The Transmission Electron Microscope (TEM) is a very important tool for
many aspects of science. A biologist might use a TEM to study the membrane of a subcellular
organelle and the chemist might use it to identify a crystalline substance.
TEM images are created using electrons instead of the photons of light we use to see the normal
world around us, or even the microscopic world as seen through the familiar light
microscope. Electrons are often more useful than photons for imaging because they
can travel at shorter wavelengths than light. This permits magnification and imaging of a
specimen by electrons up to 800 times greater than the best light microscope. The theoretical
limit of magnification with the resolution of light microscopy is about 2,000X whereas a high
quality TEM can magnify and resolve a specimen greater than 1,500,000X!
TEM is generally used to study ultra-thin sections of specimens, and allows scientists to
observe and analyze internal microstructures. These ultra-thin sections are typically 50-60
nanometers which is about one-third thinner than the shortest wavelength of visible light!!
Ultra-thin sections are produced using a special sectioning machine equipped with a diamond
knife. The most common imaging process begins by passing, or transmitting, a beam of
electrons through the ultra-thin section to a detector below the specimen. The specially prepared
sections are treated with heavy metal compounds that adhere selectively to the various
microstructures of the specimen. The areas with the highest concentrations of heavy
metal compounds are more dense than surrounding areas. Therefore, fewer electrons pass
through to the detector there, and the area appears darker in the final image. The signal from
the detector is processed and displayed on a fluorescent material for observation, recorded on
photographic film as a permanent record, or digitally captured for analysis and/or storage.
You can think of this process as standing over a screen with areas of differing mesh sizes,
the ultra-thin section of a specimen, and dropping a handful of BBs coated with wet white paint,
the electron beam, through the screen onto a piece of black paper, detector. The areas of the
black paper that get white paint from the BBs would be a representation of the pattern of mesh
openings in the screen.
Other, more specialized, imaging techniques include shadow casting, which means coating
specimens having microtopography so that shadows are imaged when the electron beam passes
through the specimen. In another technique, crystalline structure can be determined and
crystalline substances identified by studying the patterns of electron diffraction when the
electron beam passed through an ultra-thin layer of crystals.
Our Zeiss transmission electron microscope with a charge-coupled device (CCD) camera
interfaced to a computer workstation provides a comprehensive image collection and analysis
capability. This system is interfaced with the Information Technology Services' Network.