Scanning electron microscopy examines structure by bombarding the specimen with a scanning beam of electrons and then collecting slow moving secondary electrons that the specimen generates. These are collected, amplified, and displayed on a cathode ray tube (CRT, typically a slower version of the picture tube of a television set). The electron beam and the cathode ray tube scan synchronously so that an image of the surface of the specimen is formed. Specimen preparation includes drying the sample and making it conductive to electricity, if it is not already. Photographs are taken at a very slow rate of scan in order to capture greater resolution. All our SEMs use digital imaging. SEM is typically used to examine the external structure of objects that are as varied as biological specimens, rocks, metals, ceramics and almost anything that can be observed in a dissecting light microscope. The Zeiss 960 is used for larger samples that require lower resolution. Maximum sample size is about the size of a baseball, but certain accomodations may have to be made. Specimens are typically mounted on "specimen stubs".
ZEISS DSM-960A SEM Characteristics
This microscope joined the lab in 2003 and is the microscope used for instruction in various courses and also the Oklahoma Ugly Bug contests (see http://www.uglybug.org/). The DSM is among the first digital SEMs made. This microscope is a composites of parts of SEMs donated by Conoco-Phillips and the OSBI.
The ZEISS 960 accepts conventional specimens that are mounted on stubs, which are 1 cm wide discs that have a pin-mount on the base of the disc. Specimens are mounted on to the stub before being made conductive -- usually by sputter coating the specimens with gold/palladium. Because of the palladium content, specimens typically appear silverish. Although the coating is coarse by TEM or JSM-880 standards, the grains cannot be resolved in the DSM-960.
Below are some examples of stubs used for the DSM-960 from the Oklahoma Uglybug Contest which is an annual outreach program for OU's SRNEML, as well as other EM labs around the State. Specimens are kept in a desiccator under vacuum and often with silica beads to keep them dry so that they do not change or decay. Mothballs are used as an effective deterrent for biological activity when sheets of specimens are too large to allow storage in a vacuum.
Specimen storage for conventional SEM is facilitated by using commercially available Plexiglass sheets with holes for the specimen pins. Hundreds of specimens can be mounted, indexed and stored on such a sheet.
Here are some insects from the Oklahoma Uglybug contest at higher magnification.