Application of X-ray absorption near-edge structure (XANES) using bent crystal analyzer to speciation of trace Os in iron meteorites

Abstract

X-ray absorption near-edge structure (XANES) spectroscopy at the Os L III-edge was measured for the speciation of trace Os in two iron meteorites, Negrillos and Bennett County, which contain 65.7 and 58.3 mg/kg of Os, respectively, while major elements, Fe + Ni, are more than 95 wt.%. During Os XANES measurement, intense Fe–K and Ni–K fluorescence and scattered X-rays existed as background noise, which were more than four orders higher than Os Lα fluorescence. To reject the unwanted X-rays, a log spiral bent crystal analyzer in Bragg geometry for X-ray fluorescence detection combined with a single-element pure Ge detector and slits was used. The system enabled us to obtain the Os L III-edge XANES spectra of the two iron meteorites with greater quality compared with those recorded in a conventional fluorescence mode by a 19-element Ge detector without analyzer crystals. In addition, a commercially available crystal analyzer (BCLA, Oxford danfysik) in Laue geometry was also tested, and it also gave similar high-quality XANES spectra. The Bragg geometry reduced the noise due to background X-rays more effectively than the Laue system, while the Laue geometry was efficient to collect target fluorescence X-ray, suggesting that a better spectrum in terms of statistics could be obtained using a Laue analyzer coupled with a multi-element Ge detector with a large acceptance area for fluorescence detection. The reliable spectral shape of XANES measured enabled us to simulate XANES spectra of Os in iron meteorites in a highly quantitative manner. A system using log spiral crystal analyzer coupled with a Ge detector is a powerful tool for the speciation of trace elements in natural minerals and rocks.