Selectivity of alkylamide bonded-phases with respect to organic acids under reversed-phase conditions

It was shown previously by Czajkowska et al. [J. Chromatogr. A, 691 (1995) 217] that alkylamide phases are better for separating pyridinecarboxylic acids than conventional alkyl packings. They allow separation of these compounds from hydrophobic substances over a wide range of mobile phase compositions. In the current work the retention data for pyridinedicarboxylic acids were measured on monomeric and polymeric alkylamide phases at 35 degrees C by using the acetonitrile-water eluent and utilized to calculate the nonspecific (methylene) and specific (carboxylic and amine) selectivities. Subsequently, the selectivity data were plotted against the pH of the mobile phase. It was shown that while the methylene selectivity does not change much at the pH range from 2 to 5, the specific selectivites studied depend strongly on the pH of the mobile phase. The magnitude of this dependence is determined by the type of the bonded phase and silica support as well as the length of the terminal alkyl groups.