Mass spectrometric analysis of the electrospray plume from an externally wetted tungsten ribbon emitter

Angle-resolved mass spectrometry in conjunction with retarding potential energy analysis provides information on the mechanisms of ion field evaporation and spatial distributions of the emitted ions and droplets, the latter being of particular importance in the emitter array configuration. In the present study, we conduct mass spectrometric, retarding potential, and angular distribution measurements for ions emitted from [Emim][Im] (1-ethyl-3-methylimidazolium bis(trifluromethylsulfonyl)imide) when sprayed from an externally wetted tungsten ribbon emitter. A tungsten ribbon of 750 μm width and 50 μm thickness is electrochemically etched to produce the ribbon emitter tip ∼20 μm wide and ∼1 μm thick. The angle resolved measurements indicate that the spray comprises a mixture of droplets and ions. The major ionic species identified are Emim⁺([Emim][Im])_n and Im ^-([Emim][Im])_n, with n=0,1,2 in the positive and negative polarity modes, respectively. At low extraction voltage (1070-1170 V), the dominate ion intensity comprises a narrow distribution in the center of the spray, with additional intensity emitted at larger angles (>±10 degrees). The anionic species observed are Im^- and Im ^-([Emim][Im]) with approximately equal intensity. At higher extraction voltages (∼1300-1400 V), a broader angular distribution is found, extending to 40 degree half-angles. The distribution is peaked at center of the spray. In this case, Im^-([Emim][Im]) and Im^-([Emim][Im]) ₂ are the major species observed. The total current and deposition measurements are consistent with the observation that the higher extraction voltage creates a larger flow rate than that produced from the lower extraction voltage, leading to emission that contains a larger fraction of droplets versus ions, and also produces droplets of a larger size.