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X-ray absortption spectroscopy for speciation analysis

(23.12.2024)

X-ray absorption spectroscopy (XAS) is a powerful technique used for speciation analysis, which involves identifying and quantifying the different chemical forms of an element in a sample.

Basics of X-Ray Absorption Spectroscopy

XAS involves measuring the absorption of X-rays as a function of energy near and above the core-level binding energies of an element. The technique is divided into three main regions (see figure ):
  1. The absorption threshold determined by the transition to the lowest unoccupied states.
  2. X-ray Absorption Near Edge Structure (XANES, also called NEXAFS): This region, close to the absorption edge, provides information about the oxidation state and coordination environment of the absorbing atom.
  3. Extended X-ray Absorption Fine Structure (EXAFS): This region, extending beyond the absorption edge, provides detailed information about the distances, coordination numbers, and types of neighboring atoms around the absorbing atom.



Figure: Three regions of XAS data for the K-edge


Steps in Speciation Analysis Using XAS

Sample Preparation: Samples can be in various forms, such as solids, liquids, or thin films. Proper preparation is crucial to ensure representative measurements.
  • Data Collection:
    XAS spectra are collected using synchrotron radiation sources, which provide intense and tunable X-ray beams. The photon beam is tuned by using a crystalline monochromator, to a photoon energy to a range where core electrons can be excited (0.1-100 keV). The sample is exposed to these beams, and the absorption is measured as the X-ray energy is varied. The edges are, in part, named by which core electron is excited: the principal quantum numbers n = 1, 2, and 3, correspond to the K-, L-, and M-edges, respectively.
  • XANES Analysis:
    Oxidation State Determination: The edge position in the XANES region shifts with changes in the oxidation state of the element. By comparing the edge position with reference compounds of known oxidation states, the oxidation state of the element in the sample can be determined.       
    Coordination Environment: The shape and features of the XANES spectrum can indicate the coordination geometry and type of ligands around the absorbing atom.
  • EXAFS Analysis:
    Local Structure: The EXAFS region provides information about the distances between the absorbing atom and its neighbors, the number of neighboring atoms (coordination number), and the type of neighboring atoms. This is achieved through Fourier transform analysis of the EXAFS oscillations.
  • Quantitative Fitting:
    The experimental EXAFS data is fitted with theoretical models to extract quantitative structural parameters. Software tools like ATHENA and ARTEMIS (part of the Demeter software package) are commonly used for this purpose.
        
The experimental analysis focusing on both the XANES and EXAFS regions is called X-ray absorption fine structure (XAFS).

Applications of XAS in Speciation Analysis

  • Environmental Science: Determining the speciation of heavy metals (e.g., arsenic, lead, mercury) in soils, sediments, and water to understand their mobility, bioavailability, and toxicity.
  • Biological Systems: Studying the speciation of metal ions in biological systems, such as metalloproteins and enzymes, to understand their functional roles and interactions.
  • Catalysis: Investigating the active sites and oxidation states of catalysts during reactions to improve catalytic processes.
  • Material Science: Analyzing the speciation of elements in complex materials like glasses, ceramics, and alloys to understand their properties and behavior.

Advantages of XAS for Speciation Analysis

  • Element-Specific: XAS is highly element-specific, allowing for the selective study of individual elements in complex matrices.
  • Non-Destructive: The technique is generally non-destructive, preserving the sample for further analysis.
  • Applicable to Diverse Samples: XAS can be applied to a wide range of sample types, including solids, liquids, and gases.
  • In Situ Capabilities: XAS can be performed under in situ conditions, enabling the study of dynamic processes in real-time.

Limitations

  • Requires Synchrotron Source: High-quality XAS measurements typically require access to synchrotron radiation facilities, which may not be readily available to all researchers.
  • Complex Data Analysis: The analysis and interpretation of XAS data can be complex and often require specialized software and expertise.

In summary, X-ray absorption spectroscopy is a versatile and powerful tool for speciation analysis, providing detailed information about the chemical state, local structure, and environment of specific elements in a wide range of samples.




Related Information: Synchrotron facilities

As of the most recent data, there are approximately 50 operational synchrotron light sources worldwide. These facilities are distributed across various regions and serve as crucial resources for a wide range of scientific research fields, including materials science, biology, chemistry, environmental science, and physics.

Here are some prominent synchrotron facilities by region:

North America




South America



Europe

Asia






Tutorial web material related to X-ray absorption fine structure

 Grant Bunkers's Tutorials on XAFS


Further information sources related to X-ray absorption fine structure




 Reviews of X-ray absorption spectroscopy (newest first)

Mark A., Newton, Patric Zimmermann, Jeroen A. van Bokhoven, X-Ray Absorption Spectroscopy (XAS): XANES and EXAFS, in: I.E. Wachs, M.A. Ba�ares (eds.), Springer Handbook of Advanced Catalyst Characterization, Springer, 2023, 565-600. DOI: 10.1007/978-3-031-07125-6_27

