Negative Differential Resistance and Ion Emission from The Taylor Cone of Undiluted Ionic Liquid Generated by a Voltage-Driven High-Pressure Electrospray

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)_nEmim⁺ 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.