DETERMINATION OF ION MULTIPLIER SENSITIVITIES AS A FUNCTION OF IONIC MASS FOR USE IN QUANTITATIVE MASS-SPECTROMETRY

An experimental method has been developed for the determination of ion multiplier sensitivity to ions of different mass to charge ratio using the electron-ion coincidence technique of dipole (e,e+ion) spectroscopy. Time-of-flight mass spectra of each of the noble gas atoms He, Ne, Ar, Kr, and Xe are collected using the ion multiplier under investigation. In the present work we have determined relative efficiencies, at 7.5-8 keV ion impact energy, over the mass range 1-140 amu for two different large active area multipliers, a Johnston focused mesh multiplier and a microchannel-plate detector, both operated in the saturated pulse counting mode. Significant variations in detector sensitivity occur at low m/e values for both multipliers. The microchannel-plate detector shows a level response function above m/e = approximately 30, while the Johnston multiplier exhibits a slow decrease in sensitivity in the same mass range. The applicability of the measured response functions to the correction of molecular mass spectra and experimental photoion branching ratio data has been investigated. It appears that the detection efficiencies for atomic and polyatomic ions of the same m/e may be different.