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Principals and Limitations of Electron Microscopy

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´╗┐Principals and Limitations of Electron Microscopy There are many types of microscopes, which all have different functions and uses in science. One of the most recently used ones were light microscopes which were popular before the 1930s. However they had a poor resolution due to the long wavelength of light. In the 1930s, a type of microscope was developed which used a beam of electrons rather than light. These were therefore called electron microscopes. There were two types of electron microscopes; they are Scanning Electron and Transmitting Electron Microscopes. They have two main advantages: it has a high resolving power due to the short wavelength of electron beam and because the electrons are negatively charged the beam can be focused using electromagnets. Since the use of modern electron microscopes, they can resolve objects which are just 0.1nm apart which is nearly 2000 times better than what a light microscope could do. Due to the absorption of electrons by the molecules in the air, a near-vacuum need to be generated the chamber of the electron microscope in order for it to work effectively. Transmission Electron Microscopes--------------------------------------------------------------------------------------------------- The Transmission Electron Microscope or TEM was invented over 70 years ago. ...read more.


Moreover, the image may contain artefacts. Artefacts are the things that results in the preparation of the samples. They are not part of the actual specimen but appear on the photomic-rographs. So we can?t be sure what we see on the photomicrograph is what is really on the specimen. Lastly, because the specimen is really thin, the TEM only creates a 2-D image. So if you what a 3-D image they analyse a few different parts of the specimen and put them together for a 3-D image. Scanning Electron Microscope---------------------------------------------------------------------------------------------------------- The SEM pass a beam of electrons over the surface of the specimen in the form of a ?scanning? beam. Electrons are reflected off the surface of the specimen as it has been previously coated in heavy metals. It is these reflected electron beams that are focussed of the fluorescent screen in order to make up the image. The reason a three-dimensional image is created is because a SEM scans the beam of electrons back and forth across a sample, which is known as raster scanning. The electrons hitting a substance are either reflected or absorbed by the substance. ...read more.


7. 6) Cryofixation ? freezing a specimen so rapidly, to liquid nitrogen or even liquid helium temperatures, that the water forms vitreous (non-crystalline) ice. This preserves the specimen in a snapshot of its solution state. 8. 7) Sectioning ? produces thin slices of specimen, semitransparent to electrons. These can be cut on an ultramicrotome with a diamond knife to produce ultrathin slices about 60-90 nm thick. Disposable glass knives are also used because they can be made in the lab and are much cheaper. There are some common and other disadvantages of Electron microscopy as well as the technicality of it. Electron microscopes are very expensive to buy and maintain. They are dynamic rather than static in their operation: requiring extremely stable high voltage supplies, extremely stable currents to each electromagnetic coil/lens, continuously-pumped high/ultra-high vacuum systems and a cooling water supply circulation through the lenses and pumps. As they are very sensitive to vibration and external magnetic fields, microscopes aimed at achieving high resolutions must be housed in buildings with special services. A significant amount of training is required in order to operate an electron microscope successfully and electron microscopy is considered a specialised skill. 1. 2. 3. 4. 5. 6. 7. ------------------------------------------------------------------------------------------------------------------- ________________ Tercy Ann Thomas Page ...read more.

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