Prediction of internal standards in reversed-phase liquid chromatography. III. Evaluation of an alternative solvation parameter model to correlate and predict the retention of ionizable compounds

This paper describes the results of the evaluation of an alternative solvation parameter model for ionizable compounds. The new model is described as Log(k) = Int + rR2 + spi2(H) + asigmaalpha2(H) + bsigmabeta2(H) + mVx + U/(1 + V10 (+/-(pH-Pk))). The first six terms are the usual solvation parameter equation for neutral solutes, and the last term represents the contribution to retention from the ionization of solutes. Retention data obtained for 30 solutes in acetonitrile/aqueous buffer mobile phases are used to evaluate the capability of the function using different pH/pK scales. Because the function is not linear, nonlinear least-squares analysis is used to perform the data processing. It is concluded that the model function describes similarly the retention of ionizable compounds to the literature model without the need to accurately measure the mobile phase pH and solute's pK. Accordingly, the function simplifies the application of linear solvation energy relationships (LSERs) to ionizable compounds, and allows us to easily predict their retention for chromatographic optimization.