Synchrotron-based X-ray Fluorescence Research Articles

Preliminary investigation of trace element in Pterygium using Synchrotron radiation micro-beam X-ray fluorescence analysis (-XRF)

We have assessed the relative content and distribution of Iron and Zinc elements using microbeam synchrotron radiation X-ray fluorescence technique. One such technique is X-ray fluorescence (XRF), which has been used previously to map trace elements distribution in Physical samples. In this article a compromise is suggested in issue Pterygium samples. In this study, a prospective randomized clinical trial was conducted. Serial frozen sections of pterygium tissues and conjunctival tissues of 40 μm thickness were collected from 8 patients £¨10 eyes£©undergoing pterygium excision combine with limbal stem cell transplantation. A synchrotron-based XRF microprobe was used to map the distribution of Fe and Zn in whole frozen pterygium sections. The frozen sections were tested by synchrotron radiation X-ray fluorescence technique. These experiments were performed at BL15U in Shanghai, China. Then, the results have palyed that Iron and Zinc were present in both pterygium tissues and normal conjunctiva tissues (relevance ratio 100%). The contents of Iron and Zinc in normal conjunctiva tissues were significantly lower than in pterygium tissues (P < 0.05). The microelements were mostly clusteredin the pterygium tissues, while sparsely distributed in the normal conjunctiva tissue. Finally, we found that XRF imaging will be useful for mapping elemental distribution in Pterygium tissues. 40 μm frozen section on 6 μm mylar film is good for the test on BL15U. The contents of Iron and Zinc in pterygium tissue were significantly higher than in the control tissues. The results seem to be valuable in that Iron and Zinc may play a role in the development process of Pterygium.

Quantitative determination of metal and metalloid spatial distribution in hydrated and fresh roots of cowpea using synchrotron-based X-ray fluorescence microscopy

Many metals and metalloids, jointly termed metal(loid)s, are toxic to plants even at low levels. This has limited the study of their uptake, distribution, and modes of action in plant roots grown at physiologically relevant concentrations. Synchrotron-based X-ray fluorescence microscopy was used to examine metal(loid)s in hydrated cowpea (Vigna unguiculata L.) roots exposed to Zn(II), Ni(II), Mn(II), Cu(II), Hg(II), Se(IV), Se(VI), As(III), or As(V). Development of a mathematical model enabled in situ quantitative determination of their distribution in root tissues. The binding strength of metals influenced the extent of their movement through the root cylinder, which influenced the toxic effects exerted—metals (e.g. Cu, Hg) that bind more strongly to hard ligands had high concentrations in the rhizodermis and caused this tissue to rupture, while other metals (e.g. Ni, Zn) moved further into the root cylinder and did not cause ruptures. When longitudinal distributions were examined, the highest Se concentration in roots exposed to Se(VI) was in the more proximal root tissues, suggesting that Se(VI) is readily loaded into the stele. This contrasted with other metal(loid)s (e.g. Mn, As), which accumulated in the apex. These differences in metal(loid) spatial distribution provide valuable quantitative data on metal(loid) physiology, including uptake, transport, and toxicity in plant roots.

Application of X-ray fluorescence imaging to ceramics from the American Southwest

We have used synchrotron-based X-ray fluorescence imaging (XRFI) to examine sherds of painted ceramics from prehistoric cultures of the American Southwest. These sherds are very well suited to XRFI analysis. Paints can generally be detected and distinguished by the fluorescence intensities of their constituent elements. Spatial maps of element distributions yield the spatial distribution of pigments. Pigments can be distinguished that are (or have become) visually similar; layers that have been obscured by overpainting can be examined; and pigment residues can be distinguished from surface contaminants deposited after painting and firing. As a result, XRFI allows the painted motifs to be clarified and hidden features to be revealed. Furthermore, the very rapid scanning and high sensitivity elemental detection possible with synchrotron-based XRFI facilitate measurements on large collections of sherds, allowing an integrative rather than anecdotal analysis.

