Assessment of sulfur and iron speciation in a soil aggregate by combined S and Fe micro‐XANES: microspatial patterns and relationships

To test whether synchrotron‐based spectromicroscopy can be used to identify spatial patterns of sulfur (S) and iron (Fe) speciation as well as relationships between the speciation of S and Fe in soil colloids or aggregates at the micrometre and sub‐micrometre level, an anoxically prepared dissected soil aggregate (size ～1 mm³) was analyzed by µ‐XANES at the K‐edges of S (2472 eV) and Fe (7112 eV). The experiment included (i) elemental mapping at the S K‐edge (S, Si, Al) and the Fe K‐edge (Fe, Si), (ii) acquisition of 300 µm × 300 µm images of the region of interest with X‐ray energies of 2474 eV (addressing reduced organic and inorganic S), 2483 eV (total S), 7121 eV (divalent Fe) and 7200 eV (total Fe), as well as (iii) acquisition of S and Fe µ‐XANES spectra at two different positions, where image analysis suggested the dominance of reduced and oxidized S and Fe, respectively. Image analysis revealed a heterogeneous distribution of total Si, S and Fe as well as of different S and Fe species in the aggregate. Microregions which were either enriched in reduced or in oxidized S and Fe could be identified. A microregion with a large contribution of oxidized S (sulfate, sulfonate) to total S contained exclusively Fe(III) oxyhydroxides (probably ferrihydrite) as S‐bearing phase, whereas another microregion with a large contribution of reduced organic S (thiol, organic disulfide) to total S contained a small amount of Fe(II)‐bearing silicate in addition to the dominating Fe(III) oxyhydroxides. Our results show that combined S and Fe µ‐XANES is a powerful tool for studying microscale spatial patterns of S and Fe speciation as well as microscale relationships between the speciation of S and Fe in soil aggregates.     

K‐edge XANES analysis of sulfur compounds: an investigation of the relative intensities using internal calibration

Sulfur K‐edge XANES (X‐ray absorption near‐edge structure) spectroscopy is an excellent tool for determining the speciation of sulfur compounds in complex matrices. This paper presents a method to quantitatively determine the kinds of sulfur species in natural samples using internally calibrated reference spectra of model compounds. Owing to significant self‐absorption of formed fluorescence radiation in the sample itself the fluorescence signal displays a non‐linear correlation with the sulfur content over a wide concentration range. Self‐absorption is also a problem at low total absorption of the sample when the sulfur compounds are present as particles. The post‐edge intensity patterns of the sulfur K‐edge XANES spectra vary with the type of sulfur compound, with reducing sulfur compounds often having a higher post‐edge intensity than the oxidized forms. In dilute solutions (less than 0.3–0.5%) it is possible to use sulfur K‐edge XANES reference data for quantitative analysis of the contribution from different species. The results show that it is essential to use an internal calibration system when performing quantitative XANES analysis. Preparation of unknown samples must take both the total absorption and possible presence of self‐absorbing particles into consideration.     

Relationship between the structural distortion and the Mn electronic state in La1−xCaxMnO3: a Mn K‐edge XANES study

A theoretical study of the X‐ray absorption near‐edge structure (XANES) spectra at the Mn K‐edge in the La_1?xCa_xMnO₃ series is reported. The relationship between the edge shift, the Ca–La substitution and the distortion of the MnO₆ octahedra in these systems has been studied. It is shown that, by correctly considering these effects simultaneously, the experimental XANES data are consistent with the presence of two different Mn local environments in the intermediate La_1?xCa_xMnO₃ compounds. By taking into account the energy shift associated with the modification of the MnO₆ distortion as Ca substitutes for La, it is possible to reproduce the XANES spectra of the intermediate‐doped compounds starting from the experimental spectra of the end‐members LaMnO₃ and CaMnO₃. These results point out the need to re‐examine the conclusions derived in the past from the simple analysis of the Mn K‐edge XANES edge‐shift in these materials. In particular, it is shown that the modification of the Mn K‐edge absorption through the La_1?xCa_xMnO₃ series is well reproduced by considering the simultaneous presence of both distorted and undistorted octahedra and, consequently, that the existence of charge‐ordering phenomena cannot be ruled out from the XANES data.     

