Synchrotron X-ray Fluorescence Analysis of Dust Particles

Introduction: Synchrotron x-ray fluorescence (SXRF) studies carried out on a microprobe endstation allow non-destructive analysis of both major and trace element abundances in particles only microns in size. The technique has been applied to IDPs captured in the stratosphere to study atmospheric entry effects such as stratospheric contamination and frictional heating [1, 2]. SXRF will also be applied to the analysis of samples from NASA’s Stardust mission [3] returning to Earth in January of 2006 with cometary dust particles captured in silica aerogel collectors during its flyby of the comet Wild-2. For Stardust particles, as for IDPs, trace element analysis of volatiles may provide an indication of the degree of thermal processing during capture in the aerogel collectors. Assuming negligible processing, trace elements can be used to study petrogenetic relationships between Stardust particles and other known extraterrestrial materials. In general, analysis of trace element abundances in extraterrestrial materials is highly relevant because variations in concentration patterns of these elements may act as signatures for origin and conditions of formation of the carrier mineral phases [4]. A number of challenges must be overcome for trace element studies by SXRF: sample preparation, contamination, choice of sample mount, potential for sample damage in the x-ray beam and positional stability of the beam on the sample. IDP SXRF studies to date offer only limited discussion of experimental setup and analytical limitations. We discuss here the experimental requirements for obtaining high quality SXRF data on small particles for trace element analysis and our progress in achieving this goal. Experimental Methods: We collected SXRF data from several chondritic porous IDPs on a scanning fluorescence microprobe endstation currently being commissioned on Beamline 6-2 at the Stanford Synchrotron Radiation Laboratory (SSRL). The final focus is provided by a Kirkpatrick-Baez mirror pair capable of focusing monochromatic xrays into a micron-sized spot. The current nonoptimized photon flux for a ~3 μm spot is ~5x10 photons/s, and the x-ray energy range extends past the Br K absorption edge. We have previously demonstrated SXRF mapping of major elements at low and high spatial resolutions [5]. For this early trace element work during endstation commissioning, there was unusual beam motion at the source, and we used a ~10 μm spot size to encompass entire IDP particles and sections. In future work, we will combine trace element mapping with TEM imaging and EDS analysis as discussed below. Experimental Requirements and Results: To obtain the best quality SXRF data on small particles, we want no sample damage and high sample stability with no extraneous fluorescence to obscure peaks of interest and reduce signal-to-noise ratios.