Retention of ionisable compounds on high-performance liquid chromatography XVI.: Estimation of retention with acetonitrile/water mobile phases from aqueous buffer pH and analyte pKa

In agreement with our previous studies and those of other authors, it is shown that much better fits of retention time as a function of pH are obtained for acid-base analytes when pH is measured in the mobile phase, than when pH is measured in the aqueous buffer when buffers of different nature are used. However, in some instances it may be more practical to measure the pH in the aqueous buffer before addition of the organic modifier. Thus, an open methodology is presented that allows prediction of chromatographic retention of acid-base analytes from the pH measured in the aqueous buffer. The model presented estimates the pH of the buffer and the pK(a) of the analyte in a particular acetonitrile/water mobile phase from the pH and pK(a) values in water. The retention of the analyte can be easily estimated, at a buffer pH close to the solute pK(a), from these values and from the retentions of the pure acidic and basic forms of the analyte. Since in many instances, the analyte pK(a) values in water are not known, the methodology has been also tested by using Internet software, at reach of many chemists, which calculates analyte pK(a) values from chemical structure. The approach is successfully tested for some pharmaceutical drugs. (c) 2006 Elsevier B.V. All rights reserved.