Correctness of the van 't Hoff analysis for homogeneous or heterogeneous retention

Abstract

It is already known that the physical meaning of the numerical values of the calculated enthalpy (ΔH) and entropy (ΔS) change in chromatography via the van 't Hoff plot analysis is rather questionable. In the former work of these authors, it has been demonstrated that by serially coupling two reversed-phase columns of the same inner diameter but of different retention mechanism, the obtained thermodynamic values are not related to the numerical results obtained on the individual columns respectively. Since surface heterogeneity of the stationary phase is intrinsically present in chiral chromatography (enantioselective and non-selective sites), the calculation of ΔH and ΔS should be revisited in that field.In this study, more details of the pressure dependence were investigated. Using special POPLC columns, the effect of the column length on the calculated thermodynamic values was investigated. The calculated values differ by 30–400% when the column length is increased from 4 cm to 12 cm. The dependence of the calculated results on the applied flow rate was already highlighted earlier, and here the emphasis was put on the instrument on which the separation was performed. It turns out that there is a difference when using a Shimadzu HPLC system compared to using a Waters Acquity UPLC system.Because the Eyring method serves as an alternative route to calculate the thermodynamic values from chromatographic data, the differences obtained by the two methods were also investigated.