Speciation analysis XANES

In 2006, Lobinski et al.1 reported on the imaging and speciation analysis of trace elements to study the element distribution, oxidation state, metal site and metal structure in biological environments using mass spectrometric techniques (LA-ICP-MS, SIMS, MALDI-MS) and non-mass-spectrometric techniques such as micro-PIXE (proton induced X-ray emission), XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) -the latter two techniques are very sensitive due the use of a more intense synchrotron beam.1... 

In order to better understand the mechanism of antiwear functions it is essential to identify the compounds, as well as the elements, constituting the tribofilm. With the exception of XPS and X-ray diffraction techniques, all other physical techniques to date have focused on elemental analysis. Extended X-ray absorption, fine structure EXAFS and, in particular, X-ray absorption near-edge structure XANES techniques have, been shown to be very sensitive for structural and chemical speciation. The K-edge EXAFS spectroscopy has been used to study antiwear additives and tribofilms of some metals (Koningsberger and Prins, 1988 Martin et al., 1986a Belin et al., 1989). 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

Figure 13. Normalized, As-K edge XANES spectra of (a) As(]]l) and (b) As(V) in different model coordination environments, illustrating the power of XANES spectroscopy for chemical fingerprinting. Often the low bulk concentrations of As in aquifer sediments preclude a quantitative EXAFS analysis fortunately, quantitative oxidation state and semi-quantitative speciation information can be obtained from high-quality XANES spectra.

Huffman GP, Mitra S, Huggins FE, Shah NS, Vaidya N, Lu F (1991) Quantitative analysis of all major forms of sulfur in coal by X-ray absorption fine structure spectroscopy. Energy Fuels 5 574-581 Huffman GP, Shah NS, Huggins FE, Stock LM, Chatterjee K, Kilbane JJ, Chou M, Buchanan DH (1995) Sulfur speciation of desulfurized coals by XANES spectroscopy. Fuel 74 549-555 Huggins FE, Huffman GP (1995) Chlorine in coal anEXAFS spectroscopic investigation. Fuel 74 556-569 Hundal LS, Carmo AM, Bleam WL, Thompson ML (2000) Sulfur in biosolids-derived fulvic acidic characterization by XANES spectroscopy and selective dissolution approaches. Environ Sci Technol 34 5184-5188... 

Figure 1.10 Schematic flowchart for the two alternate workflows for high-throughput X-ray absorption spectroscopy (HT-XAS). Synchrotron radiation analysis consists of mapping the metal distribution using X-ray fluorescence (XRF), using XANES for metal speciation and using EXAFS for metal-site structural analysis of the metalloproteome. 2005 International Union of Crystallography.

Besides the straightforward fingerprints studies, XANES can also be applied to quantitative speciation. This is because XAS is a local probe teehnique, which implies no long-range order in the sample is required. Therefore, if the absorbing atoms are present in the sample at two different sites, the XANES spectrum obtained from this material can be represented by the weighted addition of the spectra of suitable reference samples. For many systems, XANES analysis based on linear combinations of known spectra from model compounds is sufficient to estimate ratios of different species. More sophisticated linear algorithms, such as principle component analysis and factor analysis, can also be applied to XANES spectra. ... 

Figure 6.28 Examples of the data collected to test the feasibility of the procedure of HTXAS procedure. Individual gene product samples from the Pyrococcus furiosus genome were loaded into a 5x5 spatial array of 3mL samples (middle left). A single raster scan was used to monitor both Ni (lower left) and Zn (upper left) K emission, identifying the presence of these metals in the indicated wells. Ni XANES speciation (lower right) clearly differentiates the t es of Ni binding sites and Zn EXAFS structural analysis (upper right) shows the sensitivity available for determining metal-site structures. 2005 International Union of Crystallography.

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