Rational method development strategies on a fluorinated liquid chromatography stationary phase: Mobile phase ion concentration and temperature effects on the separation of ephedrine alkaloids

Fluorinated, silica-based stationary phases are becoming increasingly popular alternatives to traditional alkyl phases owing to their differential selectivity and retention for a variety of analyte classes. In this report, the ion-exchange mechanisms characteristic of a fluorinated phase are exploited to rapidly develop separation conditions for ephedrine alkaloids and synephrine using a mobile phase compatible with mass spectrometry. A linear relationship of basic analyte retention with the reciprocal of ammonium acetate concentration is first established. This linear relationship can then be used to optimize retention and selectivity in just two experiments. The relationship of retention with temperature is also explored. Greater retention with increasing temperature is demonstrated on the fluorinated phase at high percentages of organic modifier, which is in contrast to behavior observed in typical reversed-phase separations. The unexpected observation is explicated based on the reduction in solvent solvating power with increasing temperature. As solvation power of the mobile phase decreases, decreased solvation of both mobile phase and ionized surface groups of the stationary phase leads to stronger interactions between analyte and stationary phase. Both mobile phase ion concentration and temperature are shown to be powerful tools for the manipulation of analyte retention and selectivity.