the electron beam is affected by air and water molecules, so the sample must be placed in a vacuum. the sample must also be conductive to allow the electrons not reflected or absorbed to be conducted away. this gives some limits on the type of sample that can be used. in the case of samples from plants or animals, it often means that they must first be coated in gold before they can be placed in the microscope. this is done in order to preserve the sample and keep it from changing or decaying throughout the scanning process.