Beam Characteristics of Positively and Negatively Charged Droplets Generated by Vacuum Electrospray of an Ionic Liquid

To develop a vacuum-electrospray beam source for secondary ion mass spectrometry (SIMS), beam characteristics of charged droplets electrosprayed in vacuum were investigated in the negative-ion mode as well as the positive-ion mode. A quaternary ammonium ionic liquid was tested. Experimental results showed that there are differences as well as similarities between the positive-ion mode and the negative-ion mode. Beam current changed greatly with capillary voltage and the flow rate of the ionic liquid. Transient response analysis showed that the vacuum electrospray generated a mixed beam consisting of charged particles of smaller $m/z$ values ($m/z\sim 10^{3}$) and charged droplets of larger $m/z$ values ($m/z\sim 10^{5}$ to $10^{6}$). It turned out that the $m/z$ values of the charged droplets diminished with increasing capillary voltage. Using a three-dimensional positioning stage, the capillary position dependence on the beam characteristics was measured. It proved to be of great importance to align the central axis of a capillary with those of apertures in order to maximize the current component of the charged droplets of the larger $m/z$ values and minimize the ratio of the current component of the smaller $m/z$ values. A high alignment accuracy proved to be required at small gap lengths between a capillary tip and a counter electrode.