Selenium Metabolism in Cancer Cells: The Combined Application of XAS and XFM Techniques to the Problem of Selenium Speciation in Biological Systems

Determining the speciation of selenium in vivo is crucial to understanding the biological activity of this essential element, which is a popular dietary supplement due to its anti-cancer properties. Hyphenated techniques that combine separation and detection methods are traditionally and effectively used in selenium speciation analysis, but require extensive sample preparation that may affect speciation. Synchrotron-based X-ray absorption and fluorescence techniques offer an alternative approach to selenium speciation analysis that requires minimal sample preparation. We present a brief summary of some key HPLC-ICP-MS and ESI-MS/MS studies of the speciation of selenium in cells and rat tissues. We review the results of a top-down approach to selenium speciation in human lung cancer cells that aims to link the speciation and distribution of selenium to its biological activity using a combination of X-ray absorption spectroscopy (XAS) and X-ray fluorescence microscopy (XFM). The results of this approach highlight the distinct fates of selenomethionine, methylselenocysteine and selenite in terms of their speciation and distribution within cells: organic selenium metabolites were widely distributed throughout the cells, whereas inorganic selenium metabolites were compartmentalized and associated with copper. New data from the XFM mapping of electrophoretically-separated cell lysates show the distribution of selenium in the proteins of selenomethionine-treated cells. Future applications of this top-down approach are discussed.

Iron and zinc complexation in wild-type and ferritin-expressing wheat grain: implications for mineral transport into developing grain

We have used synchrotron-based X-ray fluorescence and absorption techniques to establish both metal distribution and complexation in mature wheat grains. In planta, extended X-ray absorption fine structure (EXAFS) spectroscopy reveals iron phytate and zinc phytate structures in aleurone cells and in modified aleurone cells in the transfer region of the grain: iron is coordinated octahedrally by six oxygen atoms and fewer than two phosphorous atoms. Zinc is coordinated tetrahedrally by four oxygen atoms and approximately 1.5 phosphorus atoms in an asymmetric coordination shell. We also present evidence of modified complexation of both metals in transgenic grain overexpressing wheat ferritin. For zinc, there is a consistent doubling of the number of complexing phosphorus atoms. Although there is some EXAFS evidence for iron phytate in ferritin-expressing grain, there is also evidence of a structure lacking phosphorus. This change may lead to an excess of phosphorus within the storage regions of grain, and in turn to the demonstrated increased association of phosphorus with zinc in ferritin-expressing grains. Derivative X-ray absorption spectra also suggest that mineral complexation in the transfer region of ferritin-expressing grains is quite different from that in wild-type grain. This may explain why the raised levels of minerals transported to the developing grain accumulate within the crease region of the transgenic grain.

In Situ Study of ALD Processes Using Synchrotron-based X-ray Fluorescence and Scattering Techniques

In this paper, we present an x-ray based approach for in situ characterization during ALD processes. In a first case study the growth of HfO2 on H-terminated and oxidized Si and Ge surfaces is studied. X-ray fluorescence (XRF) is used to monitor the amount of deposited Hf atoms, while grazing incidence small angle x-ray scattering (GISAXS) provides information on the evolution of surface roughness. A second case study concerns the growth of TiO2 in a porous titania film containing ink-bottle mesopores. XRF and GISAXS measurements are used to monitor the Ti uptake and the density of the porous film, respectively.

Spatial distribution and speciation of copper in root tips of cucumber revealed by μ-XRF and μ-XANES

The localization, biotransformation, and chemical speciation of copper in root tips of cucumber (Cucumis sativus) were investigated using synchrotron-based micro X-ray fluorescence (μ-XRF) and micro X-ray absorption near edge structure (μ-XANES). The highest content of Cu was found in root cap and meristematic zone whereas low Cu content in elongation and maturation zone. There was a dramatic increase of Cu content in root cap and meristematic zone after treatment with 100 μM CuSO4 for 72 h. The μ-XANES analysis revealed that most of Cu in root tip was bound with alginate, citrate, and cysteine-like ligands whereas rarely deposited in form of CuO. From root cap to maturation zone, the proportion of Cu bound with alginate-like ligands increased whereas that bound with citrate-like ligands decreased. The proportion of Cu bound with cysteine-like ligands increased from root cap to elongation zone but sharply declined in maturation zone. The results suggested that Cu was chelated by S ligands in the cell walls which protect protoplasm against possible damage caused by Cu excess.

