Ion induced microscopic mass spectrometry

Abstract

A microprobe analytical technique has been developed. It is based on mass spectrometric analysis of the ions desorbed from the impact area of a collimated microbeam of high energy heavy projectiles (84 MeV ^84Kr^+7). This is the first application of the technique of particle induced desorption mass spectrometry (PDMS) for spatially resolved analyses. The mass spectrometry capabilities were validated with the analysis of standard targets. A mass resolution of up to 300 was obtained for masses up to 653 (Cs3I2+). The absolute detection limit for the detection of cesium in a thin film of CsI is estimated at [about] 5 x 10^6 atoms; relative desorption yields of up to 69% have been observed. Comparison of isotopic peaks in various alkali halide targets show a relative sensitivity of at least seven shown the technique capable of identifying positively and negatively charged atomic, isotopic, and molecular ions of both inorganic and organic species in simultaneous multimass determinations. The scanning mode of the microprobe was demonstrated by the analysis of targets of known spatial composition. The smallest feature identified was [about] 60 μm wide. Verification of the scans was performed by comparison with electron microprobe scans. To analyze smaller areas with fewer ions, cluster ion beams were evaluated for use as primary projectiles. These experiments were carried out on the cluster accelerator at the Institut de Physique Nucleaire, Lyon, France. The beams studied were beams of H5+ to H23+ at 400 to 600 keV in energy. The cluster ions were found to be at least twice as effective as an equal number of individual atoms of equivalent energy. As the mass of the cluster ion beams increases, the desorption yields increase with the electronic stopping power raised to the fourth power, (dE/dX)^4. Overall, the PDMS microprobe technique developed provides spatially resolved qualitative and semiquantitative mass spectrometry analyses. High desorption yields allow analysis with a few heavy ions; this provides for nondestructive analyses. The technique fills a unique niche in that it can provide mass information from areas potentially beyond the current ion microprobe techniques.