Gradient HPLC in the determination of drug lipophilicity and acidity

The linear-solvent strength (LSS) model of gradient elution in high-performance liquid chromatography (HPLC) has been demonstrated to provide parameters of lipophilicity and acidity of analytes. pK(a) and log k(W) values are determined in three gradient runs. The first two experiments use an aqueous buffered eluent with a wide-range organic modifier gradient at pH of buffer, providing suppression of ionization of the analyte. That experiment allows an estimate of contents of the organic modifier in the mobile phase (%B), producing requested retention coefficient, k, for the nonionized form of the analyte. The next experiment is carried out with the latter %B and a pH-gradient of the aqueous component of the eluent that is sufficient to overlap possible pK(a) value of the analyte. The initial pH of the buffer used to make the mobile phase is selected to insure that the analyte is in nonionized form. The resulting retention time allows an estimate of pK(a) in a solvent of the given %B. The log k(W) parameter obtained correlated well with the corresponding value obtained by the standard procedure of extrapolation of retention data determined in a series of isocratic measurements. The correlation between log k(W) and the reference parameter of lipophilicity, log P, was very good for a series of test analytes. The values of pK(a), were found to correlate with the literature pK(a) data determined in water for a set of aniline derivatives studied.