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What is a Scanning Electron Microscope?

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  • Written By: Emma Lloyd
  • Edited By: Bronwyn Harris
  • Last Modified Date: 11 November 2016
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The scanning electron microscope is a piece of equipment which uses high-energy electron beams to generate information about a microscopy sample. The information that is generated is then resolved into an image of the sample. Scanning electron microscopes are up to 250 times more powerful than light microscopes, and can magnify images up to 500,000 times.

A standard scanning electron microscope can resolve images of items as small as five nanometers in size. One nanometer is one billionth of a meter, or approximately four billionths of an inch. These microscopes can generate accurate images of organisms as small as viruses, and even bacteriophages, which are viruses that infect bacteria.

In addition to its ability to magnify such small specimens, another useful characteristic of the scanning electron microscope is that it can produce three-dimensional images. This is because the microscopes have a wide depth of field, allowing objects in the background and foreground to remain in focus simultaneously. This makes scanning electron microscopy highly useful for determining the surface structure and the 3D shape of samples.

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Due to the way the machine works, proper sample preparation is a vital aspect of scanning electron microscopy. There are two important parts to preparation. First is the fact that samples must be coated in an electrically conductive substance such as gold, platinum, or chromium. This is important to reduce the build-up of electrostatic during the process. The second important aspect is that samples are examined in a vacuum, meaning they must be completely dry. For this reason, biological samples are chemically fixed with a substance such as formaldehyde to preserve tissue structure.

The operation of the scanning electron microscope involves an electron gun, magnetic lenses, and an electron detector. Once the specimen is placed on the microscope stage and the process begins, the electron gun begins to fire. The gun fires an electron beam through an anode, then through two magnetic lenses, and then the electron detector.

In conjunction with the condenser lens of the microscope, this process effectively concentrates the beam of electrons so that it can accurately strike the specimen. When this happens, the electrons begin to interact with the sample, and the detectors in the microscope count the number of interactions which occur. The number of interactions then dictates how pixels appear on the monitor that displays images. The more interactions that occur, the brighter the pixels appear. The contrast in the brightness of pixels makes up the image.

Scanning electron microscope images are generated without the use of light waves; therefore the images are always in black and white. These are highly detailed three-dimensional images, and despite the lack of color they are extremely accurate. Images can be colorized to make them appear more vivid and improve the contrast.

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