Vacuum electrospray ionization study of the ionic liquid, [Emim][Im

Abstract

An analysis is presented of positive and negative vacuum electrospray currents from the ionic liquid, [Emim][Im], wetted on a sharp tungsten needle. The source is operated in a cone–jet configuration established with a low extraction voltage of approximately ±1 kV. Emission current and mass flow are measured as a function of the emitter angle with respect to the instrument axis. Mass spectra of field evaporated ions and mass-resolved energy distributions of emitted charges are recorded as a function of emitter angle. The measurements at both source polarities indicate that the cone–jet emits a mixture of ions and charged droplets. While the charged droplets are emitted with a narrow angular distribution centered on axis, the angular distributions of ions are significantly broader than those of the droplets and centered off axis. Pure ion emission is observed at emission angles larger than ∼15°. The main mass spectral peaks are from X ±{[Emim][Im]} n ions (X = Emim or Im for positive and negative polarities, respectively) with n = 0, 1, and 2. Quantum chemical calculations of the n = 0 and 1 ions and the isolated ion pairs are presented, yielding thermochemical information on the observed ionic species. The present results demonstrate that the cone–jet emits droplets from the tip of the jet, and provide direct evidence that ions are produced in an Iribarne–Thomson field-evaporation mechanism at the transition region between the Taylor cone and jet. Electrochemical modification of the cone–jet emitter properties are observed when maintaining the emitter at a positive polarity for a prolonged (minutes to hours) period. New ions emerge consisting of clusters containing a neutralized Im −. The associated current suppression can be mitigated through polarity alternation at 1 Hz. Measurements are also presented with the tungsten needle heated to 373 K, thereby reducing the liquid viscosity and increasing the flow rate. The average mass of the droplets is observed to increase with temperature while the mass distributions of the ions are not found to change noticeably.