Chromatographic behavior of ionizable compounds in liquid chromatography. Part 2. Standardization of potentiometric sensors and effect of pH and ionic strength on the retention of analytes using acetonitrile-water mobile phases

Ln this study several properties of acetonitrile-water (ACN-water) mixtures were examined which have an influence on the liquid chromatography (LC) be havior of ionizable compounds, such as Debye-Huckel parameters, pH scale, L C-useful pH range and solvatochromic parameters. Factor analysis techniques were applied to the correlation between standard pH(S) values of NIST buff er solutions and solvatochromic parameters in acetonitrile (ACN)-water. The relationships obtained allow the calculation of standard pH(S) values in a ny ACN-water mixture and thus permit correct pH measurement in these media. Through correct pH, pK(a) and activity coefficient values, a model describ ing the effect of pH on retention in LC is established and tested for sever al diuretic acids and peptide ampholytes using an octadecylsilica column an d ACN-water mixtures of 7, 30, 35, 40, 45 and 50% (v/v) ACN as mobile phase s. The proposed model uses the pH values in ACN-water mobile phases instead of in water, and takes into account the effect of activity coefficients. T he usefulness of the proposed equations is twofold: they permit the determi nation of pK(a) of analytes in the hydroorganic media used as mobile phases , and also permit the prediction of the optimum pH for the separation metho dologies using a minimum number of measurements. The proposed equations can be combined with the previously derived equation, which relates the retent ion to the composition of the mobile phase, in order to establish a general model relating elution behavior of the solutes with the significant mobile phase properties: composition, pH and ionic strength. (C) 1999 Elsevier Sc ience B.V. All rights reserved.