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electron microscope

An instrument that uses electromagnetic lenses to focus a parallel beam of electrons and produce an image by differential electron scattering. The *resolving power of an electron microscope is far greater than that of a *light microscope because the wavelength of an electron beam is 0.005 nm as compared to about 600 nm for yellow light. In theory this should mean that objects only 0.0025 nm apart could be distinguished. However in practice aberrations in the lenses (electromagnetic fields), deterioration of the specimen during observation, and other technical difficulties reduce resolution to about 1 nm. This is still some 300 times better than that using light. Two principal types of electron microscope are used. In the transmission electron microscope (TEM) the electron beam, which is usually produced from a tungsten filament, passes through a condenser lens system and is focused onto the specimen, which is normally an ul-trathin section held on a fine copper grid. The image of the specimen, focused by the objective lens and magnified by the projector lens, is then projected onto a fluorescent screen or photographic plate. The entire microscope column is under high vacuum to prevent the beam of electrons from being scattered by the molecules in air. The casing of the microscope must therefore be completely dry and the specimen dehydrated. The magnification is altered and the   focusing   adjusted   by  varying   the current through the electromagnetic lenses.
In the more recently developed scanning electron microscope (SEM), commercially available since the mid 1960s, the surface of the specimen is observed and a three-dimensional appearance is obtained. The beam of incident electrons ejects secondary electrons and back-scattered electrons from the specimen, which is usually treated by *coating. These reflected electrons are collected by a scintillator and an image is built up on a high-resolution cathoderay tube as the electron beam passes over the specimen in a sequence of scanning movements. The resolution obtained in the scanning electron microscope is lower than in the transmission electron microscope but is constantly being improved upon and instruments capable of resolving 2 nm are now available. Some electron microscopes are capable of operating in several different modes, either as scanning or transmission electron microscopes or in a scanning transmission electron microscope (STEM) mode. Microscopes combining optical and transmission electron microscopy are also available for making high and low magnification observations of the same specimen.
Since the electron beam produces x-rays from the specimen, attachments are also available that permit the composition of the specimen, or part of it, to be determined by comparison of the radiation produced.
One of the main limitations of the electron microscope apart from its expense is that living material cannot be viewed. The specimen has to be fixed, dehydrated,   etc.,   before   observation.
Hydrated specimens can, however, be examined by *low-temperature scanning electron microscopy and one direction of current research is aimed at producing a special specimen chamber tha could be operated at atmospheric pres-sure and isolated from the high vacuum system of the microscope. See also coating , staining , freeze drying , freeze fracturing , replica plating , shadowing.

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