Phosphorus L2,3‐edge XANES: overview of reference compounds

Synchrotron‐based X‐ray absorption near‐edge structure (XANES) spectroscopy is becoming an increasingly used tool for the element speciation in complex samples. For phosphorus (P) almost all XANES measurements have been carried out at the K‐edge. The small number of distinctive features at the P K‐edge makes in some cases the identification of different P forms difficult or impossible. As indicated by a few previous studies, the P L_2,3‐edge spectra were richer in spectral features than those of the P K‐edge. However, experimentally consistent spectra of a wide range of reference compounds have not been published so far. In this study a library of spectral features is presented for a number of mineral P, organic P and P‐bearing minerals for fingerprinting identification. Furthermore, the effect of radiation damage is shown for three compounds and measures are proposed to reduce it. The spectra library provided lays a basis for the identification of individual P forms in samples of unknown composition for a variety of scientific areas.     

Reference spectra of important adsorbed organic and inorganic phosphate binding forms for soil P speciation using synchrotron‐based K‐edge XANES spectroscopy

Direct speciation of soil phosphorus (P) by linear combination fitting (LCF) of P K‐edge XANES spectra requires a standard set of spectra representing all major P species supposed to be present in the investigated soil. Here, available spectra of free‐ and cation‐bound inositol hexakisphosphate (IHP), representing organic P, and of Fe, Al and Ca phosphate minerals are supplemented with spectra of adsorbed P binding forms. First, various soil constituents assumed to be potentially relevant for P sorption were compared with respect to their retention efficiency for orthophosphate and IHP at P levels typical for soils. Then, P K‐edge XANES spectra for orthophosphate and IHP retained by the most relevant constituents were acquired. The spectra were compared with each other as well as with spectra of Ca, Al or Fe orthophosphate and IHP precipitates. Orthophosphate and IHP were retained particularly efficiently by ferrihydrite, boehmite, Al‐saturated montmorillonite and Al‐saturated soil organic matter (SOM), but far less efficiently by hematite, Ca‐saturated montmorillonite and Ca‐saturated SOM. P retention by dolomite was negligible. Calcite retained a large portion of the applied IHP, but no orthophosphate. The respective P K‐edge XANES spectra of orthophosphate and IHP adsorbed to ferrihydrite, boehmite, Al‐saturated montmorillonite and Al‐saturated SOM differ from each other. They also are different from the spectra of amorphous FePO₄, amorphous or crystalline AlPO₄, Ca phosphates and free IHP. Inclusion of reference spectra of orthophosphate as well as IHP adsorbed to P‐retaining soil minerals in addition to spectra of free or cation‐bound IHP, AlPO₄, FePO₄ and Ca phosphate minerals in linear combination fitting exercises results in improved fit quality and a more realistic soil P speciation. A standard set of P K‐edge XANES spectra of the most relevant adsorbed P binding forms in soils is presented.     

Sample thickness and quantitative concentration measurements in Br K‐edge XANES spectroscopy of organic materials

While XANES spectroscopy is an established tool for quantitative information on chemical structure and speciation, elemental concentrations are generally quantified by other methods. The edge step in XANES spectra represents the absolute amount of the measured element in the sample, but matrix effects and sample thickness complicate the extraction of accurate concentrations from XANES measurements, particularly at hard X‐ray energies where the X‐ray beam penetrates deeply into the sample. The present study demonstrates a method of quantifying concentration with a detection limit approaching 1 mg kg^?1 using information routinely collected in the course of a hard X‐ray XANES experiment. The XANES normalization procedure unambiguously separates the signal of the absorber from any source of background. The effects of sample thickness on edge steps at the bromine K‐edge were assessed and an empirical correction factor for use with samples of variable mass developed.     

