SEM topography preparation techniques
Similarly to the ultrastructure preparations described above this technique ensures the preservation of fine ultrastructural details and stabilisation of the specimen. However, the specimen is not in any way embedded. Since charging is much more of an issue than in TEM it is crucial to render the specimen conductive to avoid image distortions.
Therefore, a combination of postfixation with conductive heavy metal osmium and coating of specimen surface with gold is applied. Depending on application, both or only one of these measures is necessary. In order for it to be suitable for high vacuum imaging the specimen has to be dry. After dehydration, the solvent is replaced with a low surface tension agent (HMDS or liquid CO2) which ensures that no tears are inflicted on the specimen surface upon evaporation.
In certain cases the specimen can be neither dehydrated, postfixed with metals nor coated. It is possible to image such specimens in variable pressure mode (VP).
