‐phase modifiers on the enantioselective retention behavior of methyl mandelate with an amylose 3,5‐dimethylphenylcarbamate chiral stationary phase under reversed‐phase conditions

Abstract

In this study, methanol, ethanol, n‐propyl alcohol, isopropyl alcohol, acetone, and tert‐butanol were used as organic modifiers in reversed‐phase mode chiral liquid‐chromatography to systematically investigate the effects of mobile phase components on the enantioselective retention behavior of methyl mandelate with immobilized amylose 3,5‐dimethylphenylcarbamate‐based sorbent called Chiralpak IA. A two‐site enantioselective model was used to obtain information on the recognition mechanisms by observing the dependence of the enantioselectivity and retention factor difference on the modifier content. Similar enantioselective retention behaviors were observed for all modifiers, and characteristic modifier concentration points (PL, PM, and PH) were identified. At modifier concentrations up to PM, the weakened hydrophobic environment resulted in polymer structural relaxation, which changed the recognition mechanisms. By contrast, at concentrations beyond PH, considerably different enantioselectivity behaviors were observed, indicating that the existence of dipole‐dipole interaction, which was stronger at higher modifier concentrations, contributed to the retention mechanisms. The concentrations at which these characteristic points occurred were dependent on the carbon number of the modifier molecule. Modifiers with more carbon numbers facilitated the transition in the enantioselective behaviors. These results demonstrated that the proposed method can provide a physically consistent quantitative description of enantioselective retention behavior in reversed‐phase mode.