An attempt at kidney stone analysis with the application of synchrotron radiation

X‐ray absorption near‐edge spectroscopy (XANES) is a spectroscopic technique using synchrotron light to determine the valence state of excited atoms as well as the electronegativity of their neighbouring atoms. XANES spectra can provide information about the chemical bond in the second coordination shell of the excited atom. In this study, XANES spectra of unknown compounds from human kidney stones were recorded around the K‐edges of sulfur, phosphorus and calcium. The XANES results agree well with the diffractogram data of the same stones obtained through an X‐ray powder diffraction (XRPD) technique. By comparing the measurement techniques presented here, it is shown that XANES requires a smaller amount of each sample than XRPD for analysis.     

Polycapillary‐optics‐based micro‐XANES and micro‐EXAFS at a third‐generation bending‐magnet beamline

A focusing system based on a polycapillary half‐lens optic has been successfully tested for transmission and fluorescence µ‐X‐ray absorption spectroscopy at a third‐generation bending‐magnet beamline equipped with a non‐fixed‐exit Si(111) monochromator. The vertical positional variations of the X‐ray beam owing to the use of a non‐fixed‐exit monochromator were shown to pose only a limited problem by using the polycapillary optic. The expected height variation for an EXAFS scan around the Fe K‐edge is approximately 200 µm on the lens input side and this was reduced to ～1 µm for the focused beam. Beam sizes (FWHM) of 12–16 µm, transmission efficiencies of 25–45% and intensity gain factors, compared with the non‐focused beam, of about 2000 were obtained in the 7–14 keV energy range for an incoming beam of 0.5 × 2 mm (vertical × horizontal). As a practical application, an As K‐edge µ‐XANES study of cucumber root and hypocotyl was performed to determine the As oxidation state in the different plant parts and to identify a possible metabolic conversion by the plant.     

A setup for synchrotron‐radiation‐induced total reflection X‐ray fluorescence and X‐ray absorption near‐edge structure recently commissioned at BESSY II BAMline

An automatic sample changer chamber for total reflection X‐ray fluorescence (TXRF) and X‐ray absorption near‐edge structure (XANES) analysis in TXRF geometry was successfully set up at the BAMline at BESSY II. TXRF and TXRF‐XANES are valuable tools for elemental determination and speciation, especially where sample amounts are limited (<1 mg) and concentrations are low (ng ml^?1 to µg ml^?1). TXRF requires a well defined geometry regarding the reflecting surface of a sample carrier and the synchrotron beam. The newly installed chamber allows for reliable sample positioning, remote sample changing and evacuation of the fluorescence beam path. The chamber was successfully used showing accurate determination of elemental amounts in the certified reference material NIST water 1640. Low limits of detection of less than 100 fg absolute (10 pg ml^?1) for Ni were found. TXRF‐XANES on different Re species was applied. An unknown species of Re was found to be Re in the +7 oxidation state.     

Solid energy calibration standards for P K‐edge XANES: electronic structure analysis of PPh4Br

P K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy is a powerful method for analyzing the electronic structure of organic and inorganic phosphorus compounds. Like all XANES experiments, P K‐edge XANES requires well defined and readily accessible calibration standards for energy referencing so that spectra collected at different beamlines or under different conditions can be compared. This is especially true for ligand K‐edge X‐ray absorption spectroscopy, which has well established energy calibration standards for Cl (Cs₂CuCl₄) and S (Na₂S₂O₃·5H₂O), but not neighboring P. This paper presents a review of common P K‐edge XANES energy calibration standards and analysis of PPh₄Br as a potential alternative. The P K‐edge XANES region of commercially available PPh₄Br revealed a single, highly resolved pre‐edge feature with a maximum at 2146.96 eV. PPh₄Br also showed no evidence of photodecomposition when repeatedly scanned over the course of several days. In contrast, we found that PPh₃ rapidly decomposes under identical conditions. Density functional theory calculations performed on PPh₃ and PPh₄⁺ revealed large differences in the molecular orbital energies that were ascribed to differences in the phosphorus oxidation state (III versus V) and molecular charge (neutral versus +1). Time‐dependent density functional theory calculations corroborated the experimental data and allowed the spectral features to be assigned. The first pre‐edge feature in the P K‐edge XANES spectrum of PPh₄Br was assigned to P 1s → P‐C π* transitions, whereas those at higher energy were P 1s → P‐C σ*. Overall, the analysis suggests that PPh₄Br is an excellent alternative to other solid energy calibration standards commonly used in P K‐edge XANES experiments.     

