[Determination of imidazolium ionic liquid cations by ion-pair chromatography using gradient elution]

Determination of five imidazolium ionic liquid cations by ion-pair chromatography was carried out using ultraviolet-visible spectrophotometry (UV/VIS) detection. Chromatographic separations were performed on a ZORBAX Eclipse XDB-C18 column with 1-heptanesulfonic acid sodium (adjusted by citric acid to pH 4.0) + acetonitrile as eluent. The effects of ion-pair reagents, acetonitrile concentration and column temperature on retention of the cations were investigated. The retention factor of imidazolium cation on column was increased with the increase of the concentration of 1-heptanesulfonic acid sodium. With the increase of the concentration of acetonitrile, the retention time of imidazolium cation was obviously shortened and the peak shape was improved. Under the optimum conditions, detection limits (S/N = 3) for the cations were 0.05-0.30 mg/L. Relative standard deviations (RSDs) for peak areas were less than 0.1%. The method has been successfully applied to the determination of two ionic liquids by chemically synthesized in laboratory with the recoveries of the cations of 98.6%-102.1% after spiking.