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.


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