Synchrotron-based Spectroscopic Studies of Metal Species in Solid Phases: the Case of Arsenic

The production of high-energy (1 KeV to 40 KeV) x-radiation at synchrotron laboratories around the world has enabled the spectroscopic analysis of core level electronic transitions and electronic scattering processes in trace (< 1000 ppm) metal and metalloid atoms as they are found in heterogeneous natural materials (e.g., soils, sediments, plants, and microbial mats). Environmental geochemists interested in arsenic (As) have found this information of particular interest, because the oxidation state(s) and chemical coordination of As can be studied in materials that have been difficult to study using other techniques. The primary technique used for the past 10 years has been bulk X-ray absorption fine structure spectroscopy (XAFS), but this method is becoming increasingly augmented/supplanted by microbeam XAFS and X-ray fluorescence techniques. X-ray absorption fine structure (XAFS) spectroscopy has characteristics that make it the technique of choice for the analysis of environmentally-relevant solid materials: • Can be “element-specific”; • Low detection limits (relative to lab-based X-ray fluorescence and electron microprobe techniques); • Species can be analyzed under ambient or sample-specific conditions (presence/absence of O2, frozen, wet, or dry); • Direct information about As oxidation state; • Direct information about As coordination environment (number and distance of neighboring atoms) [1].