The conventional type of SEM uses a high vacuum with pressures as low as 10-5 Torr. This means that to get a good image, the specimens to be viewed must be electrically conductive, and devoid of water and solvents that could vaporize in the vacuum causing problems in the column. Therefore, biological tissue samples have to be absolutely dry before they could be put in the conventional SEM. As you know, being devoid of water is not compatible with living samples!
|To preserve the 3-D structure of biological samples, the samples have to be carefully dried using liquid carbon dioxide in a machine called a critical point dryer. Then the samples have to be coated with a thin layer of metal such as gold to make the sample electrically conductive. All this takes a lot of time - many hours.|
Our SEM is special in that it images the sample under different vacuum conditions. The JEOL JSM-5800LV scanning electron microscope is called a Low Vacuum SEM and it can work with a maximum operating pressure of up to 2 - 4 Torr (250 Pascals)! This means that we can look at wet samples quickly in the SEM. The procedure is that we cut a piece of tissue, stick it down on tape and it's ready for viewing in the SEM. Our SEM can also view samples in the conventional mode, but they need to be prepared as for conventional SEMs.
The low vacuum SEM has a panel on the front which carefully controls the vacuum pressure. This picture shows the front panel of the SEM. It is set to 80 Pascals using the controller button, and is in the low vacuum mode as evidenced by the LV button being lit. If the SEM was in the high vacuum mode, the LV button would not be lit and would read 0 Pascals when under high vacuum.
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David J. Wright and Shirley J. Wright
Department of Biology, The University of Dayton.
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Copyright © 1999 Shirley J. Wright (Shirley.Wright@notes.udayton.edu)