Using Auger Spectroscopy to Characterize Sub-micrometer Presolar Grains in Situ: an Overview

Introduction: Presolar grains have isotopic compositions that clearly identify them as stellar condensates which survived prolonged residency in the interstellar medium before being incorporated into Solar System materials. Many such grains are first identified during in situ isotope imaging searches in various types of primitive extraterrestrial materials. To constrain a given particle’s possible origins, it is beneficial to get compositional information beyond its isotopic makeup. Since most presolar grains are sub-micrometer in size, only a few analytical techniques are suitable for this purpose. We have previously shown that scanning Auger spectroscopy represents an excellent tool for the elemental characterization of presolar materials that have been identified by NanoSIMS isotopic imaging searches [1]. We have now used this combination of techniques for in situ isotopic and elemental characterizations of sub-micrometer presolar grains in primitive meteorites [2, 3, 4], Antarctic micrometeorites [5], and interplanetary dust particles (IDPs) [2, 6]. Here, we give an overview of these studies and discuss the qualitative and quantitative identification of presolar grain types. Analytical Details: A scanning Auger spectrometer is an electron beam instrument similar to an electron microprobe, but it uses the characteristic energies of Auger electrons and not those of x-rays for the determination of a sample’s composition. The elemental information obtained during scanning Auger imaging measurements has a spatial resolution of tens of nanometers which is comparable to what can be achieved in NanoSIMS isotope measurements (50 100 nm). Such a spatial resolution is significantly higher than what can be reached with energy dispersive x-rayanalysis and is more than sufficient for the characterization of typical (200 500 nm) presolar silicate and oxide grains. Another advantage of using Auger spectroscopy in connection with NanoSIMS measurements is that the analyses can be performed on the same, unmodified sample mount and that virtually any sample that is suitable for the NanoSIMS can also be analyzed in the Auger instrument. To test spatial resolution, we prepared a mix of (terrestrial) sub-micrometer quartz and diamond particle standards and then used the Auger spectrometer to image the elemental distributions of C, O and Si (Fig. 1). In this simplified case, the identification of the two grain types from the Auger data is straightforward and the spatial resolution of the elemental images is almost as high as that of the secondary electron image of the same sample. Although the creation of Auger electrons by backscattered electron ionization can lead to a decrease of the spatial resolution (especially at high primary beam energies), this does not interfere with the grain type identification at the given particle size range.