Role of stationary phase and eluent composition on the determination of log P values of N-hydroxyethylamide of aryloxyalkylen and pyridine carboxylic acids by reversed-phase high-performance liquid chromatography.
The partition coefficients, P, between n-octanol and water of a number of growth stimulating substances, N-hydroxyethylamide of aryloxyalkylen- and pyridine carboxylic acids were obtained from Pomona College (C log P), and Rekker's (log P(Rekker)) revised fragmental constant system was used to calculate log P data sets. Both of these data sets were correlated with two different substance lipophilicity parameters, log k(w) and phi0. Log k(w) was obtained by extrapolation of log retention factor (k) to 0% organic modifier measured in reversed-phase liquid chromatography (RPLC) systems. Phi0 values were obtained from the slopes and intercepts of these relationships. The RPLC experiments were performed on four commercially available reversed-phase columns. Binary mixtures of methanol-water, methanol-phosphate buffer (pH 7.0), methanoltricine buffer (pH 7.0) and acetonitrile-water were used as mobile phases for the determination of log k(w) values. For the methanolic eluents linear regression provided satisfactory correlations (r>0.99) for the relationships log k vs. organic modifier content in the eluent, while for the acetonitrile-containing eluents a second-degree polynominal regression was necessary. For all four RPLC columns, by linear regression satisfactory correlations (r>0.99) were obtained between log k(w) and log P data using methanolic eluents. In such eluents phi0 values were shown to be the second-best lipophilicity parameters. For acetonitrile-containing eluents the use of second-degree polynominal regression was necessary and, in contrast to methanol, significant influence of the applied column on regression results was observed. For acetonitrile-containing eluents the phi0-index does not provide satisfactory results for our substances. No difference in regression results between the use of buffered and non-buffered eluents was observed.