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 secondary ion mass spectrometry (static SLMS) although the data are relevant to any ion-detection system. ii model is developed that shows how the efficiency falls with increased mass and decreased ion impact energy at the front of 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 that 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 increasing the ion impact energy. Therefore, up to a mass of 4000 amu, the spread for ions of 100% observed for 5-keV ion impact energy is reduced to a negligible spread for ions of 20-keV impact energy, where the efficiency is approximately unity, independent of the composition. A simple method is provided to determine the correct voltage to operate the MCP for a given efficiency. This operating voltage should be determined for the highest mass ions in the required range. (Int J Mass Spectrom 202 (2000) 217-229) Crown copyright (C) 2000. published by Elsevier Science B.V.