Low Temperature Scanning Electron Microscope

Hitachi S-4100 field emission Scanning Electron Microscope

Techniques developed for observing snow and ice crystals with low-temperature scanning electron microscopy are relatively easy to use and have been found to be successful in a wide range of snow and ice environments. Samples of snow, ice and associated life forms are collected by dislodging the crystals or biota from the face of a snow pit or the surface of the snow onto copper metal sample plates containing precooled methyl cellulose solution. Within fractions of a second these plates are plunged into a reservoir of liquid nitrogen which rapidly cools them to -196C and attaches these prefrozen materials to the plates. Due to the low surface tension of liquid nitrogen and the extreme hardness of materials cooled to these temperatures, very fragile samples can be shipped by aircraft, in dry shipping dewars from study sites throughout the country. After arrival at the Beltsville Electron Microscopy facility, the copper plates can be stored at -196C in storage dewars. Selected samples are transferred to the preparation chamber of an Oxford CT 1500 HF Cryotrans System for sputter coating with platinum. This renders them electrically conductive and they are placed on the precooled (-170C) stage of a Hitachi S-4100 field emission Scanning Electron Microscope where they are imaged and photographed. Hydrologists study photographs of the grain sizes, shapes and associations in relation to passive microwave remote sensing in an effort to determine the water content of the winter snow pack. This information is critical to the determination of the nation's water supply as well as protection from flooding.

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