Editorial Feature

High-Precision Elemental Mapping of Metals with XRF Spectrometry

Elemental mapping is an imaging technique used to visualize the spatial distribution of elements within a material. X-ray fluorescence (XRF) spectrometry is a widely used analytical technique for this purpose due to its ability to provide information on a  broad range of elements, making it an ideal tool for studying compositional zonation and determining the elemental composition of a material.

This article explores the application of XRF in the elemental mapping of metals and its significance in the field.

XRF Spectrometry, Elemental Mapping of Metals

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XRF Spectrometry

In 1907, a British physicist, Charles G. Barkla, found a correlation between X-rays radiating from a sample and its atomic weight. Henry G. J. Moseley employed X-rays in 1913 to assign numerical values to the elements. He noted that the K-line spectral transitions in X-rays shifted uniformly as the atomic number increased by one, a fundamental principle in XRF physical theory. As a result, Mosley was credited with revising the periodic tables based on increasing atomic weight.

XRF spectrometry is an analytical technique that allows rapid and non-destructive analysis of solid or liquid samples. The sample is irradiated by an intense X-ray beam, resulting in the emission of fluorescent X-rays, which are detected using either a wavelength or an energy-dispersive detector.

Thus, either the wavelength or energy of the emitted X-rays is used for elemental identification in an analyte and the concentrations are determined by the intensity of the X-rays. It provides broad applications, can detect all elements between sodium and uranium in a periodic table, and has detection limits of 1–10 mg kg− l. This method is highly precise and accurate, making it a reliable tool for determining the elemental compositions of various types of samples.

How Does XRF Spectrometry Work?

To accurately determine the analyte concentration using XRF spectroscopy, it is crucial to first convert the measured intensity values of the characteristic radiation into the concentrations of the elements in question. This conversion process depends on the measured characteristic X-rays, the intensity of the excitation source, the concentration of the analyte, the overall composition of the sample, and absorption properties.

The core concept of XRF analysis is the excitation of electrons through the interaction of the sample with incident X-rays, causing the excitation of core electrons to an excited state. The de-excitation of these electrons to their ground states results in fluorescence, also known as secondary X-rays. The emitted fluorescence energy and wavelength spectrum are unique to specific elements and act as fingerprints for determining their relative abundances.

The objective of XRF analysis is to precisely identify elements in a sample with a high level of accuracy. Hence, while analyzing an XRF spectrum, it is imperative to detect any unique peaks in the sample above the background. Since the XRF signal intensity is specific to an atomic species concentration, it directly correlates with the number of atoms present.

Elemental Mapping in Biological Samples

XRF spectrometry is commonly used to examine the elemental composition of various biomedical samples, including soft tissues, bones, and teeth. This analytical technique helps investigate samples derived from various body organs such as the liver, heart, brain, kidneys, breasts, and lungs.

XRF can provide quantitative information on the distribution of metallic elements in biological samples. In addition to determining the actual chemical composition of a sample, quantitative XRF analysis allows the generation of distribution maps from the obtained XRF spectra. The thickness of the sample is a crucial factor for an accurate quantitative XRF analysis. The sensitivity of XRF can be further enhanced by combining it with synchrotron radiation, resulting in synchrotron radiation X-ray fluorescence (SR-XRF).

Elemental Mapping of Earth Materials

 XRF spectroscopy is also used to determine the geochemical compositions of rocks, sediments, and other earth material samples. It can measure the concentrations of major and trace elements at the parts per million (ppm) level and has been successfully applied to the analysis of geological, archaeological, and industrial samples.

The primary reason for the application of XRF spectrometry in the elemental mapping of earth materials is due to its ability to analyze the bulk chemical contents of major elements found in earth materials. The X-ray emission generated by the XRF method is characterized by its simplicity, systematic nature, insensitivity to the chemical state of the sample, and uniform excitation and absorption that depend solely on the atomic number of the element.

The geochemistry of many earth materials is influenced by climate, parent rock, and human interactions, making XRF a significant advantage for determining the major oxide/element composition. As a result, XRF remains the standard method for investigating the mineral and chemical compositions of Earth's solid materials.

Recent Studies

A study published in Eng reported micrometric two-dimensional (2D) mapping of distinct elements in distinct soil grain-size fractions of a sample using the micro-XRF (µ-XRF) technique.

The sample was collected from a geographical area containing massive sulfide minerals. High levels of distinct metal and metalloid elements were found where natural geochemical abnormalities were expected. The study used clustering and k-means statistical analysis on red–green–blue (RGB) pixel proportions in 2D micrometric image maps to determine the elemental distribution of the elements in 2D space.

The results showed the variation in elemental composition at the micrometric scale per grain-size class, observed as irregular spatial distribution and dependent on mineral spatial distributions. The composition of elements varied more in coarser grain-size classes (larger grain sizes). Although the analysis was complex, the results showed that µ-XRF is useful for studying natural, heterogeneous samples on a small scale, making it a promising high-resolution technique.

An article published in Plant and Soil employed Herbarium XRF Ionomics, a recently developed XRF spectrometry approach for obtaining quantitative elemental data from herbarium specimens.

Following XRF screening, which identifies a potential hyperaccumulator, the next step is to confirm whether the result is accurate because there could be several reasons for anomalous measurements caused by artifacts. The application of μ-XRF analysis to herbarium specimens has paved the way for subsequent research that employs specimens specifically prepared for micro-analytical examination.

Conclusion

Overall, XRF spectrometry allows for precise mapping of metal elements in a variety of biological, geological, and archaeological samples, offering detailed and accurate information on their spatial distribution.

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References and Further Reading 

Jurowski, K., Buszewski, B., Piekoszewski, W. (2015). The analytical calibration in (bio) imaging/mapping of the metallic elements in biological samples–definitions, nomenclature and strategies: state of the art. Talanta, 131, 273-285. https://doi.org/10.1016/j.talanta.2014.07.089

Oyedotun, T. D. T. (2018). X-ray fluorescence (XRF) in the investigation of the composition of earth materials: a review and an overview. Geology, Ecology, and Landscapes, 2(2), 148-154. https://doi.org/10.1080/24749508.2018.1452459

Barbosa, S., Dias, A., Pacheco, M., Pessanha, S., & Almeida, J. A. (2023). Investigating Metals and Metalloids in Soil at Micrometric Scale Using µ-XRF Spectroscopy—A Case Study. Eng, 4(1), 136-150. https://doi.org/10.3390/eng4010008

van der Ent, A., Casey, L. W., Purwadi, I., & Erskine, P. D. (2023). Laboratory μ-X-ray fluorescence elemental mapping of herbarium specimens for hyperaccumulator studies. Plant and Soil, 1-9. https://doi.org/10.1007/s11104-023-06201-5

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Bhavna Kaveti

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Bhavna Kaveti

Bhavna Kaveti is a science writer based in Hyderabad, India. She has a Masters in Pharmaceutical Chemistry from Vellore Institute of Technology, India, and a Ph.D. in Organic and Medicinal Chemistry from Universidad de Guanajuato, Mexico. Her research work involved designing and synthesizing heterocycle-based bioactive molecules, where she had exposure to both multistep and multicomponent synthesis. During her doctoral studies, she worked on synthesizing various linked and fused heterocycle-based peptidomimetic molecules that are anticipated to have a bioactive potential for further functionalization. While working on her thesis and research papers, she explored her passion for scientific writing and communications.

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