Variable Pressure Electron Microscopes

Variable pressure electron microscopes are equipped with environmental chambers, which allow them to maintain a pressure differential between the high vacuum levels required in the gun and column area and the relatively low pressures used in the chamber. This facility means that the microscope can be used to examine uncoated specimens such as type material, pinned insects, mineralogical specimens, and fossils. Variable pressure electron microscopes allow the pressure in the sample chamber to be maintained at a much higher level than in conventional SEMs. The presence of molecules of air (or inert gas) in the chamber has two major effects. First, it significantly reduces outgassing from samples that are hydrated or oily. This allows these types of samples to be examined without the need for complex sample preparation, such as freeze-drying or critical point drying, etc. The second important benefit is that the air molecules also help dissipate the build up of charge on the surface of nonconducting specimens, which means that they no longer need to be coated with a conducting layer before examination.