Retention of ionizable compounds on HPLC.: Modelling retention in reversed-phase liquid chromatography as a function of pH and solvent composition with methanol-water mobile phases

The retention of ionizable compounds in reversed-phase liquid chromatography with methanol-water mobile phases has been related to the pH, composition and polarity of the mobile phase. The ionization of the compound is calculated from the pH measured in the mobile phase ((w)(w)pH) and the pK(a) of the compound in the same mobile phase ((s)(w)pK(a)) which can be estimated from the aqueous pK(a) ((w)(w)pK(a)) by means of general equations previously established. The retention factors are calculated from the degree of ionization and the retention factors of the neutral and ionic forms of the compounds. Two different linear models have been tested to relate the retention factor of the acid-base forms of the studied compounds to methanol-water composition or polarity.