Electrons produced by heating a filament (Tungsten or LaB6) at voltages ranging from 60-120kV, are fired towards the sample down a column held under vacuum. Electromagnetic lenses allow the beam to be focused and magnification to be modulated. The images formed are resolved on a phosphor fluorescent screen and can be recorded using either digital or film-based cameras.
In order to visualise the specimen, the sample must be thin enough (typically less than 200 nm) to allow the electrons to pass through it. This often involves sectioning samples using an ultramicrotome, alternatively particle suspensions and cell fractions can be added directly to the EM grid for processing. Contrast is generated by staining the sample with electron-absorbing heavy metals allowing cellular ultrastructure to be more clearly observed. For example, osmium tetroxide stains lipids providing a bold outline of all of the cellular compartments. In addition, modern TEMs are increasingly capable of visualising ultrastructure in very low contrast samples such as those associated with frozen hydrated samples.
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