|transmission electron microscopy (TEM) and cryo-TEM
The resolution of the TEM is dependent on the accelerating voltage used, as this in turn determines the wavelength of the electrons. Using an accelerating voltage of 100 kV the resolution can be improved 1000-fold compared to light (electrons thereby have a wavelength of 0.0038 nm compared to about 550 nm for light).
A very important precondition is that the examined samples are thin enough (max. 100 nm) in order for the electron beam to pass through. The part of the incident electrons that are scattered by the interaction with atoms of the sample contributes to the image formation. The number of scattered electrons depends on the sample thickness. Their scattered angle and therefore the contrast are directly proportional to the mass density.
Because of their low average atomicity biological samples (main components of which are carbon, nitrogen, oxygen and hydrogen) have only a minor appropriate contrast. To overcome this disadvantage, a set of preparation techniques was developed in the past decades. In our laboratory we offer a broad spectrum of these techniques to prepare the samples for the high vacuum conditions in the TEM column.
||scanning electron microscopy (SEM)
The SEM allows the non-destructive imaging of the surface of large samples (several mm to cm). The electron beam is focused on the electrically conductive sample in the specimen chamber by scanning the surface line by line. Due to the interaction with the sample secondary and backscattered electrons are generated, detected and assembled to an artificial image.
Thereby surface structures with a high depth of field can be made visible in the size range of nm to mm. In comparison to a light microscope the SEM has a 1000-fold higher resolving power and a 100-fold higher depth of field.
The beam broadening in the sample and the generation of secondary and backscattered electrons limits the resolving power.
To examine samples in a SEM they have to be vacuum resistant and electrically conductive. Samples without these requirements have to be prepared with special techniques.