Final answer:
Specimens for SEM must be dehydrated to prevent physical damage and maintain image integrity. Water evaporates in the vacuum, causing specimen distortion and interfering with the electron beam.
Step-by-step explanation:
When viewing a specimen in a Scanning Electron Microscope (SEM), it is essential that there is no water present due to the operating conditions of the microscope.
The main reason is that the specimen is observed in a vacuum within the SEM, which means any water present would rapidly evaporate.
This sudden evaporation causes physical damage to the specimen, altering its surface structure, and thus leading to inaccurate imaging.
Secondly, water within the specimen can lead to the formation of vapor inside the SEM's vacuum chamber, which can interfere with the electron beam and deteriorate the quality of the image.
Sputter-coating with metal, such as gold or palladium, is a critical part of preparing a specimen for SEM because it adds conductivity to the sample and prevents it from becoming charged under the electron beam, which would otherwise distort the final image.
Preparative techniques like critical point drying with liquid carbon dioxide are used to ensure the specimen is completely free of water before undergoing sputter-coating and subsequent observation in the SEM.