Source of heavy molecular ions based on Taylor cones of ionic liquids operating in the pure ion evaporation regime

Abstract

The full spray emitted by Taylor cones of the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMI+BF4−) held in a vacuum is investigated at room temperature by time of flight mass spectrometry. The current is composed mainly of ions under most conditions studied, but contains a small component of nanometer drops that tends to dominate the emitted mass flow. Exceptionally, drop ejection vanishes close to the smallest flow rate at which the Taylor cone is steady. The present discovery of a stable strictly ionic regime in Taylor cones of substances other than liquid metals owes much to earlier observations with sulfuric acid, where most but not all the current was ionic. Most striking is the fact that this purely ionic regime is obtained at an electrical conductivity K of only 1.3 S/m, much smaller than that of sulfuric acid, and smaller than that at which formamide electrolytes with K>2 S/m do still emit substantial drop currents. The ion emission includes primarily the dimer (EMI–BF4)EMI+, accompanied by fair currents of the monomer EMI+ and the trimer ions. The modest spread of ion energies observed and considerable currents attained (0.6 μA) suggests the use of this and other ionic liquids as ion beam sources with a much wider range of mass/charge and chemical compositions than previously available. The present data provide a direct proof of the reality of the often doubted mechanism of ion evaporation from liquid surfaces.