Comparison of stationary phases for packed column supercritical fluid chromatography based upon ionic liquid motifs: a study of cation and anion effects

A class of stationary phases for packed column supercritical fluid chromatography (SFC), referred to as immobilized ionic liquids (IILs), is evaluated with a two-part study: (1) a cation effect study and (2) an anion effect study. The former study compares six different IILs (tripropylphosphonium, tributylphosphonium, methyl-imidazolium, benzyl-imidazolium, triphenylphosphonium, and 4,4′-bipyridyl) on silica gel, evaluating their performance in SFC with all the counter anions as trifluoroacetate (TFA⁻). In the latter study, the stationary phase consisted of a bonded tributylphosphonium cation and varying counter anions (acetate, TFA⁻, chloride, NTf₂⁻, and perchlorate). An order of retentivity was established for the cation effect study, and the favorable behavior of phosphonium-based stationary phases is reported for the first time in SFC. It was not possible to always assign a retentivity order for the anion effect study, but wide variations in selectivity are noted for different anions showing the tunable nature of this class of stationary phases.