Simultaneous effect of pH, temperature and mobile phase composition in the chromatographic retention of ionizable compounds

The retention of 22 monoprotic acid–base solutes in 12 buffers (pH from 2 to 12) at 3 temperatures (25, 40 and 55 °C) and in 3 mobile phase compositions (20, 40 and 60% acetonitrile) was measured. The retention data for each solute, temperature and mobile phase compositions was fitted to pH by means of the common sigmoidal equation and the retention and acid–base parameters were obtained (log kHA, log kA and pKa). The dependence of these parameters on temperature (van’t Hoff plots), mobile phase composition (ϕ  , volume fraction of acetonitrile) and mobile phase polarity (PmN parameter) was investigated. Linear plots of the parameter values against the reverse of the absolute temperature, on one hand, and ϕ   or PmN, in the other hand, were generally obtained. From this analysis we propose 6-parameter equations to relate retention to pH and T at constant mobile phase composition, and to pH and ϕ   or PmN at constant temperature. A general 12 parameter equation is also proposed to relate retention simultaneously to pH, T and ϕ   or PmN. The general constancy of some terms of the equations allow to simplify the 12 parameter equation to a 8 parameter equation able to predict retention of the studied solutes. The accuracy of the proposed method provided excellent results with the advantage of modeling the effects of various optimization variables (modifier concentration, mobile phase pH and temperature) using a single equation, based on only eight fitting parameters.