A form of microscopy that uses beams of electrons instead of beams of light and is able to achieve very high magnification, up to 1,000,000X (magnification by a factor of a million). SEM also has high depth of field, meaning that a large portion or depth of the image remains in focus no matter how high the magnification. This contrasts with traditional light microscopy, with which the higher the magnification, the shallower the depth of focus. SEM and the associated X-ray techniques (electron dispersive spectroscopy [also called energy dispersive] or X-ray diffraction [XRD]) are becoming increasingly important in forensic microscopy and have been used for gunshot residue (gsr), building materials, paint, dust, and other types of trace evidence.
A simple schematic of an SEM system is shown in the figure. Inside a vacuum chamber, an electron gun supplies a tightly focused beam of incident electrons that interact with the sample. Although many types of interactions result, it is the emission of back-scattered and secondary electrons that is used to create an image. On the sampleâ€™s surface, elements with higher atomic numbers (refer to the periodic table, Appendix III) will scatter more incident electrons and appear brighter than elements of lower atomic numbers. This difference will be discernible in the final image. Secondary electrons, which are actually emitted from the sample rather than scattered, are used to obtain information about surface features (topography). To generate an image, the electron beam is moved over the surface, scanning it as the back-scattered and secondary electrons are collected. The image is a display that shows the relative intensity of the electrons collected at a given location. Older systems used cathode ray tubes (CRTs, similar to older televisions and to CRT computer monitors) to display the image, while newer systems typically incorporate some type of digital imaging. Since the signal is only related to electron detection and not to the detection of light, as in traditional microscopy, the image is not colored. However, coloring (called false coloring) can be added to improve the visualization.
Another advantage of SEM is its ability to determine the elemental composition (elemental analysis) of the sample. When the incident electron beam interacts with atoms on the surface, inner shell electrons are ejected, and other electrons fall into those shells to fill the gaps. As a result, electromagnetic energy in the X-ray range of the electromagnetic spectrum is released. The wavelength and energy of the emitted electrons are characteristic of the elements from which they came, and this relationship allows for the identification of the elements. Energy dispersive spectroscopy (EDS) determines the element by measuring the energy of the emitted X-ray, while wavelength dispersive spectroscopy (WDS) does so by measuring its wavelength. Most SEM instruments use the EDS system, while electron microprobe instruments often incorporate the WDS detection system. These devices use electrons for elemental analysis but not for imaging, as in SEM. An SEM system can also be used for X-ray diffraction (XRD) to identify crystalline compounds.
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