Combined effect of organic modifier concentration and column temperature on retention in reversed-phase ion-pair liquid chromatography

Summary The retention behavior of phenylamine and naphthylamine sulphonic acid was evaluated in reversed-phase ion-pair liquid chromatography as a function of organic modifier concentration and column temperature. It has been observed that the logarithm of capacity factors decrease linearly with organic modifier concentration, and there is a good linear relationship between the intercept and slope for this relationship. Phenylamine and naphthylamine sulphonic acid retention decreases with increase in column temperature. A linear dependence of lnk_ip on the reciprocal of the absolute temperature, the Van't Hoff plot, was observed over the column temperature range studied, and the standard enhalpic change (H^o) for these sulphonic acid transfers from the mobile phaser to the stationary phase was determined. H^o was dependent on the solute structure and in the range from 2.5 Kcal/mol to 5.5 Kcal/mol, which is close to that observed in RP-HPLC. The enthalpy/entropy compensation effect was evaluated by plotting lnk_ip(T) vs. –H^o, and the apparent differences in retention mechanisms between the analytes were observed, which may arise from the significant differences in their configuration, hydrophobicity and the charges of the solutes as well as the complex retention processes of RP-IPC.