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09.06.2023New methodological studies on the analysis of materials using electron microprobe
Two new methodological studies on the analysis of materials using electron microprobe have recently been published, with the participation of Dr. Xavier Llovet, the head of the Electron Microprobe Laboratory at the CCiTUB.
The first study was published in the journal Chemical Geology, titled "Secondary fluorescence effect quantification of EPMA analyses of olivine grains embedded in basaltic glass" while the second study appeared in the journal Microscopy and Microanalysis under the title "Assessing the Accuracy of Mass Attenuation Coefficients for Soft X-ray EPMA"
Correcting the secondary fluorescence effect
Electron microprobe is an analytical technique that allows for the analysis of trace and minor elements in minerals such as olivine, thanks to its excellent spatial resolution (on the order of a few microns), analytical precision, and detection limits. Olivine is one of the primary minerals that crystallize in most magmas and is one of the most abundant components of the Earth's mantle.
The minor and trace elements incorporated into olivine provide valuable information about the conditions of magma generation and evolution. However, in an electron probe microanalysis system, the primary photons generated by the incident electron beam can produce secondary fluorescence in a mineral phase relatively far from the point of electron beam impact. Quantification methods used in commercial equipment do not take this effect into account, resulting in analytical results being affected by a systematic error that can distort the obtained petrogenetic information.
While the secondary fluorescence effect can be corrected using the Monte Carlo simulation method, this method is computationally expensive and time-consuming. In this study, the authors utilized the Monte Carlo simulation method to calculate the secondary fluorescence for the elements Al, Ca, and Ti in olivine crystals up to a hundred microns in diameter and a wide range of compositions in contact with different types of basaltic glass. They obtained a set of analytical expressions that enable the correction of this effect quickly and practically.
Reliability of the mass attenuation coefficient
In the second of the mentioned articles, the accuracy of existing tabulations of mass attenuation coefficients for the so-called soft X-rays, which are X-rays with energies below 1 keV, is investigated. The mass attenuation coefficient is a fundamental and key parameter for the analysis of light elements (Li, Be, B, C, N, O, F) using analytical techniques based on X-ray spectroscopy, such as electron probe microanalysis (EPMA), (micro)X-ray fluorescence, or particle-induced X-ray emission (PIXE).
The novelty of the study lies in not only utilizing experimental data from conventional measurements of mass attenuation coefficients (based on X-ray transmission experiments conducted in synchrotron radiation laboratories) but also incorporating indirect measurements made with electron probe microanalysis. Furthermore, the study examines mass attenuation coefficients derived from very recent first-principles-based calculations and explores the region of the so-called ultralow-energy X-rays, with energies below 100 eV.