Aqueous-nonaqueous solvent-switching ion chromatography of halide impurities in ionic liquids.

We here propose an efficient solvent-switching preconcentration method for the ion-chromatographic (IC) determination of halide impurities contained ionic liquids (ILs). Because halide impurities strongly affect the physicochemical properties of ILs, their analysis is an important task for the successful utilization of ILs. Although IC is an efficient method for this purpose, its application still involves significant challenges. The major halide impurities, such as F and Cl, show much smaller retention in aqueous anion-exchange chromatography than IL component anions. Therefore, if an IL sample is directly analyzed by IC with aqueous mobile phases, the halide impurities are eluted earlier, whereas the IL component anion is hardly eluted and gives a large peak once eluted. Thus, the introduction of the IL component anions into the IC separation column should be avoided for efficient analyses and also for preventing the degradation of the column by the accumulation of the IL anions in it. This problem, which arises from the ion-exchange selectivity in aqueous media, is solved by a solvent switching preconcentration method. The anion-exchange selectivity in aqueous media is reversed by a use of an aprotic solvent, such as acetonitrile (MeCN). Hence, we have come up with the idea of preconcentrating anions in MeCN and stripping them with an aqueous mobile phase for IC analysis. The introduction of the IL component anions into the IC separation column is substantially reduced while maintaining high sensitivity for the halide impurities. Sub μM impurities are detectable in the mM level of ILs.