Elution characteristics of natural cyclodextrins on porous graphitic carbon

The retention behavior of natural alpha-, beta-, and gamma-cyclodextrins on a porous graphitic carbon (PGC) stationary phase is investigated. Unusual retention properties for reversed-phase chromatographic conditions are observed with acetonitrile-methanol and water-methanol mixtures as mobile phases. It is assumed that the retention process is governed not only by the standard solvophobic effect but also by specific interactions described as "CD-PGC" effect. The retention factor versus the volumetric methanol fraction in the mobile phase show second-order curves expressing this double mechanism hypothesis. van't Hoff plots demonstrate the contribution of these two retention processes. The retention factor of each natural cyclodextrin is shown to depend on the mobile phase property to act as a proton acceptor, according to the solvent selectivity classification described by Snyder. The "CD-PGC" effect is interpreted as an equilibrium between different interactions: cyclodextrin-PGC stationary phase, London dispersion forces, and cyclodextrin-mobile phase hydrogen bonding. The balance of these interactions may monitor the orientation of the cyclodextrin molecule facing the carbon surface, which is therefore suspected to be the major parameter of this retention mechanism.