Molecular anions as surface probes

The group at the INEL has developed ion sources producing 5- to 7-atom molecular anion beams for use in secondary ion mass spectrometry (SIMS) of insulting materials. Anion sources were chosen because the buildup of electrical charge on insulating samples is much more readily controlled with an anion probe than with a cation probe. Both sulfur hexafluoride and perrhenate molecular anions have proven well adapted to SIMS analysis of insulators, particularly when combined with pulsing of the secondary ion extraction voltage. Pulsed extraction with an anion beam permits the sample surface to be easily held constant to within a few tenths of a volt. The authors have successfully used these beams for analysis of a wide variety of bulk plastics, biological specimens, organics, pharmaceuticals, and polymer films. In particular, the studies have shown sulfur hexafluoride anions are a factor of 30 times more efficient than cesium ions for desorbing tertiary amines