Measurements of dead time and characterization of ion counting systems for mass spectrometry

A method for characterization of electron multiplier-based pulse counting systems is presented. Specific attention is paid to measurement of the counting system dead time. The systems discussed here are typically used to measure the abundance of ions passing through a mass spectrometer. A method for the determination of dead time by measurement of at least two isotopic ratios of an element with at least three isotopes is given. This method differs from the "text book" method typically used to measure the dead time, The theoretical basis for the measurements is given and an example of measurements of Ti isotopes by secondary ion mass spectrometry (SIMS) is shown. Dead time measurements on SIMS instruments can be performed in a few hours of machine time. A brief description of the pulse detection and counting circuit and its operation is also given. Guidelines for the proper measurement of pulse shapes and the determination of reasonable settings for electron multiplier gain, preamplifier gain, and discriminator threshold are also discussed. [S0034-6748(98)01703-1].