Modelling retention in liquid chromatography of polyphenolic acids

The retention behavior of a series of polyphenolic acids has been modeled in water-acetonitrile mobile phases, in order to optimize the chromatographic resolution between these compounds. The retention model is based on the relationships between retention factors of solutes with the pH and solvatochromic parameter of the mobile phase (ETN), and between the dissociation constants of the compounds and the mole fraction of organic modifier. The retention of these ionizable compounds has been studied at three levels of acetonitrile percentage (10, 20 and 30%), and at six different pH values ranging between 3.5 and 7. Three different fitting procedures have been applied: model I consists of a nonlinear regression to all data points, whereas models II and III use only four and two data points, respectively, taken from the extreme pH/ETN values. The results obtained from these models indicate a good agreement between real and predicted retention factors for each compound, with errors about 5% for model I and about 10% for models II and III. The results of model III are of particular interest, because of only two experiments for each compound are needed to describe the retention behavior in a relatively large pH and percentage of acetonitrile intervals; this is especially valuable in the optimization of the mobile phase for chromatographic separation of several compounds.