An electrospray technique can generate a continuous stream of charged droplets at currents exceeding 1000 nA. We expect that it has a great potential in Secondary Ion Mass Spectrometry (SIMS). Consequently, we investigated beam generation characteristics using the electrospray technique operated at atmospheric pressure and under reduced pressure (i.e., vacuum).

When electrospraying is performed at atmospheric pressure or above, emitted charged droplets and ions are required to be sampled through an aperture into a vacuum chamber, and then accelerated toward a target. It was confirmed that a beam current was considerably reduced in the process passing through the aperture; for instance, passing efficiency through the 120-mm aperture was on the order of 10^-3. This loss mainly results from the spreading of electrosprayed ions at atmospheric pressure.

When electrospraying is done in a vacuum, freeze and evaporation of a solution will be a serious problem. In other words, in the case of frequently-used solutions such as water and ethanol, it is difficult to achieve stable electrospraying in a vacuum because of the freeze and evaporation around a capillary tip. To solve the problem, we have proposed a vacuum electrospray beam source using pure ionic liquids for SIMS application. Ionic liquids have very low vapor pressure as well as high ionic condu ctivity; its use will ena ble us to perform stable electrospraying in a vacuum. We investigated the electrospray characteristics of an ionic liquid in a high vacuum environment with pressures around 10^-5 Pa using the stainless-steel capillary. The experimental results demonstrated that stable electrosprayed currents of the order of 1000 nA can be continuously generated in both positive and negative modes. Experimental results indicate that the electrospray technique seems to be applicable to SIMS.