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SEM | TEM | Confocal Compound Light Microscope | Compound Light
Inverted Microscope | Compound Light Microscope with DIC |
Compound Light Microscope with Phase | Stereo Light Microscope(Olympus) |
Stereo Light Microscope (Leica) | Stereo Light Microscope with Leica Camera |
Zeiss Stemi | Compound Light Microscope with Phase | Digital SLR Camera
Why would You want to Use this Microscope: The SEM offers much greater depth of field, resolution, and much greater range of magnification (from about 40 to 50,000x) than light microscopy. The SEM images surface features. The low vacuum and ESEM modes of imaging require little or no specimen preparation thereby eliminating time and potential artifacts due to chemical fixation, drying and sputter coating.
1. Our Scanning Electron Microscope (SEM) is capable of three different modes of SEM imaging:
Conventional SEM (CSEM): CSEM involves the greatest amount of specimen preparation where typical fragile biological specimens are often fixed in aqueous fixatives like glutaraldehyde, dehydrated in ethanols, dried chemically or in a critical point dryer, mounted on stubs, and then sputter coated with gold/palladium so they become conductive to SEM beam electrons. Robust specimens like hard sclerotized beetles or ants can be air dried and then sputter coated.
Low Vacuum (LV): The Quanta 200 is a variable pressure SEM that uses a series of pressure apertures that can allow normal very high vacuum levels (low pressures) for the electron gun, but much higher pressures (much weaker vacuum) in the specimen area. The actual surface imaging in secondary electron mode is enhanced by water vapor molecules that lead to an amplified cascade of secondary electrons that are detected. Since water is actually necessary for imaging here, it means that specimens do not have to be completely dry. Also specimens do not have to be coated. While contrast levels of photographs maybe lower than specimens observed in CSEM, this can often be addressed by post-processing images in Adobe Photoshop. We find that back-scattered electron imaging with suitably adjusted analog and digital scope contrast often minimizes the need for Photoshop enhancement. LV mode greatly speeds up the imaging process and essentially wet or dry specimens like plant parts, hard insects, clam shells, rocks, cotton fibers, diatoms, foraminiferans, etc. can be imaged directly. A further benefit of variable pressure SEM is that charging is greatly minimized by the water vapor.
Environmental SEM (ESEM): The Quanta 200 possesses a specialized gaseous secondary electron detector (GSED) that bears a terminal aperture that allows pressures to achieve up to 20 Torr (2.6% of one atmosphere). Typically we get best results at 5 Torr (0.66 % of one atmosphere) and 2 degrees centigrade specimen temperature. ESEM allows not only the direct imaging of wet specimens but also the maintenance of specimens under user controlled humidity levels that can keep the specimen hydrated. At 5 Torr and 2oC, specimens are maintained at 95% Relative Humidity. Thus ESEM is a good choice for observing fragile live or recently dead specimens directly with or without fixation and with no dehydration or drying at all, thus saving considerable specimen processing time and dehydration/drying artifacts. Careful subtle microscope adjustments and also post-processing of the digital images in Adobe Photoshop (levels and smart sharpen) can result in excellent images in terms of detail and contrast.
2. Our SEM also includes an Oxford Inca x-act energy dispersive x-ray elemental detector for routine microanalysis of elements including Boron and above. This is a very useful tool for Geology and has potential for use in Biology. For example, some insects like Fire Ants strengthen their mandibular tips with Zinc. In our scope you can view the lighter Zinc tips in backscattered mode and you can confirm the presence of Zinc and Chlorine in the tips with the Inca x-act. The software is quite easy and intuitive to use and reports are easily made in Microsoft Word.
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