Energy Dispersive Spectroscopy (EDS or EDX) is a technique for the investigation of the elemental composition of solid surfaces. It plays an important role in evaluating and understanding the nature of materials in various areas, such as microelectronics, catalysis, metallurgy, environmental and biomedical sciences.
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Together with many other analytical techniques, namely X-ray photoelectron spectroscopy (XPS), secondary ions mass spectroscopy (SIMS), and atomic force microscopy (AFM), EDS is crucial for the characterization of solid materials to obtain information about their surface properties.
How it Works – Basic Principles of EDS
Atoms contain electrons in discrete energy levels (or shells). Such shells are typically assigned the letters K, L, M, and so on, depending on their energy levels, with K being the inner shell closest to the atomic nucleus.
When a sample is bombarded with a beam of high-energy electrons, the electrons in the target atoms are expelled from their inner shell, producing an electron vacancy. Subsequently, the electrons on the higher energy levels are transferred to the inner shell in a transition that releases energy in the form of X-rays.
Depending on the shells involved in the transition, a characteristic X-ray is released. For instance, when an electron transitions from the L to the K shell, it is known as Kα. Similarly, a transition from the M shell to the K shell is a Kβ X-ray.
Each element on the periodic table has a unique electronic configuration and thus has different energy levels. Therefore, the generated X-rays are characteristic of every element. By measuring the characteristic X-ray energy and intensity released by each element, it is possible to have information on the elemental composition of a sample.
Strengths and Weaknesses of EDS Analysis
An EDS apparatus consists of four basic components: the excitation source (electron beam or X-ray beam), the X-ray detector, the pulse processor, and the analyzer.
The X-ray detector converts X-rays into electric signals. The pulse processor measures the detected signals and transfers them to the analyzer, where data is processed and displayed. The ultimate result is a spectrum that reports the X-ray intensity versus the energy (expressed in kiloelectron volts, keV).
EDS is non-destructive and is often coupled with electron microscopy techniques, such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to provide insights into the structure and morphology of the sample. Most of the time the analysis is relatively fast and can be performed in minutes.
EDS can be used to perform both qualitative and quantitative elemental characterization. The area under the peaks is proportional to the number of X-rays that generated the signals. Moreover, by adjusting the excitation voltage it is possible to gather information at different depths in the sample.
Among the limitations of the technique, the accuracy of EDS relies on the accuracy of the measurement of the X-ray intensity and it usually decreases when moving from the heavier elements to lighter elements. EDS fails to accurately detect elements whose atomic number is below eleven.
Another factor that may be detrimental to the analysis, is sample preparation. Since the technique employs just a small fraction of the sample, the surface roughness directly affects the results obtained. Therefore, samples must be carefully prepared to acquire surface smoothness.
The dependence of EDS on standards to identify the resulting peaks is another limitation. The need for standards with a composition as similar as possible to the sample under investigation is a negative aspect when investigating new materials.
Applications in Forensic, Biomedical, and Environmental Sciences
EDS is widely employed in forensic science – generally coupled with SEM – for the investigation of evidence, namely explosives, gunshot residue (GSR), or biological samples.
Forensic scientists can use EDS to investigate the elemental compositions of different explosives. In an example, it was concluded that the explosive mixture under examination was based on chlorate and consisted of trinitrotoluene and tetryl.
GSR is among the most common types of forensic samples analyzed via EDS. The technique coupled with SEM can examine GSR to discriminate between various kinds of ammunition on residues collected from the hands of the shooters.
An analysis revealed different element combinations (e.g., PbSbBa, SbBa, SbSn, etc.) from the residues collected and it was concluded that different kinds of ammunition had different compositions and ratios of elements.
EDS can also be a powerful tool in biomedical research and diagnosis, for instance, the detection of nanoparticles in drug delivery studies. Furthermore, EDS can be employed to investigate the accumulation of heavy metals in tissues.
It is possible to determine whether heavy metals accumulation in bone tissues could be related to the altered bone metabolism of osteoporotic patients. An EDS analysis demonstrated a specific accumulation of Pb, Cr, and Cd in head biopsies.
Lately, EDS has also found increasing applications for the study of heavy metals pollution and its effects on both the environment and the living organisms. Thanks to EDS analysis it was possible to demonstrate that the underground subway stations in Shanghai allowed the exposition to metal through aerosol. The concentrations of Fe, Mn, and Cr were found to be higher than the averages of the corresponding aboveground urban areas.
Although there are some difficulties involved, EDS is an extremely useful, non-destructive technique to investigate material composition, and it finds applications in several sectors. With further innovative technological developments, such as the design of new detectors exhibiting higher efficiencies and resolutions, these shortfalls could be overcome.
- Hamuyuni, J., Daramola, M. O. & Oluwasina, O. O. 2017. Energy-Dispersive X-Ray Spectroscopy: Theory and Application in Engineering and Science. Encyclopedia of Physical Organic Chemistry.
- Hussain, C. M., Rawtani, D., Pandey, G. & Tharmavaram, M. 2021. Chapter 15 - Energy dispersive X-ray (EDX) coupled microscopy in forensic science. In: HUSSAIN, C. M., RAWTANI, D., PANDEY, G. & THARMAVARAM, M. (eds.) Handbook of Analytical Techniques for Forensic Samples. Elsevier.
- Brozek-Mucha, Z. & Jankowicz, A. (2001). Evaluation of the possibility of differentiation between various types of ammunition by means of GSR examination with SEM–EDX method. Forensic Science International, 123, 39-47.https://doi.org/10.1016/S0379-0738(01)00518-7
- Scimeca, M., Bischetti, S., Lamsira, H. K., Bonfiglio, R. & Bonanno, E. (2018). Energy Dispersive X-ray (EDX) microanalysis: A powerful tool in biomedical research and diagnosis. Eur J Histochem, 62, 2841.10.4081/ejh.2018.2841