Trends in X-ray Fluorescence Microscopy

Modern synchrotron-based X-ray fluorescence microscopy (XFM) has become a critical tool for many a research program, addressing extremely broad and highly relevant scientific questions. Their ability to map trace elemental content and probe local chemical state has been applied to numerous scientific areas in the life sciences [1–3], the environmental and earth sciences [4, 5], the materials sciences, as well as in cultural heritage studies. The newest generation of instruments utilizes high-brightness X-ray sources and incorporate state-of-the-art focusing optics and detector systems. Advances in X-ray sources and nanofocusing optics, for example, have allowed these instruments to achieve spatial resolutions of 20–30 nm using diffractive optics such as Fresnel zone plates and 200 nm using reflective optics such as Kirkpatrick-Baez mirrors. New beamlines, now in the design stage, aim to achieve similar (and better) resolutions within the next five years.

Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

A phosphorus diffusion gettering model is used to examine the efficacy of a standard gettering process on interstitial and precipitated iron in multicrystalline silicon. The model predicts a large concentration of precipitated iron remaining after standard gettering for most as-grown iron distributions. Although changes in the precipitated iron distribution are predicted to be small, the simulated post-processing interstitial iron concentration is predicted to depend strongly on the as-grown distribution of precipitates, indicating that precipitates must be considered as internal sources of contamination during processing. To inform and validate the model, the iron distributions before and after a standard phosphorus diffusion step are studied in samples from the bottom, middle, and top of an intentionally Fe-contaminated laboratory ingot. A census of iron-silicide precipitates taken by synchrotron-based X-ray fluorescence microscopy confirms the presence of a high density of iron-silicide precipitates both before and after phosphorus diffusion. A comparable precipitated iron distribution was measured in a sister wafer after hydrogenation during a firing step. The similar distributions of precipitated iron seen after each step in the solar cell process confirm that the effect of standard gettering on precipitated iron is strongly limited as predicted by simulation. Good agreement between the experimental and simulated data supports the hypothesis that gettering kinetics is governed by not only the total iron concentration but also by the distribution of precipitated iron. Finally, future directions based on the modeling are suggested for the improvement of effective minority carrier lifetime in multicrystalline silicon solar cells.

Detection and role of trace impurities in high-performance organic solar cells

Trace impurities in organic solar cells, such as those from residual catalyst material in conjugated polymers, are often ignored but are known to deleteriously affect device performance. Batch-to-batch variations in the nature and quantity of such impurities leads to widespread issues with irreproducible optoelectronic function, yet to date no technique has emerged that is reliably capable of identifying the character of impurities or their concentration in organic photovoltaic active layer blends. Here we focus on state-of-the-art, high-performance bulk heterojunction blends and show that synchrotron-based X-ray fluorescence can detect and quantify trace concentrations of metal impurities in these systems. Adopting a strategy of artificially introducing known quantities of additional catalyst into polymer/fullerene blends, we identify both the threshold concentration at which performance degrades and the mechanism for the degradation. With the knowledge of a target impurity concentration and a technique in hand to accurately measure their presence, researchers can implement materials preparation processes to achieve consistent, high performance in organic solar cells.