Development of a two‐dimensional imaging system of X‐ray absorption fine structure

A two‐dimensional imaging system of X‐ray absorption fine structure (XAFS) has been developed at beamline BL‐4 of the Synchrotron Radiation Center of Ritsumeikan University. The system mainly consists of an ionization chamber for I₀ measurement, a sample stage, and a two‐dimensional complementary metal oxide semiconductor (CMOS) image sensor for measuring the transmitted X‐ray intensity. The X‐ray energy shift in the vertical direction, which originates from the vertical divergence of the X‐ray beam on the monochromator surface, is corrected by considering the geometrical configuration of the monochromator. This energy correction improves the energy resolution of the XAFS spectrum because each pixel in the CMOS detector has a very small vertical acceptance of ～0.5 µrad. A data analysis system has also been developed to automatically determine the energy of the absorption edge. This allows the chemical species to be mapped based on the XANES feature over a wide area of 4.8 mm (H) × 3.6 mm (V) with a resolution of 10 µm × 10 µm. The system has been applied to the chemical state mapping of the Mn species in a LiMn₂O₄ cathode. The heterogeneous distribution of the Mn oxidation state is demonstrated and is considered to relate to the slow delocalization of Li⁺‐defect sites in the spinel crystal structure. The two‐dimensional‐imaging XAFS system is expected to be a powerful tool for analyzing the spatial distributions of chemical species in many heterogeneous materials such as battery electrodes.     

Application of Synchrotron X‐Ray Microbeam Spectroscopy to the Determination of Metal Distribution and Speciation in Biological Tissues

Abstract

Resolving the distribution and speciation of metal(loid)s within biological environmental samples is essential for understanding bioavailability, trophic transfer, and environmental risk. We used synchrotron x‐ray microspectroscopy to analyze a range of samples that had been exposed to metal(loid) contamination. Microprobe x‐ray fluorescence elemental mapping (µSXRF) of decomposing rhizosphere microcosms consisting of Ni‐ and U‐contaminated soil planted with wheat (Triticum aestivum) showed the change in Ni and U distribution over a 27‐day period, with a progressive movement of U into decaying tissue. µSXRF maps showed the micrometer‐scale distribution of Ca, Mn, Fe, Ni, and U in roots of willow (Salix nigra L.) growing on a former radiological settling pond, with U located outside of the epidermis and Ni inside the cortex. X‐ray computed tomography (CMT) of woody tissue of this same affected willow showed that small points of high Ni fluorescence observed previously are actually a Ni‐rich substance contained within an individual xylem vessel. µSXRF and x‐ray absorption near‐edge spectroscopy (XANES) linked the elevated Se concentrations in sediments of a coal fly ash settling pond with oral deformities of bullfrog tadpoles (Rana catesbeiana). Se distribution was localized within the deformed mouthparts, and with an oxidation state of Se (?II) consistent with organo‐Se compounds, it suggests oral deformities are caused by incorporation of Se into proteins. The range of tissues analyzed in this study highlight the applicability of synchrotron X‐ray microspectroscopic techniques to biological tissues and the study of metal(loid) bioavailability.     

Magnesium K‐edge XANES spectroscopy of geological standards

Magnesium K‐edge X‐ray absorption near‐edge structure (XANES) spectra have been investigated to develop a systematic understanding of a suite of Mg‐bearing geological materials such as silicate and carbonate minerals, sediments, rocks and chemical reagents. For the model compounds the Mg XANES was found to vary widely between compounds and to provide a fingerprint for the form of Mg involved in geologic materials. The energy positions and resonance features obtained from these spectra can be used to specify the dominant molecular host site of Mg, thus shedding light on Mg partitioning and isotope fractionation in geologic materials and providing a valuable complement to existing knowledge of Mg geochemistry.     

