Ion detection efficiency in SIMS: dependencies on energy, mass and composition for microchannel plates used in mass spectrometry

The effects of ion energy, mass and composition on the detection efficiency of a microchannel plate (MCP) have been studied in detail, using a time-of -flight (TOF) mass spectrometer. This spectrometer is used for static secon dary ion mass spectrometry (static SLMS) although the data are relevant to any ion-detection system. ii model is developed that shows how the efficien cy falls with increased mass and decreased ion impact energy at the front o f the MCP. At an impact energy of 20 keV, the efficiency for the detection of cationised PS oligomers of mass 10,000 amu is approximately 80%, whereas at 5 keV it has fallen to similar to5%. The model is extended to estimate the effect of ion composition on the detection efficiency. It was found tha t ions with a high hydrogen content have a lower efficiency than those that consist of a cluster of high atomic number atoms. The spread of detection efficiencies arising from both composition and mass may be reduced by incre asing the ion impact energy. Therefore, up to a mass of 4000 amu, the sprea d for ions of 100% observed for 5-keV ion impact energy is reduced to a neg ligible spread for ions of 20-keV impact energy, where the efficiency is ap proximately unity, independent of the composition. A simple method is provi ded to determine the correct voltage to operate the MCP for a given efficie ncy. This operating voltage should be determined for the highest mass ions in the required range. (Int J Mass Spectrom 202 (2000) 217-229) Crown copyr ight (C) 2000. published by Elsevier Science B.V.