The influence of mobile phase pH on the retention and selectivity of related basic compounds in reversed-phase liquid chromatography

Retention change with the pH of the mobile phase is shown for several basic compounds of pharmaceutical importance (drotaverine, ketotifen and chlorphenamine) and some of their related compounds. The partition model is used to explain the pH dependence of the retention for these compounds. Based on the experimental retention data, the dependence between the retention factor (k’) and pH of the mobile phase were plotted. Sigmoid shapes were obtained confirming thus the applied theoretical model. The pKa values were
estimated for all the investigated compounds using the inflexion curve parameter. Comparison with theoretical pKa values obtained with specialized software leads to good correlation for some of the studied compounds.