A homologous series of chiral imidazolinone herbicide was previously resolved on Chiralcel OJ column in high performance liquid chromatography. However, the mechanism of the chiral separation remains unclear. In this study, chromatographic behaviors of five chiral imidazolinone herbicides were characterized by thermodynamic and extrathermodynamic methods in order to enhance the understanding of the chiral separation. Thermodynamic parameters of this study were derived from equilibrium constant ( ) that was estimated from the moment analysis of the chromatographic peak. Van't Hoff plots of ( versus ) were linear at a range of 15–50°C, only nonlinear at a range of 5–15？°C with n-hexane (0.1%, trifluoroacetic acid)-2-propanol 60/40 (v/v) mobile phase. The enantiomer retention on the chiral column was entropy-driven at a lower temperature (5°C) and enthalpy-driven at a higher temperature (10 to 50°C). Enantioseparations of four of the five imidazolinone herbicides were enthalpy-driven, only entropy-driven for imazaquin. Enantioseparation mechanisms were different in between 5–10°C and 15–50°C probably due to the conformational change of the OJ phase. Enthalpy-entropy compensation showed similar mechanisms in retention and chiral separation for the five or enantiomers. Several extrathermodynamic relationships were able to be extracted to address additivity of group contribution. 1. Introduction Chiral separation in high-performance liquid chromatography (HPLC) is keeping to be of great interest in diverse areas such as the agrochemicals and pharmaceutical industry [1]. To achieve better resolution and improve chiral stationary phase, enantioseparation mechanisms have been studied extensively using various methods [2–4]. A thermodynamic study is found to be especially valuable for providing information on the separation mechanism [5–7]. A link between chromatographic behavior and thermodynamic parameters such as enthalpy ( ), entropy ( ), and Gibbs free energy ( ) is distribution constant ( ) that describes the transfer of an analyte from mobile phase to stationary phase [8]. Moment analysis of elution peak based on a kinetic model provides an approach to estimate the value [9]. The present study is concerned with using the value to characterize retention and chiral separation, a case where there has been so far few works [10]. Extrathermodynamic relationships are empirical correlations of thermodynamic parameters with molecular structures, chromatographic conditions, and enthalpy-entropy compensation (EEC) and have been among the most important tools in
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