MnxSi1-x磁性薄膜的结构研究

利用X射线衍射(XRD)和X射线吸收近边结构(XANES)方法研究了在Si(100)衬底上及600℃温度条件下用分子束外延(MBE)共蒸发方法生长的Mn_xSi_1-x磁性薄膜的结构.由XRD结果表明,只有在高Mn含量(8％和17％)样品中存在着Mn₄Si₇化合物物相.而XANES结果则显示,对于Mn浓度在0.7%到17%之间的Mn_xSi_1-x样品,其Mn原子的XANES谱表现出了一致的谱线特征.基于多重散射的XANES理论计算进一步表明,只有根据Mn₄Si₇模型计算出的理论XANES谱才能够很好的重构出Mn_xSi_1-x样品的实验XANES谱.这些研究结果说明在Mn_xSi_1-x样品中,Mn原子主要是以镶嵌式的Mn₄Si₇化合物纳米晶颗粒存在于Si薄膜介质中,几乎不存在间隙位和替代位的Mn原子. 关键词： xSi_1-x磁性薄膜')" href="#">Mn_xSi_1-x磁性薄膜 分子束外延 XRD XANES     

Investigation of annealing‐induced oxygen vacancies in the Co‐doped ZnO system by Co K‐edge XANES spectroscopy

To clarify the mechanism of the observed room‐temperature ferromagnetism (RTF), many studies have been focused on dilute magnetic semiconductor systems. Several investigations have demonstrated that oxygen vacancies play a significant role in mediating the RTF behavior so that much effort has been devoted to confirm their presence. In this investigation, X‐ray absorption spectroscopy was combined with ab initio calculations of the electronic structure of Co and Zn in the Zn_0.9Co_0.1O system before and after annealing, which has been recognized as an effective method of originating oxygen vacancies. A feature at about 20 eV after the rising edge of the Co K‐edge XANES that disappears after annealing has been associated with the presence of an oxygen vacancy located in the second shell surrounding the Co atom. Moreover, Zn K‐edge XANES spectra point out that this oxygen vacancy affects the electronic structure near the Fermi level, in agreement with density functional theory calculations.     

Local electronic structures at selected sites of intermetallic perovskites Mn3MeX (Me=divalent metal,X=N,C)

Summary We have measured the XANES (X-ray absorption near edge structure) spectra on the manganese and metal MeK-edges of the intermetallic perowskites Mn₃MeX (X=N, Me=Mn, Cu, Zn, Ga, Sn; X=C, Me=Ga, Sn) to investigate the distribution on the partial density of the empty states. The information on the local electronic structure at the two differents sites, obtained from the spectra in the first 10 eV above the onset, is discussed in terms of one-electron band model. The relative shape changes in the spectra between the different compounds are explained by the different magnitude of hybridization between the electronic states of different atoms and/or the filling of the empty band by the electrons supplied by the substituted metal. Moreover, information about the local geometrical structure has been obtained from the (10÷50) eV range of the spectra.

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Riassunto La distribuzione della densità parziale degli stati vuoti nelle perovskiti intermetalliche Mn₃MeX (X=N, Me=Mn, Cu, Zn, Ga, Sn; X=C, Me=Ga, Sn) è stata studiata mediante spettroscopia XANES (X-ray absorption near edge structure) alle soglieK del Mn e del metallo Me. Le informazioni sulla struttura elettronica locale di entrambi i siti, ottenute dall’analisi dei primi 10 eV degli spettri XANES, sono discussed in termini di un modello proposto. Il cambiamento relativo della forma negli spetttri dei diversi composti è posto in relazione alla differente ibridizzazione tra stati di atomi diversi e/o dal riempimento degli stati della banda vuota con elettroni forniti dal metallo sostituente. Sono state anche ottenute informazioni sulla struttura geometrica locale analizzando gli spettri nell’intervallo da 10 a 50 eV.

