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.

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.