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The NanoSIMS as the Newest Tool for the Microanalysis of Extraterrestrial Materials
The new NanoSIMS at Washington University's Laboratory for Space Sciences may be the best demonstration of Bob Walker's ongoing commitment to groundbreaking research in extraterrestrial materials analysis and beyond. It was his combination of scientific instincts, steadfastness and enthusiasm which made the acquisition of this revolutionary new ion microprobe possible. The NanoSIMS was installed in our laboratory in December 2000 after several years of intensive testing with a prototype instrument at the manufacturer's factory in Paris. From our perspective, there are three important characteristics which make this instrument uniquely suited for our isotopic studies of sub-micrometer particles: (1) a primary ion beam with a diameter as low as 30 nm, (2) a secondary ion transmission at high mass resolution that is almost two orders of magnitude higher than that of our older ims3f ion microprobe, and (3) the capability to measure several secondary ion signals simultaneously. Although we had a number of technical problems to deal with during the first year of operation, we have been very successful with the NanoSIMS and were able to perform a number of measurements that would not have been possible with any other instrument. Among the new results from our group are measurements of individual sub-micrometer presolar grains, detailed isotopic images of the sub-components of interplanetary dust particles and presolar graphite grains and the discovery of presolar silicates in interplanetary dust particles. One of the most fascinating aspects of the work with the NanoSIMS is its potential to be used as a complement to the transmission electron microscope (TEM). Previous generations of ion microprobes could determine the isotopic compositions of micrometer-sized (and larger) dust grains, but it was not possible to determine the mineralogical context of the observed anomalies due to differences in size scales and sample mounting requirements. With the NanoSIMS it is now not only possible to measure isotopes at a spatial resolution that comes close to that of the TEM, but it is possible to do so in the same samples. We have developed a sample mounting technique that allows us to perform NanoSIMS measurement directly on TEM slices, without any modifications to the sample itself. A sample can first be characterized in the TEM and then be transferred to the NanoSIMS for raster imaging SIMS analysis. It is thus possible to directly correlate the isotopic and mineralogical information about a sample and its sub-components on a scale of 100 nm. This combination of two powerful microanalytical techniques may become the one of the strongest tools for the analysis of extraterrestrial materials. |