     

Classification of Fe‐bearing species from K‐edge XANES data using two‐parameter correlation plots

Solid iron compounds are extremely common in the environment as well as in meteorites and comets. Fe K‐edge XANES (X‐ray absorption near‐edge structure) measurements can be carried out quickly, theoretically allowing one to categorize many areas within a sample or set of samples in a short time. However, interpretation of such data is not straightforward unless one has the appropriate reference spectra, hence a way of classifying an unknown spectrum to a family group (trivalent, divalent, oxide, silicate etc.) is required. Methods of abstracting Fe XANES spectra to produce pairs of variables which, when plotted, cluster in distinct regions depending on the family are presented. For instance, divalent minerals fall in a different region than trivalent minerals, and sulfides in a different region than oxides.     

Influence of crystallographic orientation of biogenic calcite on in situ Mg XANES analyses

Micro X‐ray absorption near‐edge spectroscopy at the Mg K‐edge is a useful technique for acquiring information about the environment of Mg²⁺ in biogenic calcite. These analyses can be applied to shell powders or intact shell structures. The advantage of the latter is that the XANES analyses can be applied to specific areas, at high (e.g. micrometre) spatial resolution, to determine the environment of Mg²⁺ in a biomineral context. Such in situ synchrotron analysis has to take into account the potential effect of crystallographic orientation given the anisotropy of calcite crystals and the polarized nature of X‐rays. Brachiopod shells of species with different crystallographic orientations are used to assess this crystallographic effect on in situ synchrotron measurements at the Mg K‐edge. Results show that, owing to the anisotropy of calcite, in situ X‐ray absorption spectra (XAS) are influenced by the crystallographic orientation of calcite crystals with a subsequent potentially erroneous interpretation of Mg²⁺ data. Thus, this study demonstrates the importance of crystallography for XAS analyses and, therefore, the necessity to obtain crystallographic information at high spatial resolution prior to spectroscopic analysis.     

Development of micro‐XANES mapping in the diamond anvil cell

Energy‐dispersive X‐ray absorption spectroscopy is now a well established method that has been applied to a broad range of applications. At the energy‐dispersive EXAFS beamline of the ESRF, ID24, the recently achieved 5 × 5 µm focal spot combined with fast acquisition has allowed complex and non‐uniform samples to be mapped and images to be obtained where each pixel contains full XAS information. This method has been applied to a study under extreme conditions of pressure and temperature in a diamond anvil cell in transmission mode. The case study was the investigation of the Fe K‐edge XANES of (Mg,Fe)SiO₃‐perovskite and (Mg,Fe)O‐ferropericlase on decomposition of the spinel‐structured olivine [γ‐(Mg,Fe)₂SiO₄] at 78 (3) GPa after laser heating at 2200 (100) K.     

Solid‐phase cadmium speciation in soil using L3‐edge XANES spectroscopy with partial least‐squares regression

Cadmium (Cd) has a high toxicity and resolving its speciation in soil is challenging but essential for estimating the environmental risk. In this study partial least‐square (PLS) regression was tested for its capability to deconvolute Cd L₃‐edge X‐ray absorption near‐edge structure (XANES) spectra of multi‐compound mixtures. For this, a library of Cd reference compound spectra and a spectrum of a soil sample were acquired. A good coefficient of determination (R²) of Cd compounds in mixtures was obtained for the PLS model using binary and ternary mixtures of various Cd reference compounds proving the validity of this approach. In order to describe complex systems like soil, multi‐compound mixtures of a variety of Cd compounds must be included in the PLS model. The obtained PLS regression model was then applied to a highly Cd‐contaminated soil revealing Cd₃(PO₄)₂ (36.1%), Cd(NO₃)₂·4H₂O (24.5%), Cd(OH)₂ (21.7%), CdCO₃ (17.1%) and CdCl₂ (0.4%). These preliminary results proved that PLS regression is a promising approach for a direct determination of Cd speciation in the solid phase of a soil sample.