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Electron microscope

The electron microscope can magnify very small details due to the use of electrons rather than light to scatter off material, magnifying at levels up to 500.000 times.

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History The first electron microscope was built in 1931 by Ernst Ruska. It was greatly developed through the 1950s and has allowed great advances in the natural sciences. The advantage of an electron beam is that it has a much smaller wavelength (see wave-particle duality), which allows a higher resolution - the measure of how close together two things can be before they are seen as one. Light microscopes allow a resolution of about 0.2 micrometres, whereas electron microscopes can resolutions below 1 nanometer.

Process Electron beams from a cathode are focused by magnetic lenses on to the specimen. They are then magnified by a series of magnetic lenses until they hit photographic plate or light sensitive sensors - which transfer the image to a computer screen. The image produced is called an electron micrograph (EM).

Types The Transmission electron microscope (TEM) produces 2D images (for example of cells) while the Scanning electron microscope (SEM) produces 3D images or models. As its name implies the TEM image is produced by detecting electrons that are transmitted through the sample. By contrast the SEM usually monitors secondary electrons which are emitted from the surface due to excitation by the primary electron beam. Generally, the TEM resolution is about an order of magnitude better than the SEM resolution, however, because the SEM image relies on surface processes rather than transmission it is able to image thicker samples and gives better 3D contrast.

Treatment Samples viewed under an electron microscope have to be treated in many ways:

  • Fixation - is preserving the sample to make it more realistic. Glutaraldehyde - for hardening - and osmic acid[?] - which stains lipids black - are used.
  • Dehydration - is the removing of water to be replaced with an embedding medium such as ethanol or propanone.
  • Embedding - supports the tissue for sectioning in a resin such as araldite[?].
  • Sectioning - produces thin slices for mounting. These can be cut on an ultramicrotome[?] with a diamond knife to produce very thin slices.
  • Staining - uses metals such as lead and uranium to reflect electrons to give contrast between different structures.

Disadvantages The samples have to be viewed in vacuums, as air would scatter the electrons. This means that no living material can be studied.
The samples have to be prepared in many ways to give proper detail resulting in artifacts - objects purely the result of treatment, and this gives the problem of distinguishing artifacts from biological material. Electron microscopes are also very expensive to buy and maintain.


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