ABUNDANCE AND DETECTION SENSITIVITY IN SECONDARY-NEUTRAL MASS-SPECTROMETRY WITH ELECTRON-GAS POST-IONIZATION

Post-ionization of sputtered neutral atoms using the electron componen t of a special low-pressure plasma (''electron-gas''), and detection o f these ions in a high-transmission double-focusing mass spectrometer provides a bulk detection efficiency in the low parts-per-billion (ppb = 10(-9)) range if mass interferences are absent. This is demonstrate d for the trace elements in different metallic (Fe-. Ni-, Ti- and Cu-b ased) standards and a Te-doped GaAs specimen. The relative sensitivity factors derived for the elements in these standards (with concentrati ons in the low and sub-ppm range) agree within the various data sets. The Te-isotope (from the GaAs) with the lowest natural abundance (Te-1 20) corresponds to an atomic concentration of 2.85 ppb and is identifi ed at a signal level of similar to 2 counts/s, while the (mass-indepen dent) background amounts to <0.1 counts/s. The measured intensities fr om all specimens exhibit a close one-to-one correlation with the respe ctive atomic concentrations extending over a range of nearly nine orde rs of magnitude which exemplifies the homogeneity of the post-ionizati on scheme employed. This large abundance sensitivity is accessible in a single mass spectrum with an acquisition time of a few 100 s. (C) 19 98 Elsevier Science B.V.