第七届青年地学论坛

Confocal micro-X-ray fluorescence imaging (CMXRFI), an emerging technique using hard X-rays, is gaining increased use for characterizing interior elemental distributions of intact particles without physically breaking down the particles. The attenuated intensity of X-ray fluorescence (XRF) requires correction prior to data interpretation. Here a beam-path based method was developed using the Beer-Lambert Law by accounting for the geometry of the CMXRFI setup. The method is developed for application to both heterogeneous and homogeneous samples. The method was verified by two NIST standard reference materials and was applied for two heterogeneous environmental samples (biochars). The mass distribution of elements is assumed to be the same as the XRF intensity distribution of the element in each pixel, and the heterogeneity of a sample is expressed based on this assumption. Density and elemental compositions are calculated for each pixel. The beam path in each pixel is calculated based on the geometry of the CMXRFI setup. The attenuated intensity is then corrected using the Beer-Lambert Law. The XRF intensity used to calculate the mass distribution is updated iteratively using the corrected values from the previous step, until the difference between the current and previous step is less than a threshold value.
One of the potential applications of CMXRFI is redox mapping of elements of interest with depth information. Redox mapping, a type of chemical mapping using X-ray absorption spectroscopy, has been used for speciation mapping of thin-sections using traditional micro-XRF mapping. However, the obtained maps do not provide depth information of the distribution of the element of interest. Here, a procedure is presented for data collection and processing (linear combination fitting) for redox mapping using CMXRFI. The procedure was applied to a biochar particle reacted with Cr(VI)-spiked water. Redox mapping was conducted at 33 energies. Linear combination fitting was performed for the spectra from each pixel, and the fraction and absorbance of each Cr species were calculated. The results indicate Cr(III) is the primary Cr species with fractions ranging from 0.6 to 1 and that this fraction is greater in the interior pixels of the particle than at the surface; in contrast, the Cr(VI) fraction is greater at the surface than for interior pixels. The absorbance map indicates total Cr, Cr(III), and Cr(VI) primarily accumulate at the surface. The method can also be applied to data collection and processing of redox or chemical mapping obtained using conventional µ-XRF and for other elements. The method can potentially be applied in chemistry, biology, and the material, earth, and environmental sciences.

同步辐射X射线,共聚焦,氧化还原成像