The Scanning Electron Microscope (SEM) provides an image of surfaces and is capable of both high magnification and good depth of field, so that even at low magnifications it is often more useful than an optical light microscope.
1. How is the SEM different from a light microscope?
Unlike a light microscope, the SEM uses electrons instead of white light to view the specimen. With the SEM you can only view inanimate materials, but you can magnify over 100,000 times. Rather than seeing "through and inside" a living organism, as you would with a light microscope, you are viewing the surface details. SEM images are in black and white because only light carries colour information, but the images can have false colour added to them using computer software.
2. How does it work?
The scanning electron microscope (SEM) provides a three-dimensional high-resolution image of cells and tissues. In SEM the surface of the tissue is studied.
Tissues prepared for SEM are usually chemically fixed using Glutaraldehyde and then dehydrated in ethanol before being dried. Drying at critical point has become the preferred method. The tissue is introduced into liquid carbon dioxide which is then brought to its critical point, which is the combination of pressure and temperature at which the fluid and gaseous phases exist together without an interface or meniscus. Thus there is no surface tension. The presence of surface tension during drying is disruptive to a tissue and causes visible distortions.
After drying, the surface of the tissue is sputter coated in a vacuum with an electrically conductive layer of gold.
The coated dry sample is now placed in a vacuum so that the electron beam can move without interference. The electrons are generated from a thin tungsten wire in the "gun" of the SEM. Electricity is passed through the wire and then focused by magnets onto the sample. When the electrons from the gun strike the surface coating of gold, electrons are reflected back off the specimen to a detector, this is transmitted to a TV screen where the image is viewed and photographed.
Only inanimate items can be viewed under an SEM: nothing can survive the preparation stage which involves water and air deprivation plus being wrapped in gold!
Preparation times
Depending on the sample, this can vary from 30 minutes for air dried samples eg insects, to 3-4 days if sample has to fixed, dehydrated and critical point dried. See link below for sample procedure.
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