Negative Differential Resistance and Ion Emission from The Taylor Cone of Undiluted Ionic Liquid Generated by a...

The characteristics of the spray current, flow rate, effect of ambient gas pressure, and generated ion species from the Taylor cone of an undiluted ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate, Emim-BF₄) are investigated using a voltage-driven high-pressure electrospray ion source coupled to an Orbitrap mass spectrometer. By using a relatively large-bore emitter capillary (micropipette tip with an outlet inner diameter of ∼0.4 mm) under a high-pressure condition that is free of electric discharge, a wide dynamic range of flow rate Q from <0.1 nL/min to ∼40 nL/min is attained by modulating the emitter voltage V with the monitoring of spray current I. The I-V measurement reveals a pressure-dependent characteristic and an intriguing negative differential resistance in the region between the high and low flow rate regimes. While the well-known I ∝ Q^1/2 scaling law is observed at a high flow rate (Q > 10 nL/min), deviation from the scaling law is noticed for the low flow rate region (Q < 1 nL/min) where the total spray current became dominated by the ions evaporated directly from the Taylor cone. The ions generated from the undiluted Emim-BF₄ consist of Emim⁺ and cationized molecule (Emim-BF₄)Emim⁺ as major species, followed by the less abundant but detectable cluster ions (Emim-BF₄)Emim⁺ with n up to 19. In the region of low voltage, the generated ion beam is highly collimated, and the intensity for all ion species reached its maximum at a spray current slightly above the minimum current, suggesting that ion evaporation from the tip of the Taylor cone may account for most detected ion species including the high-order clusters at low flow rate.