Iron distribution through the developmental stages of Medicago truncatula nodules

Paramount to symbiotic nitrogen fixation (SNF) is the synthesis of a number of metalloenzymes that use iron as a critical component of their catalytical core. Since this process is carried out by endosymbiotic rhizobia living in legume root nodules, the mechanisms involved in iron delivery to the rhizobia-containing cells are critical for SNF. In order to gain insight into iron transport to the nodule, we have used synchrotron-based X-ray fluorescence to determine the spatio-temporal distribution of this metal in nodules of the legume Medicago truncatula with hitherto unattained sensitivity and resolution. The data support a model in which iron is released from the vasculature into the apoplast of the infection/differentiation zone of the nodule (zone II). The infected cell subsequently takes up this apoplastic iron and delivers it to the symbiosome and the secretory system to synthesize ferroproteins. Upon senescence, iron is relocated to the vasculature to be reused by the shoot. These observations highlight the important role of yet to be discovered metal transporters in iron compartmentalization in the nodule and in the recovery of an essential and scarce nutrient for flowering and seed production.

Strontium Substitution for Calcium in Lithogenesis

Strontium has chemical similarity to calcium, which enables the replacement of calcium by strontium in biomineralization processes. Incorporating strontium into human bone and teeth has been studied extensively but little research has been performed of the incorporation of strontium into urinary calculi. We used synchrotron based x-ray fluorescence and x-ray absorption techniques to examine the presence of strontium in different types of human kidney stones. Multiple unique human stone samples were obtained via consecutive percutaneous nephrolithotomies/ureteroscopies. A portion of each stone was sent for standard laboratory analysis and a portion was retained for x-ray fluorescence and x-ray absorption measurements. X-ray fluorescence and x-ray absorption measurements determined the presence, spatial distribution and speciation of strontium in each stone sample. Traditional kidney stone analyses identified calcium oxalate, calcium phosphate, uric acid and cystine stones. X-ray fluorescence measurements identified strontium in all stone types except pure cystine. X-ray fluorescence elemental mapping of the samples revealed co-localization of calcium and strontium. X-ray absorption measurements of the calcium phosphate stone showed strontium predominately present as strontium apatite. Advanced x-ray fluorescence imaging identified strontium in all calcium based stones, present as strontium apatite. This finding may be critical since apatite is thought to be the initial nidus for calcium stone formation. Strontium is not identified by standard laboratory stone analyses. Its substitution for calcium can be reliably identified in stones from multiple calcium based stone formers, which may offer opportunities to gain insight into early events in lithogenesis.

Feasibility of direct mapping of cerebral fluorodeoxy‐D‐glucose metabolism in situ at subcellular resolution using soft X‐ray fluorescence

Glucose metabolism is difficult to image with cellular resolution in mammalian brain tissue, particularly with (18) fluorodeoxy-D-glucose (FDG) positron emission tomography (PET). To this end, we explored the potential of synchrotron-based low-energy X-ray fluorescence (LEXRF) to image the stable isotope of fluorine (F) in phosphorylated FDG (DG-6P) at 1 μm(2) spatial resolution in 3-μm-thick brain slices. The excitation-dependent fluorescence F signal at 676 eV varied linearly with FDG concentration between 0.5 and 10 mM, whereas the endogenous background F signal was undetectable in brain. To validate LEXRF mapping of fluorine, FDG was administered in vitro and in vivo, and the fluorine LEXRF signal from intracellular trapped FDG-6P over selected brain areas rich in radial glia was spectrally quantitated at 1 μm(2) resolution. The subsequent generation of spatial LEXRF maps of F reproduced the expected localization and gradients of glucose metabolism in retinal Müller glia. In addition, FDG uptake was localized to periventricular hypothalamic tanycytes, whose morphological features were imaged simultaneously by X-ray absorption. We conclude that the high specificity of photon emission from F and its spatial mapping at ≤1 μm resolution demonstrates the ability to identify glucose uptake at subcellular resolution and holds remarkable potential for imaging glucose metabolism in biological tissue.

