A recent acquisition in 2000, the Environmental SEM (ESEM) allows researchers to examine samples with little or no preparation. This has opened up whole new avenues of research and allows the microscopic examination of samples that otherwise could not be imaged in the SEM.
In a traditional SEM the electron beam would interact with and scatter in the presence of air. This means the entire electron path including the sample must be in a vacuum environment. Since most living material is hydrated to some extent, some sort of preparation is necessary to make it compatible with the vacuum requirements of the SEM.
In addition most biologic samples do not conduct electrons well. When exposed to the electron beam they build a negative charge preventing further imaging. For this reason the samples are usually coated with a thin layer of electrically conductive noble metal-gold or gold/palladium alloy being preferred in this facility. In the museum setting though it is not desirable to permanently coat or destructively prepare the collection materials.
With the ESEM, most of the electron path is still in vacuum while the sample chamber is maintained at a higher pressure. Beam scatter is minimized by maintaining vacuum throughout its path just until it reaches the sample surface. When an ionizable gas, such as water, is leaked into the sample chamber, it interacts with electrons excited from the sample surface amplifying the signal and negating charge build up. Scattering of the beam is still present, but minimized by the short path through the gas field. This allows NMNH scientists to examine the collections nondestructively and in some cases has allowed examination of living tissues. Depicted to the right are images of living insects collected with the ESEM.
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