Valentina Bonanni, Alessandra Gianoncelli, Soft X-ray Fluorescence and Near-Edge Absorption Microscopy for Investigating Metabolic Features in Biological Systems: A Review, Int. J. Mol. Sci., 24 (2023) 3220. DOI: 10.3390/ijms24043220

Anne Marie Aucour, Geraldine Sarret, Hester Blommaert, Matthias Wiggenhauser, Coupling metal stable isotope compositions and X-ray absorption spectroscopy to study metal pathways in soil-plant systems: a mini review, Metallomics, 15/4 (2023) mfad016. DOI: 10.1093/mtomcs/mfad016

Mina Magdy, X-Ray Techniques Dedicated to Materials Characterization in Cultural Heritage, ChemistrySelect, 8/33 (2023) 202301306. DOI: 10.1002/slct.202301306

B.V. Kerr, H.J. King, C.F. Garibello, P.R. Dissannayake, A.N. Simonov, B. Johannessen, D.S. Eldridge, R.K. Hocking, Characterization of Energy Materials with X-ray Absorption Spectroscopy - Advantages, Challenges, and Opportunities, Energy & Fules, 36/5 (2022) 2369-2389. DOI: 10.1021/acs.energyfuels.1c04072

Dominique Bazin, Solenn Reguer, Delphine Vantelon, Jean-Philippe Haqymann, Emmanuel Letavernier, Vincent Frochotd, Michel Daudon, Emanuel Esteve, Hester Colboc, XANES spectroscopy for the clinician, C.R. Chimie, 25/SI (2022) 189-208. DOI: 10.5802/crchim.129

Mihai R. Gherase, David E.B. Fleming, Probing Trace Elements in Human Tissues with Synchrotron Radiation, Crystals, 10/1 (2020) 12. DOI: 10.3390/cryst10010012

Roberto Terzano, Melissa A. Denecke, Gerald Falkenberg, Bradley Miller, David Paterson, Koen Janssens, Recent advances in analysis of trace elemnets in environmental samples by X-ray based techniques (IUPAC Technical Report), Pure Appl. Chem., 92/6 (2019) 1029-1063. DOI: 10.1515/pac-2018-0605

M. Newville, Fundamentals of XAFS, Rev. Mineral. Geochem., 78/1 (2014) 33-74. DOI: 10.2138/rmg.2014.78.2

Grant S. Henderson, Frank M.F. de Groot, Banjamin J.A. Moulton, X-ray Absorption Near-Edge Structure (XANES) Spectroscopy, Rev. Mineral. Geochem., 78/1 (2014) 75-138. DOI: 10.2138/rmg.2014.78.3
 
Mark A. Newton, Wouter van Beek, Combining synchrotron-based X-ray techniques with vibrational spectroscopies for the in situ study of heterogeneous catalysts: a view from a bridge, Chem. Soc. Rev., 39/12 (2010) 4845-4863. DOI: 10.1039/b919689g

J.J. Rehr, A.L. Ankudinov, Progress in the theory and interpretation of XANES, Coord. Chem. Rev., 249/1-2 (2005) 121.140. DOI: 10.1016/j.ccr.2004.02.014

C. Hardacre, Application of EXAFS to molten salts and ionic liquid technology, Annu. Rev. Mater. Res., 35 (2005) 29-49. DOI: 10.1146/annurev.matsci.35.100303.121832

J. J. Rehr and R. C. Albers, Theoretical approaches to x-ray absorption fine structure, Rev.  Modern Phys., 72 (2000) 621-892. DOI:10.1103/RevModPhys.72.621.

S.E. Fendorf, D.L. Sparks, G.M. Lamble, M.J. Kelley, Applications of X-Ray-Absorption Fine-Structure Spectroscopy to Soils, Soil Sci. Soc. Am. J., 58/6 (1994) 1583-1595. DOI: 10.2136/sssaj1994.03615995005800060001x



Further chapters on techniques and methodology for speciation analysis:

Chapter 1: Tools for elemental speciation
Chapter 2: ICP-MS - A versatile detection system for speciation analysis
Chapter 3: LC-ICP-MS - The most often used hyphenated system for speciation analysis
Chapter 4: GC-ICP-MS- A very sensitive hyphenated system for speciation analysis
Chapter 5: CE-ICP-MS for speciation analysis
Chapter 6: ESI-MS: The tool for the identification of species
Chapter 7: Speciation Analysis - Striving for Quality
Chapter 8: Atomic Fluorescence Spectrometry as a Detection System for Speciation Analysis
Chapter 9: Gas chromatography for the separation of elemental species
Chapter 10: Plasma source detection techniques for gas chromatography
Chapter 11: Fractionation as a first step towards speciation analysis
Chapter 12: Flow-injection inductively coupled plasma mass spectrometry for speciation analysis
Chapter 13: Gel electrophoresis combined with laser ablation inductively coupled plasma mass spectrometry for speciation analysis
Chapter 14: Non-chromatographic separation techniques for speciation analysis Chapter 21: Chemical vapor generation as a sample introduction technique for speciation analysis
Chapter 23: Isotopic measurements and speciation analysis



EVISA News related to X-ray absorption spectroscopy for speciation analysis (newest first)



last time modified: December 24, 2024



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