Abstract
The influence of water on the observed gas-phase population of negative ions in electrospray
mass spectrometry was studied for the undiluted ionic liquid 1,3-butyl-methyl-imidazolium
hexafluorophosphate (BMIMPF6
). During the electrospray process, electrolytic reduction of
water enhances the production of tetrafluorophosphate (F4PO), which undergoes further
reactions to produce difluorophosphate (F2PO2
) anions. These anions are observed in addition
to the pre-existing hexafluorophosphate anion. The apparent substitution of two fluorine
atoms with one oxygen is attributed to a series of reactions initiated by hydrolysis of
hexafluorophosphate. This hydrolysis reaction was enhanced by the addition of hydroxide,
formed via the hydrolysis of water or through the addition of ammonium hydroxide. The
formation of FxPOy
was studied as a function of the electrospray current and solution flow
rate. The mass spectral response shows a quantitative logarithmic relationship between
FxPOy
signal intensities (adjusted for mole equivalents of H2O required) and the amount of
water present, against which the water content could be rapidly assessed. Results were found
to be comparable to Karl Fischer titration data
