Automatised liquid chromatography buffer blending method for log P and weak electrolyte pKa determination

Determination of a compound’s log P and p K _a is advantageous in early stages of drug discovery. High pressure liquid chromatography (HPLC) is a viable, high throughput tool that is well-suited for such applications. The advantages stem from the minimal required analyte quantity and the ability to analyse samples of low purity and low water solubility. Determination of log P and p K _a via HPLC often utilises reversed phase mode, leveraging the fact that the analyte’s ionisation state affects its hydrophobicity and therefore retention. Estimation of both parameters can be performed under either isocratic or gradient elution, applying linear solvent strength theory. Hereby, we present a novel mobile phase blending method aimed to fully automatise the log P and p K _a determination process. The method uses buffer blending by pumps to produce mobile phases in the range of pH 2.5–8.6 within the liquid chromatography system. The results for commonly used classes of drugs show good correlation between experimental log P found in literature and our obtained values. The p K _a estimation shows satisfying results in isocratic measurements and solely aqueous mobile phase. The method enables a linear pH gradient generation, likewise applicable for p K _a determination.