An attempt to estimate ionic interactions with phenyl and pentafluorophenyl stationary phases in supercritical fluid chromatography.

Pentafluorophenyl-bonded silica (PFP) phases employed in high-performance liquid chromatography (HPLC) often provide very different results depending on the column manufacturer. PFP phases also provide significantly different selectivity from non-fluorinated aromatic phases. As all HPLC columns can also be employed with carbon dioxide-based mobile phases, PFP phases can also be useful to supercritical fluid chromatography (SFC). However, whether they provide adequate retention and selectivity in SFC conditions is necessary for them to find applicability with this technique. In our laboratory, a column classification for packed column SFC was designed, based on the solvation parameter model, which currently comprises data for about eighty different columns. In this paper, we present the characterization of eleven PFP phases provided by different manufacturers used with carbon dioxide-methanol mobile phases. The columns are compared to fifteen other non-fluorinated phenyl and diphenyl phases in terms of their retention and separation characteristics assessed by the solvation parameter model with five Abraham descriptors. The latter is insufficient for an accurate description of retention mechanisms on the PFP phases, thus two extra terms accounting for ionic interactions with anions and cations (D(-) and D(+)), previously developed for HPLC in hydrophilic interaction mode (HILIC), are introduced. While some approximations are necessary regarding the true pH and pKa values in CO2-based mobile phases, the retention models are significantly improved by this addition, allowing to integrate ionizable analytes in the test set for evaluation of ionic interactions in the chromatographic systems.