Investigating the ions emitted by multiply charged ionic liquids from a porous electrospray ion source

Abstract

A porous electrospray ion source was tested with three ionic liquids in order to investigate the effects of ionic liquid properties on the sizes of ion clusters emitted by purely ionic electrospray sources. Two of the ionic liquids, bis(1-ethyl-3-methylimidazolium) tetrathiocyanatocobaltate and 1,6-bis(3-methylimidazolium-1-yl) hexanebis(trifluoromethylsulfonyl)amide, were selected due to them having a dication or dianion, which were termed multiply charged ionic liquids due to them containing anions or cations with more than one charge within them. These were selected in order to investigate ionic clustering within electrospray ion emission, and were compared against one of the most common ionic liquids, EMI-BF₄. The current-voltage data showed that EMI-BF₄ emitted similar levels of current to bis(1-ethyl-3-methylimidazolium)  tetrathiocyanatocobaltate, even though the latter liquid had significantly lower conductivity and higher viscosity, suggesting an improvement in current speculated to be due to the extra charges contained by the ions. TOF and RPA data are provided, showing all three liquids emitting only ions comprising of monomers, dimers, trimers and quadramers, with some of the 1,6-bis(3-methylimidazolium-1-yl) hexanebis(trifluoromethylsulfonyl)amide data indicating heavier species emission. The data also suggested that the multiply charged ionic liquids produced ions which have two anions or cations attached, termed ‘double ions’, with these ions have not been previously reported using porous electrospray sources. Furthermore it was found that the dimers emitted by both of the multiply charged ionic liquids seemed to be more stable than EMI-BF₄ dimers, providing insight into ion cluster formation.

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