Abstract
X-ray absorption fine-structure (XAFS) spectroscopy can assess the chemical speciation of the elements providing their coordination and oxidation state, information generally hidden to other techniques. In the case of trace elements, achieving a good quality XAFS signal poses several challenges, as it requires high photon flux, counting statistics and detector linearity. Here, a new multi-element X-ray fluorescence detector is presented, specifically designed to probe the chemical speciation of trace 3d elements down to the p.p.m. range. The potentialities of the detector are presented through a case study: the speciation of ultra-diluted elements (Fe, Mn and Cr) in geological rocks from a calcareous formation related to the dispersal processes from Ontong (Java) volcanism (mid-Cretaceous). Trace-elements speciation is crucial in evaluating the impact of geogenic and anthropogenic harmful metals on the environment, and to evaluate the risks to human health and ecosystems. These results show that the new detector is suitable for collecting spectra of 3d elements in trace amounts in a calcareous matrix. The data quality is high enough that quantitative data analysis could be performed to determine their chemical speciation.
Highlights
X-ray absorption spectroscopy (XAS) is an element-selective technique, sensitive to the local coordination chemistry and to the valence state of the absorber, independently from its physical state, crystallographic phase or morphology (Koningsberger & Prins, 1988; Parsons et al, 2002; Mastelaro & Zanotto, 2018; Kuzmin & Chaboy, 2014)
XAS is widely exploited in geological and environmental sciences where the materials of interest are often highly heterogeneous both in chemical composition and physical status, presenting mixtures ranging from crystalline to amorphous phases
The variability of trace elements in rocks is a useful proxy for insights into geological events such as regional dispersion of 3d metals over paleoceans and disruptive volcanic events (Kravtsova, 2020; Terzano et al, 2019)
Summary
X-ray absorption spectroscopy (XAS) is an element-selective technique, sensitive to the local coordination chemistry and to the valence state of the absorber, independently from its physical state, crystallographic phase (crystalline, amorphous) or morphology (Koningsberger & Prins, 1988; Parsons et al, 2002; Mastelaro & Zanotto, 2018; Kuzmin & Chaboy, 2014). The MAIA detector for instance (Ryan et al, 2014; Chen et al, 2017) is an excellent example of a custom-made solution featuring an array of 20 Â 20 individual silicon diodes or small-sized silicon drift detectors with 1 mm active area Another system developed by academic and industrial partners is the ARDESIA detector, based on a monolithic array of four circular or squarish SDDs (collimated active area of 12.56 mm or 15 mm2), which has been successfully installed and tested at different synchrotron radiation beamlines (Bellotti et al, 2016; Hafizh et al, 2019). The research has focused on the study of ultra-diluted systems through XAS in fluorescence mode and aimed at achieving: (i) quantum efficiency higher than 80%, in particular in the low-energy region of the beamline (2.4–10 keV), (ii) both room-temperature operation and cooling of the sensors, suitable for automated beam time use, (iii) coverage of a wide solid angle of ! The very high dilution of the elements of interest, in particular for Cr reaching the p.p.m. concentration, represents a valuable stress test for the device
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