shadowing

Shadowing is a technique to obliquely deposit metals (platinum, etc.) of a high scattering power with a low angle to the specimen surface of a biological specimen composed of proteins, DNA, viruses, etc., under vacuum. By shadowing, the TEM image contrast due to roughness of the surface of those specimens is enhanced. Biological molecules, such as proteins, DNA and viruses, are composed of light elements of carbon, nitrogen, oxygen or hydrogen. When a specimen of these molecules is observed with a TEM, most incident electrons pass through the specimen without suffering scattering. As a result, an obtained image hardly shows contrast. To solve this problem, metals (platinum, etc.) are vacuum-deposited onto the molecule particles. This deposition makes it possible to enhance the contrast of the surface roughness of fine structures. As the deposition angle is lower, the finer structures can be observed. When the deposition angle is higher, the fine structures are more embedded in the deposited metal. The technique is called "shadowing" or "shadow casting" because the metal-deposited part in the TEM image appears as a shadow.

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