Effect of 1-alkyl-3-methylimidazolium-based ionic liquids as the eluent on the separation of ephedrines by liquid chromatography

The chromatographic behavior of ephedrines (norephedrine, ephedrine, pseudoephedrine and methylephedrine) on a C18 column was studied with different concentrations of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquids as the eluent at pH 3.0. The addition of ionic liquid has great effects on the separation of these basic compounds: decreasing band tailing, reducing band broadening, and improving resolution. The retention times of the analytes increase at first and then decrease with the increase in the concentration of ionic liquid. This effect may be attributed to the competition between imidazolium cations and the polar groups of the analytes for the silanol group on the alkylsilica surface, and also to the formation of weak bilayer electronic structure on the C18 column. Several ionic liquids with different alkyl substituents on the imidazolium cations or with different counterions as the eluents were compared.