Prediction of retention behaviour and evaluation of pKa values of peptides and quinolones in liquid chromatography

The present paper examines the effect of the solute ionisation on the retention behaviour in liquid chromatography of a series of peptide and quinolone compounds of biological interest, using acetonitrile–water media as mobile phases and a polymeric-based stationary phase. Polymeric columns with polystyrene–divinylbenzene (PS–DVB) polymer show advantages over silica-based reversed-phase packings since the former are stable in a wide pH range. SSpKavalues have been evaluated using chromatographic data in acetonitrile–water mixtures with acetonitrile percentages of 30, 35, 40 and 50% (v/v) for quinolones and 12.5 and 20% (v/v) for peptides. The quinolones show great retention on PS–DVB phase stationary. It was thus necessary to work with a higher acetonitrile content in the mobile phase than for the less retained peptides. The pH values were measured in the hydroorganic mixtures, used as mobile phases, instead of in water and account was taken of the effect of activity coefficients. The derived equations permit the chromatographic determination of SSpKa values of the peptides and quinolones in acetonitrile–water mixtures by fitting it to the experimental data in a nonlinear least-square procedure and also permit the prediction of the effect of SSpH on their chromatographic behaviour. We have also compared the obtained SSpKavalues with those previously obtained in acetonitrile–water mixtures from potentiometric measurements.