Development of High-Energy µ-X-ray Fluorescence and X-ray Absorption Fine Structure for the Distribution and Speciation of Rare Earth Elements in Natural Samples

Abstract

Micro-X-ray fluorescence and X-ray absorption fine structure (µ-XRF-XAFS) is one of the most powerful tools to identify the distribution and speciation of trace elements in natural samples with µm spatial resolution. However, conventional µ-XRF-XAFS studies applied to rare earth elements (REEs: lanthanide elements + Y in this study) are mainly limited to their L-edges and L lines (except for Y) that are subject to strong interferences from other elements (mainly transition metals). In this study, we extend µ-XRF-XAFS to the higher energy region (HE-µ-XRF-XAFS) by using an incident X-ray microbeam (size: ca. 1 × 1 µm2) between 38 and 54 keV to realize K-edge excitation lanthanide analysis without interferences from other elements at the BL37XU beamline, SPring-8 (Japan). This method enables us to simultaneously analyze (i) REE patterns (from La to Dy), (ii) XAFS spectra, and (iii) µm-scale distribution of each REE in the natural sample. The proposed method also realizes the simultaneous application of µ-XAFS at low (e.g., Fe K-edge) and high (lanthanide K-edges) energy at the same spot without changing the setup of the µ-XRF-XAFS system using the detuning technique.