A study on the effect of tantalum-impurity content on the superconducting properties of niobium materials used for making superconducting radio frequency cavities

Niobium materials in highly pure form are used in the fabrication of superconducting radio frequency cavities. We present here a study of the superconducting properties of such niobium materials that have been used in the fabrication of high accelerating gradient superconducting radio frequency cavities after determining their tantalum-impurity contents using a synchrotron-based x-ray fluorescence spectroscopy technique. Our results show that there is a small change in superconducting parameters such as TC,HC1 and HC2 when the tantalum-impurity content varies from ≈150 to ≈1300 ppm. In contrast, a buffered chemical polishing of the same niobium samples changes all these superconducting parameters more significantly. The implications of these results on the performance of niobium superconducting radio frequency cavities are discussed.

Elucidating the Role of Zinc in Malaria with Synchrotron-Based X-Ray Fluorescence Microscopy and Spectroscopy

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Distribution and speciation of trace elements in iron and manganese oxide cave deposits

Fe and Mn oxide minerals control the distribution and speciation of heavy metals and trace elements in soils and aquatic systems through chemical mechanisms involving adsorption, incorporation, and electron transfer. The Pautler Cave System in Southwest Illinois, an analog to other temperate carbonate-hosted karst systems, contains Fe and Mn oxide minerals that form in multiple depositional environments and have high concentrations of associated trace elements. Synchrotron-based micro-scanning X-ray fluorescence (μ-SXRF) shows unique spatial distributions of Fe, Mn, and trace elements in mineral samples. Profile maps of Mn oxide cave stream pebble coatings show Fe- and As-rich laminations, indicating dynamic redox conditions in the cave stream. μ-SXRF maps demonstrate that Ni, Cu, and Zn correlate primarily with Mn whereas As correlates with both Mn and Fe; As is more enriched in the Fe phase. Zn is concentrated in the periphery of Mn oxide stream pebble coatings, and may be an indication of recent anthropogenic surface activity. X-ray absorption fine structure spectroscopy measurements reveal that As(V) occurs as surface complexes on Mn and Fe oxides whereas Zn(II) associated with Mn oxides is adsorbed to the basal planes of phyllomanganates in a tetrahedral coordination. Co(III) and Se(IV) are also observed to be associated with Mn oxides. The observation of Fe, Mn, and trace element banding in Mn oxide cave stream pebble coatings suggests that these materials are sensitive to and document aqueous redox conditions, similar to ferromanganese nodules in soils and in marine and freshwater sediments. Furthermore, speciation and distribution measurements indicate that these minerals scavenge trace elements and limit the transport of micronutrients and contaminants in karst aquifer systems while also potentially recording changes in anthropogenic surface activity and land-use.

In Situ Study of ALD Processes Using Synchrotron-based X-ray Fluorescence and Scattering Techniques

not Available.

Arsenic Localization and Speciation in the Root–Soil Interface of the Desert Plant Prosopis juliflora-velutina

The bioavailability and mobility of arsenic (As) in soils depends on several factors such as pH, organic matter content, speciation, and the concentration of oxides and clay minerals, among others. Plants modify As bioavailability in the rhizosphere; thus, the biogeochemical processes of As in vegetated and non-vegetated soils are different. Changes in As speciation induced by the rhizosphere can be monitored using micro-focused synchrotron-based X-ray fluorescence (μXRF) combined with μX-ray absorption near-edge spectroscopy (μXANES). This research investigated As speciation in the rhizosphere of mesquite (Prosopis juliflora-velutina) plants grown in a sandy clay loam treated with As(III) and As(V) at 40 mg kg(-1). Rhizosphere soil and freeze-dried root tissues of one-month-old plants were analyzed by bulk XAS. Bulk XAS results showed that As(V) was the predominant species in the soil (rhizosphere and non-vegetated), whereas As(III) was dominant in the root tissues from both As(V) and As(III) treated plants. μXAS and μXRF studies of thin sections from resin embedded soil cores revealed the As(III)-S interactions in root tissues and a predominant As-Fe interaction in the soil. This research demonstrated that the combination of bulk XAS and μXAS techniques is a powerful analytical technique for the study of As speciation in soil and plant samples.

