Automated SIMS Isotopic Analysis Of Small Dust Particles

The isotopic compositions of sub-μm to μm sized dust grains are of increasing interest in cosmochemistry, nuclear forensics and terrestrial aerosol research. Because of its high sensitivity and spatial resolution, Secondary Ion Mass Spectrometry (SIMS) is the tool of choice for measuring isotopes in such small samples. Indeed, SIMS has enabled an entirely new sub-field of astronomy: presolar grains in meteorites. In recent years, the development of the Cameca NanoSIMS ion probe has extended the reach of isotopic measurements to particles as small as 100 nm in diameter, a regime where isotopic precision is strongly limited by the total number of atoms in the sample. Many applications require obtaining isotopic data on large numbers of particles, necessitating the development of automated techniques. One such method is isotopic imaging, wherein images of multiple isotopes are acquired, each containing multiple dispersed particles, and image processing is used to determine isotopic ratios for individual particles. This method is powerful, but relatively inefficient for raster-based imaging on the NanoSIMS. Modern computerized control of instrumentation has allowed for another approach, analogous to commercial automated SEM-EDS particle analysis systems, in which images are used solely to locate particles followed by fully automated grain-by-grain analysis. The first such system was developed on the Carnegie Institution’s Cameca ims-6f, and was used to generate large databases of presolar grains. We have recently developed a similar system for the NanoSIMS, whose high sensitivity allows for smaller grains to be analyzed with less sample consumption than is possible with the 6f system. The 6f and NanoSIMS systems are functionally identical: an image of dispersed grains is obtained with sufficient statistical precision for an algorithm to identify the positions of individual particles, the primary ion beam is deflected to each particle in turn and rastered in a small box around it, an isotopic measurement is performed, and once all particles are analyzed in an image the sample stage is moved and the process repeated. Up to 100s of particles can be analyzed in a day and isotopic precision is identical to that obtained by manual analyses. Unlike a similar system developed independently at the MPI for Chemistry in Mainz, our new NanoSIMS system is fully integrated into the Cameca instrument software. In principle, this will allow for greater measurement flexibility. The system has been successfully used both at Washington University and the Carnegie Institution to locate rare sub-μ m presolar oxide and SiC grains from meteorites. Although preliminary applications are in cosmochemistry, there is great potential for use in other fields (e.g., aerosol research, cell biology).