Developing SNMS for full-spectrum high-sensitivity in-situ isotopic analysis of individual comet grains col-

In anticipation of the return of comet (and ISM?) dust grains by the Stardust mission [1] in mid-January next year, Academia Sinica (AS) and Argonne National Laboratory (ANL) have entered into a collaboration to develop instrument and method for the isotopic analysis of these samples. We need to achieve the highest possible sensitivity so that we can analyze individual grains one at a time to the smallest possible size. Only by doing so can we hope to reach one of the main science goals of the mission, namely the recognition of those isotopically distinct grains each carrying the characteristic signature of a particu-lar nucleosynthetic stage of its parent star. In order to facilitate the interpretation of these grains the second requirement of our method is that the measurements must be made over the widest possible mass range before samples exhaustion. For instance, the thermo-nuclear fusion reactions that produced the isotopes of various major elements of a wide mass range required drastically different temperatures. Therefore their abundances could constrain the conditions at greatly varying depth inside the source star hence its structure and evolution. For high sensitivity, we decided to select Secondary Neutral Mass Spectrometry (SNMS) with post-ionization by VUV laser because in the secondary particles released from the sample surface bombarded by primary micro-beam of ions or photons, the neutrals outnumber the ions by a factor of 100-1000. Using a new design of the extraction optics, the ANL team has been able to demonstrate experimentally that the useful yield (i.e. ion detected over atoms consumed) of their improved SNMS with post-ionization over a large vol-ume (3x3x4 mm