Thermodynamic and Extrathermodynamic Studies of Enantioseparation of Imidazolinone Herbicides on Chiralcel OJ Column

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.

Hold-up volume and its application in estimating effective phase ratio and thermodynamic parameters on a polysaccharide-coated chiral stationary phase

As an "unretained" marker, 1,3,5-tri-tert-butylbenzene (TTBB) has been commonly used to measure the hold-up volume. Despite many racemates have been resolved on Chiralcel OJ column, the hold-up volume of the column is still not well characterized. The aim of this work was to evaluate the chromatographic behavior of TTBB on the OJ column, and its application in estimating the effective phase ratio and thermodynamic parameters. The hold-up volume was affected not only by the mobile phase composition but also the solvents used for dissolving TTBB. A higher concentration of TTBB (0.500 mg/mL) showed a better reproducibility than when used at a lower concentration. After correction for thermal expansion of the mobile phase, TTBB was found to have slight retention on the OJ phase. The effective phase ratio increased with an increase in the temperature and decrease in the strength of the mobile phase. The enthalpy and entropy of enantiomers of imidazolinone herbicides were independent of the temperature in a linear van't Hoff plot when the effective phase ratio was changed. This study shows that, based on the hold-up volume from TTBB, thermodynamic evaluation with parameters derived from the distribution constant is valuable for understanding chromatographic retention and enantioseparation mechanisms of chiral analytes.

Responses of enantioselective characteristics of imidazolinone herbicides and Chiralcel OJ column to temperature variations

Temperature affects not only the chromatographic characteristics of solute but may also alter the conformation of the stationary phase. However, temperature influences on enantioseparation of solute and conformation of chiral stationary phase (CSP) are seldom considered simultaneously. In this study, three temperature programs, a conventional heating procedure, a cyclic van't Hoff program, and a step-temperature program, were employed to evaluate temperature effects on enantioseparation of five imidazolinone herbicides on Chiralcel OJ column and the conformational state of the stationary phase. The van't Hoff plots of retention factor (k'), distribution constant (K) and separation factor (alpha) for imazapyr (1), imazapic (2), imazethapyr (3), and imazamox (4) were linear within 15-50 degrees C. Nonlinear van't Hoff plots of alpha were observed for imazaquin (5) with mobile phase of n-hexane (0.1% TFA)-2-propanol at 70/30 or 60/40 (v/v). The large molecular size of imazaquin (5) and van't Hoff plots of alpha were therefore more sensitive at detecting conformational changes of the stationary phase. Small but irreversible conformational changes occurred at 5-10 degrees C with the solvent ratio of 60/40. During the cyclic van't Hoff program, reversible conformational changes were observed at >or=15 degrees C. A switch was even visible at about 25 degrees C with the solvent ratio of 60/40 during the re-cooling cycle. The cyclic van't Hoff temperature program showed that using OJ column may yield satisfactory results at 15-50 degrees C but not at <or=15 degrees C. The step-temperature program showed that fast temperature changes over the 5-50 degrees C range may be performed on the coated OJ phase, and the recovery of enantioselectivity was relatively fast. This study shows that simultaneous evaluation of temperature effects on the chromatographic behavior of a chiral analyte and the conformational state of the chiral stationary phase provides more insights for understanding mechanisms of enantioseparation and selecting optimal separation conditions.