Retention models for ionizable compounds in reversed-phase liquid chromatography: Effect of variation of mobile phase composition and temperature

General models in reversed-phase liquid chromatography that have been extended to relate retention of ionizable compounds to mobile phase composition, pH and/or temperature are reviewed. In particular, the fundamentals and applications of the solvation parameter model, the polarity parameter model and several classical models based on empirical equations are presented and compared. A main parameter in all these models is the degree of ionization of the acid–base compound, which depends on both the pH of the mobile phase and the acid–base constant of the compound. Thus, on one hand, the different procedures for pH measurement in the mobile phase and their influence on the performance of the models are outlined. On the other hand, equations that relate the variation of the pH of the buffer and the pKa of the compound with the mobile phase composition and/or temperature are reviewed and their applicability to the retention models critically discussed.