Insights into retention mechanisms of perfluoroalkyl acids on perfluorinated sorbents. Fluorous affinity chromatography as a tool for enrichment and analysis of perfluorinated emerging contaminants.

The adsorption behavior of four perfluoroalkyl acids, including the environmentally relevant perfluorooctanoic acid, has been investigated on a straight-chain perfluorohexyl adsorbing material. The aim of this work is to provide original contributions to the potential use of fluorinated separation media for the analysis and/or the enrichment of perfluoroalkyl compounds in environmental samples. Water-acetonitrile-formic acid (0.1% v/v) mixtures were employed as mobile phases. An unusual U-shaped retention profile for all perfluorinated acids has been observed by changing the amount of acetonitrile in mobile phase and this has been correlated to the excess adsorption of the organic solvent, from binary water-acetonitrile mixture, on the adsorbent surface. In addition, the concept of perfluoromethylene selectivity, defined in terms of ability of a chromatographic system to discriminate between molecules that differ by a single perfluoromethylene group, and traditional van't Hoff analysis were employed to describe the thermodynamics of phase transfer of analytes under study. Contributions to the Gibbs free energy for the passage of a perfluoroalkyl carbon from the mobile to the stationary phase have been evaluated and their meaning is discussed.