Abstract
van ’t Hoff plots (logarithm of the retention factor, ln k, vs. the reciprocal of absolute temperature, 1/T) are commonly used in chromatographic studies to characterize the retention mechanisms based on the determined enthalpy (ΔH∘) and entropy (ΔS∘) change of analyte adsorption. In reversed phase liquid chromatography, the thermodynamic parameters could help to understand the retention mechanism. In chiral chromatography, however, the conclusions drawn based on van ’t Hoff plots can be deceptive because several different types of adsorption sites are present on the surface of stationary phase. The influence of heterogeneity, however, cannot be studied experimentally. In this study, we employed two reversed phase columns with different retention mechanisms to show that by serially coupling the columns, the obtained thermodynamic parameters are not related to the results obtained on the respective individual columns. Furthermore, our results show that the experimental conditions – such as flow-rate or choice of instrument – will strongly influence the calculated enthalpy and entropy values.
Highlights
Chiral separations have become a rather important chromatographic area in both analytical and preparative separations
The retention behavior in chiral separations is often investigated via the estimation of enthalpy and entropy changes of enantiomer separation to unfold the mechanism of chiral recognition
Fig. 3. van ’t Hoff plots of mefloquine enantiomers recorded at different temperatures on a ZWIX(+) column
Summary
Chiral separations have become a rather important chromatographic area in both analytical and preparative separations. Where k is the retention factor of the analyte, H◦ and S◦ are the standard molar enthalpy and entropy changes, respectively, R is the gas constant, T is the temperature and φ is the phase ratio (i.e. the ratio of the volume of the stationary phase and that of the mobile phase). This method assumes that by plotting ln k against 1/T a linear relationship is obtained. H◦ is calculated from the slope, whereas S◦ is derived from the intercept of Eq (1)
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