Scanning electron microscopy in forensics (SEM)
Scanning Electron Microscopy (SEM) is a powerful imaging technique increasingly utilized in forensic science for the comparison of materials. By employing a focused electron beam, SEM allows analysts to view samples at incredibly high magnifications, revealing intricate surface characteristics that may indicate whether materials share a common origin. Invented in 1938, SEM can distinguish objects just 3 nanometers apart, far surpassing the capabilities of traditional light microscopes.
In forensic applications, SEM is primarily used to analyze tool marks, fibers, minerals, soil, and gunshot residues recovered from crime scenes. The imaging process generates secondary and backscatter electrons, producing detailed images that help forensic scientists evaluate similarities and differences between known and unknown samples. Additionally, SEM can be combined with energy-dispersive X-ray spectroscopy (EDS) to provide elemental analysis, enhancing the accuracy of comparisons between seemingly identical materials, such as different pieces of paper. This dual capability helps forensic analysts determine the integrity and relationship between various forensic evidence, contributing to the overall investigative process.
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Subject Terms
Scanning electron microscopy in forensics (SEM)
DEFINITION: Technique in which a high-powered scanning electron beam is used to achieve high-resolution imaging of microscopic samples.
SIGNIFICANCE: Forensic scientists are often asked to compare various kinds of materials to determine whether the materials have a common origin. Scanning electron microscopy allows analysts to view samples at high magnification so that they can compare the surface characteristics of the samples to see if they are consistent with each other.
The scanning electron microscope was invented in 1938 by Manfred von Ardenne. The instrument is capable of distinguishing objects that are 3 nanometers apart, compared with 200 nanometers for simple light microscopes. Because of this, scanning electron microscopy (SEM) can be used for imaging and comparing very small surface characteristics.
In forensic science, SEM is typically used for comparing known samples to unknown samples recovered from crime scenes. SEM can be used to compare virtually any kind of materials, but in forensic laboratories it is most commonly used for imaging tool marks, fibers, minerals, soil samples, and gunshot residue.
The scanning electron microscope consists of an electron gun mounted atop a series of electromagnetic lenses and scanning coils, a sample holder, and a detector. The electron gun produces a high-powered electron beam that is focused by condenser lenses and scanned back and forth across the sample being imaged. The beam penetrates into the sample and causes the production of four things: low-energy secondary electrons (SEs), backscatter electrons (BSEs), X rays, and heat. The SEs are ejected from just under the surface of the sample, enter the detector, and are converted electronically to an image of the sample surface. Flat areas of the sample appear dark, and elevated areas appear lighter.
Some instruments also contain backscatter detectors that produce images using BSEs. These images have lower resolution than SE images, but they are unique in that in these images, heavier compounds appear brighter than lighter compounds. For example, a mixture of calcium carbonate and magnesium carbonate would appear simply as small grains when viewed in SE mode, but when viewed in BSE mode, the grains of calcium carbonate would appear brighter because calcium is a heavier element than magnesium.
SEM is often coupled with energy-dispersive X-ray spectroscopy (EDS or EDX), which provides an elemental profile of the section of the sample being viewed. This is especially useful for comparisons of items that look the same under SE mode, such as two pieces of paper. An analyst who is trying to determine whether two pieces of paper are consistent with each other may first view the two samples under SEM. If they appear different, the samples can be said to be inconsistent. If the samples appear similar, the analyst could perform EDS to see if the samples contain all the same trace elements. It may be found, for example, that one piece of paper has trace amounts of copper in it and the other does not. In that case, the two samples may be determined inconsistent even if they appear similar under SEM.
Bibliography
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Goldstein, Joseph, et al. Scanning Electron Microscopy and X-Ray Microanalysis. 3d ed. New York: Kluwer Academic/Plenum, 2003.
Li, Zhigang R. Industrial Applications of Electron Microscopy. New York: Marcel Dekker, 2003.
Madkour, Fatma, and Mohammed Abdelsabour-Khalaf. "Performance Scanning Electron Microscopic Investigations and Elemental Analysis of Hair of the Different Animal Species for Forensic Identification." Microscopy Research and Technique, vol. 85, no. 6, 2022, pp. 2152-2161, doi.org/10.1002/jemt.24073. Accessed 19 Aug. 2024.
Pilkington, Ben. "A Current View of Scanning Electron Microscopy in Forensics." AZO Materials, 25 Apr. 2022, www.azom.com/article.aspx?ArticleID=21613. Accessed 19 Aug. 2024.