Reversed-phase liquid chromatography analysis of alkyl-imidazolium ionic liquids: II. Effects of different added salts and stationary phase influence

Two mixtures of four 1-alkyl-3-methylimidazolium ionic liquids (ILs) salts associated to the anions tetrafluoroborate or hexafluorophosphate were analyzed by reversed-phase liquid chromatography with three different stationary phases: Kromasil C8, Zorbax Extend C18 and Zorbax Sb-Aq. The effect on retention of various inorganic salts (NaCl, NaH2PO4, NaBF4, NaClO4 and NaPF6) added to acetonitrile/water mobile phases was studied. The three columns gave similar separation profiles. In all cases, the retention of ILs increased with the increasing affinity of the inorganic anions for the apolar stationary phases; a phenomenon called chaotropicity. The chaotropic anion order is Cl− ∼ H2PO4− < BF4− ∼ ClO4− < PF6−. It is established that the presence of chaotropic anions in the mobile phase do not permit to differentiate between ILs associated to different anions. However, chloride or dihydrogenphosphate added salts do not fully screen the retention differences between ILs associated with different anions. Distorted and even split peaks may appear in the chromatogram depending on the nature and concentration of the injected ILs. In the RPLC analysis of imidazolium-based IL, it is recommended to add to the mobile phase significant amounts of a salt containing a chaotropic anion. This salt addition will improve the IL peak shapes and give reproducible retention factors. LODs in the low microgram range (∼5 nmol) were obtained with the Kromasil C8 column with a 50/50 acetonitrile–water mobile phase containing 0.01 M NaPF6 added salt and 230 nm UV detection.