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Resolution and contrast in an electron microscope image are dependent in part upon several factors in the selection or preparation of the specimen over which the electron microscopist has some control. For example, there are various ways of observing the specimen: as a dispersed powder; as a thin section; and indirectly by the use of surface replicas. One cannot expect the same limits of resolution for each of these methods, and even within each method the resolving power and contrast vary with specimen thickness, film thickness, stability of the specimen, and stability of the supporting film or replica. In order to establish some basis for selecting one method over the other, a brief discussion of some of the problems peculiar to the preparation techniques used in electron microscopy will be given. The techniques described are intended only as a guide. The individual must decide what method is best for his particular specimen and may often have to modify existing methods.

Because of the need to evacuate the lens column and specimen chamber to pressures in the range of 10-4 mm Hg, the specimen must be in a dry state. In this connection it must be noted that even the presence of thin films of organic materials oflow volatility can have a harmful effect by contributing to the formation of a carbonaceous film at the surface of the specimen due to breakdown of the structure in the electron beam; such films can seriously reduce contrast in the image and impair resolution.

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