Characterisation of reversed-phase stationary phases for the liquid chromatographic analysis of basic pharmaceuticals by thermodynamic data

This paper describes the characterization of reversed-phase liq. chromatog. (RPLC) columns using thermodn. measurements. Retention vs. 1/T data were used to construct Van 't Hoff plots. The slope of these plots indicates the std. enthalpy of transfer of the analyte from the mobile to the stationary phase. The std. entropy can be calcd. from the intercept. Van 't Hoff plots were linear for the investigated RPLC columns, meaning that for basic analytes over the temp. range studied no changes in the retention mechanism occurred. Enthalpies and entropies of transfer of basic analytes from the mobile to the stationary phase revealed information about the types of interaction of protonated and neutral compds. with the stationary phases. However, a clear view using the present set of basic compds. on how these thermodn. data may explain the obsd. substantial differences in peak symmetry cannot be given. It is considered that addn. of N,N-dimethyloctylamine (DMOA) to the eluent will results in a dynamically coating of the stationary phase. Addn. of DMOA to the eluent resulted for protonated basic compds. in a redn. of both enthalpy and entropy. In practice, with DMOA in the eluent sym. peaks were obtained. It is assumed that this is due to blocking residual silanols and/or ion exclusion effects. [on SciFinder (R)