To develop a cluster ion beam source for secondary ion mass spectrometry (SIMS), beam characteristics of charged droplets electrosprayed in a vacuum were investigated using a room-temperature molten salt (i.e., an ionic liquid). A target current exceeding 200 nA was produced at a capillary voltage of 1.5 kV even with a low flow rate of 0.05 mu L/min. Efficiency of beam generation varied with increasing capillary voltage and flow rate. Additionally, m/z values of emitted charged particles were investigated by a time-of-flight technique. In addition to masses and charge numbers, radii of the charged droplets were examined. (C) 2010 Elsevier B.V. All rights reserved.