Electrospray ionization mass spectrometry is used to detect both the cations (C+) and the anions (A-) of ionic liquids (CA). In this study, the ionic liquids are diluted with aqueous methanol before injection. In addition to the main peaks of the parent ions, fragmentation products are observed upon increasing the cone voltage, whereas aggregates of the parent ion with one or more ionic liquid molecules (e.g., C(CA)(n)(+), A(CA)(n)(-)) are observed upon decreasing the cone voltage. The ions of several ionic liquids in a mixture are also detected and the ratios of their concentrations estimated. A method is developed to determine quantitatively the concentration of an ionic liquid in solution by using the cation and anion of another ionic liquid as internal standards. By using this method, the solubilities in water at room temperature (22+/-1 degrees C) of three typical hydrophobic ionic liquids have been determined: 0.70+/-0.08 g L(-1) for methyltributylammonium bis(trifluoromethylsulfonyl)imide (MeBu(3)NNTf(2)), 6.0+/-0.5 g L(-1) for butylmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BuMePyrNTf(2)), and 18.6+/-0.7 g L(-1) for 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF(6)).

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