PARTS-PER-BILLION DETECTION WITH ELECTRON-GAS SECONDARY-NEUTRAL MASS-SPECTROMETRY

Post-ionization of sputtered neutral atoms by means of the electron co mponent of a special low-pressure plasma ('electron gas') and detectio n of these ions in a high-transmission double-focusing mass spectromet er provide a bulk detection efficiency in the low parts-per-billion (p pb = 10(-9)) range. This is demonstrated for 12 trace elements in a Cu standard (with concentrations in the low and sub-ppm range) and a Te- doped GaAs specimen. The Te isotope with the lowest natural abundance (Te-120) corresponds to an atomic concentration of 2.85 ppb and is ide ntified at a signal level of similar to 2 counts/s, while the (mass-in dependent) background amounts to <0.1 counts/s. For both specimens, me asured intensities exhibit a close one-to-one correlation with the ato mic concentrations extending over a range of nearly nine orders of mag nitude. (C) 1997 Elsevier Science B.V.