Capillary vs. externally wetted ionic liquid ion sources

Abstract

Recent studies on the use of Taylor cones of ionic liquids as ion sources have demonstrated a considerable effect of source geometry on the characteristics of the resulting ion beam. Of particular interest is the recent discovery by Lozano and Martinez-Sanchez of the excellent performance of externally wetted and electrochemically etched and sharpened tungsten tips. These sources are termed ionic liquid ion sources (ILIS), in analogy to conventional liquid metal ion sources (LMIS). Even more noteworthy is the fact that certain ionic liquids that had in a variety of prior reports always operated in the mixed ion-drop regime from capillary tips, do run exclusively in the purely ionic regime (PIR) on ILIS type sources. The present study uses time of flight mass spectrometry to compare full ion beams emitted from the two types of sources for a diversity of ionic liquids. We confirm earlier reports that internally fed capillary sources achieve purely ionic emissions more readily for liquids having high surface tension (γ > 40 dyn/cm) and high electrical conductivity (K ~ 1 S/m), and produce ion beam currents of several hundred nA with non-negligible energy spread. In contrast, externally wetted ILIS sources produce purely ionic emissions at tens of nA with liquids with substantially smaller γ and K, and much narrower energy distributions.