Examination of the distribution of arsenic in hydrated and fresh cowpea roots using two- and three-dimensional techniques.

Arsenic (As) is considered to be the environmental contaminant of greatest concern due to its potential accumulation in the food chain and in humans. Using novel synchrotron-based x-ray fluorescence techniques (including sequential computed tomography), short-term solution culture studies were used to examine the spatial distribution of As in hydrated and fresh roots of cowpea (Vigna unguiculata 'Red Caloona') seedlings exposed to 4 or 20 µm arsenate [As(V)] or 4 or 20 µm arsenite. For plants exposed to As(V), the highest concentrations were observed internally at the root apex (meristem), with As also accumulating in the root border cells and at the endodermis. When exposed to arsenite, the endodermis was again a site of accumulation, although no As was observed in border cells. For As(V), subsequent transfer of seedlings to an As-free solution resulted in a decrease in tissue As concentrations, but growth did not improve. These data suggest that, under our experimental conditions, the accumulation of As causes permanent damage to the meristem. In addition, we suggest that root border cells possibly contribute to the plant's ability to tolerate excess As(V) by accumulating high levels of As and limiting its movement into the root.

Use of microfocused X-ray techniques to investigate the mobilization of arsenic by oxalic acid

Improved linkages between aqueous phase transport and solid-phase reactions are needed to better predict and model transport of contaminants through the subsurface. Here we develop and apply a new method for measuring As mobilization in situ within soil columns that utilizes synchrotron-based X-ray fluorescence. By performing these measurements in situ during column transport experiments, we simultaneously monitor grain-scale solid phase reactions and column-scale transport. Arsenic may be effectively mobilized by oxalic acid but the geochemical and mineralogical factors that influence the rate and extent of mobilization are not well understood. Column experiments (∼4cm long×0.635cm ID) using As contaminated sediments from the Vineland Chemical Company Superfund site were performed on the laboratory bench as well as in the synchrotron beamline. Microfocused synchrotron X-ray fluorescence (μSXRF) maps for As and Fe were collected at the same location in the columns (<1mm2) before and during treatment with 10mM oxalic acid. The fraction of As and Fe removed by oxalic acid treatment was calculated from the change in flux-normalized counts for each pixel in the map images, and these data were used to calculate kinetic parameters over the studied area. Between 79% and 83% of the As was removed from the sediments by the oxalic acid treatment based on μSXRF data; these removal percentages agreed well with laboratory data based on column effluent (88–95%). Considerably less Fe was removed by oxalic acid treatment, 14–25% based on μSXRF counts, which is somewhat higher than the 7–9% calculated from laboratory column effluent concentrations. Microfocused X-ray absorption near edge spectroscopy (μXANES) on a subset of points indicates most of the Fe was oxidized and present as a mixture of goethite, hematite, and ferrihydrite on sand grain coatings. Treatment with oxalic acid led to subtle shifts in Fe (III) species following oxalic acid treatment, either removing ferrihydrite or transforming it to more stable oxides; however, Fe redox states were not impacted. Kinetics information extracted from μSXRF data compared favorably with rates of As removal from observed As breakthrough curves. The average pseudo-first order As removal rate constant was calculated to be 0.015min−1±0.002 (±average standard error, N=400) based on changes in μSXRF counts over time. The spatial variation observed in the rate constant is likely a result of differences in the mineral substrate or As retention mechanism. Geochemical models created using the calculated As removal rate constants showed agreement with As breakthrough curves for both a small column (4.25cm×0.635cm ID) and a larger column (23.5cm×4.2cm ID), indicating that the processes studied using the microprobe are representative and often can be predictive of larger systems. While this work was used to understand the processes that regulate As release and transport, the methods developed here could be used to study a wide variety of reaction processes, including contaminant removal due to chemical treatment, mineral precipitation due to changing redox characteristics, and solid phase transformations.