Optimizing experimental design, overcoming challenges, and gaining valuable information from the Sb K-edge XANES region

Abstract

There are many challenges associated with collecting, processing, and interpreting high-energy XAS data. The most significant of these are broad spectra, minimal separation of edge positions, and high background owing to the Compton tail. Studies of the Sb system are a particular challenge owing to its complex bonding character and formation of mixed oxidation-state minerals. Furthermore, in environmental samples such as stream sediment containing mine waste, different Sb phases may coexist. Ways to overcome these challenges and achieve accurate and useful information are presented. Our investigations used Sb K-edge X-ray absorption near-edge spectroscopy (XANES) to elucidate Sb geochemical behavior. Several Sb mineral spectra are presented, including Sb sulfosalts, and contrasted based on the different hosting and coordination environments around the Sb atom in the crystal structure. These comparisons lead to the recognition of how the different hosting and coordination environments are manifested in the shape of the Sb mineral spectra. In fact from the shape of the spectra, the occupation of the Sb atom in a single or in multiple crystallographic sites, regardless of whether multiple phases are present in the sample, is discernible. Furthermore, we demonstrate that quantitative information can be derived from the XANES region using linear combination fitting of the derivative spectra, rather than the energy spectra. Particularly useful to the advancement of Sb research is the demonstration that a significant amount of information can be gained from the Sb K-edge XANES region.

Speciation of antimony and arsenic in the soils and plants in an old antimony mine

“…Linear combination fitting analysis (LCF) was used to fit a combination of model compounds to unknown spectra using the Athena program in the IFEFFIT package. This approach is commonly used to determine the relative proportions of model compounds present in heterogenous samples (Fawcett et al, 2009). An energy range of −100 to 150 eV from the K-edge of Sb was used to fit the soil sample with respect to standard references.…”

Distribution, speciation and availability of antimony (Sb) in soils and terrestrial plants from an active Sb mining area

“…Linear combination fitting analysis (LCF) was used to fit a combination of model compounds to unknown spectra. This approach is commonly used to determine the relative proportions of model compounds present in heterogeneous samples (Fawcett et al, 2009). Spectra from the reference compounds Sb 2 S 3 , Sb 2 O 3 , Sb 2 O 5 and Ca[Sb(OH) 6 ] 2 were used for the LCF analysis.…”

“…This lies between the peak maximum of the reference materials Ca [Sb(OH) 6 ] 2 (30 502.9 eV) and Sb 2 O 3 (30 500.1 eV), which indicates the presence of both valence states in the samples. However, it is not possible to distinguish between mixed-oxidation state minerals and single minerals containing Sb(III) or Sb(V) (Fawcett et al, 2009). …”

An integrated study of the chemical composition of Antarctic aerosol to investigate natural and anthropogenic sources

“…While this resolution is less than for other elements with K-edges at lower energies, such as arsenic, the first derivative of the normalised energy spectra allows reliable oxidation state discrimination by linear combination fitting. [15] In contrast, distinguishing the local coordination environment is more challenging, with absorption edge energies of Sb III -O and Sb III -S, for example, separated by only 1.2 eV (Table S3). Fawcett et al [15] claimed that linear combination fitting of the first derivative of the normalised energy spectra could reliably differentiate Sb III -O and Sb III -S in a natural sediment sample.…”

“…[15] In contrast, distinguishing the local coordination environment is more challenging, with absorption edge energies of Sb III -O and Sb III -S, for example, separated by only 1.2 eV (Table S3). Fawcett et al [15] claimed that linear combination fitting of the first derivative of the normalised energy spectra could reliably differentiate Sb III -O and Sb III -S in a natural sediment sample. However, this analysis did not include a robust statistical assessment of fit quality, nor did the standards Fawcett et al used for Sb III -S (i.e.…”