Determination of trace cationic impurities in butylmethylimidazolium-based ionic liquids:: From transient to comprehensive single-capillary counterflow isotachophoresis-zone electrophoresis

Determination of impurities in ionic liquids (ILs) remains a difficult task. In this work, the hyphenation of isotachophoretic (ITP) preconcentration to zone electrophoresis (ZE) has been explored for the trace analysis of the cationic impurities Na+, Li+, and methylimidazolium (Ml(+)) in butylmethylimidazolium (BMI+)-based lLs. Simultaneous detection of UV-transparent and UV-absorbing impurities was ensured by a BGE composed of creatinine-acetate buffer. To induce ITP, three different strategies were evaluated: (i) Sample self-stacking ensured by the addition of ammonium acetate (NH4Ac) to 25-50-fold diluted IL solution (transient ITP). (ii) Complete ITP-ZE separation performed in a single capillary: ITP was realized in discontinuous electrolytes comprising an 80 mM NH4Ac, 40 mM acetic acid, 30 mM alpha-CD, pH 5.05, leading electrolyte (LE) and a 10 mM creatinine, 10 mM acetic acid, pH 4.9, terminating electrolyte (TE). To create the ZE stage, the ITP stack of analytes was moved back toward the capillary inlet by pressure and simultaneously the capillary was filled with the BGE. This protocol made it possible to accommodate a 2.5-times diluted IL sample. (iii) Complete counterflow ITP-ZE with continuous electrokinetic sample supply: the ITP stage was performed in a capillary filled with a 150 mM NH4Ac, 75 mM acetic acid, 30 mM alpha-CD, pH 5.0 LE, with 40-times diluted IL at the capillary inlet. BMI+ from IL acts as the terminating ion. The LODs reached in this latter case were at the 10 and 1 ppb levels for MI+ and Li+ in diluted IL matrix, respectively.