RIMS analysis of stardust—trace, isotopic analysis of individual micron-sized SiC grains

Abstract

There exists an important class of analytical problems that requires both sensitivity and discrimination. This class is exemplified by the increasingly stringent demands of the electronic industry for unambiguous quantitative identification of trace impurities in semiconductor materials at high lateral resolution. Recently, particulate analysis, the isotopic and elemental analysis of micron-sized grains, has also begun to interest the analytical community. The difficulty in these two cases arises from the need to make the measurement before consuming the few atoms of the element of interest while discriminating against the vast excess of bulk atoms. Consider trace analysis of one ppm by weight of zirconium in a SiC spherule of 1 μm in diameter. This grain contains approximately 11,000 Zr atoms. For a terrestrial isotopic composition, half of these atoms are 90Zr (the major isotope) while only ∼300 atoms are in the important (as we shall see later) 96Zr isotope. Analysis is, of course, complicated by the need to discriminate bulk species, some of which (such as Si3C) have nominally the same mass as the analyte. Why would such analyses be important? Some primitive meteorites contain presolar dust grains (such as graphite, silicon carbide, nano-diamonds, and corundum) that have survived the formation of the solar system. It is generally believed that these grains condensed in stellar outflows before being incorporated into meteoritic material of our solar system, and that the elemental and isotopic compositions of these grains preserve a nucleosynthetic record of their parent star. Measurements of the elemental composition and isotopic anomalies in these grains provide information both about stellar nucleosynthesis and about the conditions during circumstellar grain formation. The former can be inferred from the isotopic patterns of heavy element trace impurities such as Mo, Zr, and Sr that, although present at levels of only a few thousand atoms, have been measured for the first time using resonant ionization mass spectrometry (RIMS). Currently, isotopic analyses of such grains using the CHARISMA apparatus at Argonne National Laboratory (ANL) are revealing new and important information for the first time about the material that condensed to form our solar system. Grains consistent with formation around thermally pulsing asymptotic giant branch (AGB) stars, such as our sun, make up the majority of grains studied to date. However, more recently several grains have been identified, which are believed to be of supernovae origin. Since each grain represents a unique stellar source, complete elemental and isotopic analysis must be performed on individual grains—no averaging is possible. Only with the power of RIMS are such analyses possible.