The cation-anion interaction in ionic liquids studied by extractive electrospray ionization mass spectrometry

Ionic liquids, known as green solvents, are of sustainable interest in modern chemistry, industry, and many other fields. Here, extractive electrospray ionization has been applied to transfer various room temperature ionic liquids into the gas phase for mass spectrometry analysis. Generated mass spectra display free cations (C⁺), anions (A^-), and small salt clusters, such as C₂A⁺ and CA₂^-, from strongly diluted ionic liquid samples (<10^-8 mol/L) with high sensitivity and tolerance to chemical contamination. The eight ionic liquids based on the 1-butyl-3-methylimidazolium cation with different anions (OH^-, HSO₄^-, Cl^-, BF₄^-, AlCl₄^-, NO₃^-, Ac^-, and PF₆^-) are investigated in the present work. Interestingly, the 1-butyl-3-methylimidazolium cation signal intensity is inversely correlated with the hydrogen bonding strength between the anion and cation. Our study indicates that the direct extractive electrospray ionization mass spectrometry analysis is a convenient method to screen ionic liquid libraries with regard to chemical composition, physicochemical properties, and supramolecular organization of ionic liquids. (author)