Electron Microscope (EM)
Lab-2-
Sunday 13/12/2015
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image.
Electron microscopes (EM) have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than visible light (photons), and magnifications of up to about 10,000,000x, whereas ordinary.
The electron microscope uses electrostatic and electromagnetic "lenses" to control the electron beam and focus it to form an image.
These lenses are analogous to, but different from the glass lenses of an optical microscopes that form a magnified image by focusing light on or through the specimen.
Electron microscopes are used to observe a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals, and crystals.
Electron microscope constructed by Ernst Ruska in 1933
Electron microscope (E.M)
A 1973 Siemens electron microscope, Musée des Arts et Métiers, Paris
Electron microscope (E.M)
Electron microscope (E.M)
History of the (EM)
The electron microscope was invented and patented by Hungarian physicist Leo Szilárd who declined to construct it.
Instead, German physicist Ernst Ruska and electrical engineer Max Knoll constructed the prototype electron microscope in 1931, capable of four-hundred-power magnification.
Electron microscope constructed by Ernst Ruska in 1933.
Light Micrograph of Human Red Blood Cells
Types of the Electron Microscope (E.M)
1-Transmission Electron Microscope (TEM)
The original form of electron microscope, the transmission electron microscope (TEM) uses a high voltageelectron beam to create an image.
The electrons are emitted by an electron gun, commonly fitted with a tungsten filament cathode as the electron source.
The electron beam is accelerated by an anode typically at +100 keV with respect to the cathode, focused by electrostatic and electromagnetic lenses, and transmitted through the specimen that is in part transparent to electrons and in part scatters them out of the beam
Biological specimens typically require to be chemically fixed, dehydrated and embedded in a polymer resin to stabilize them sufficiently to allow ultrathin sectioning.
Sections of biological specimens, organic polymers and similar materials may require special `staining' with heavy atom labels in order to achieve the required image contrast.
Transmission Electron Microscope (TEM)
Transmission Electron Micrograph of Human Red Blood Cells
Transmission Electron Micrograph of Mitochondrion
Transmission Electron Micrograph of a Cross Section OF Cilia
2-Scanning Electron Microscope (SEM)
Unlike the (TEM), where electrons of the high voltage beam carry the image of the specimen, the electron beam of the scanning electron microscope (SEM) does not at any time carry a complete image of the specimen.
The (SEM) produces images by probing the specimen with a focused electron beam that is scanned across a rectangular area of the specimen(raster scanning).
When the electron beam interacts with the specimen, it loses energy by a variety of mechanisms.
Scanning Electron Microscope (SEM)
Main structures in the (SEM)
Scanning Electron Micrograph of Human Red Blood Cells
Scanning Electron Micrograph of a Blood Clot
Scanning Electron Micrograph of the Mucous Membrane Lining the Trachea
Types of (SEM)
A-Environmental Scanning Electron Microscope (ESEM)
Can produce images of sufficient quality and resolution with the
Samples being wet or contained in low vacuum or gas.
This greatly facilitates imaging biological samples that are unstable in the high vacuum of conventional electron microscopes.
B-Low Temperature Scanning Electron Microscope (LTSEM)
Can use in the studying of the freezing specimen and crystals.
3-Reflection Electron Microscope (REM)
In the reflection electron microscope (REM) as in the (TEM), an electron beam is incident on a surface, but instead of using the transmission (TEM) or secondary electrons (SEM), the reflected beam of elastically scattered electrons is detected.
4-Scanning Transmission Electron Microscope (STEM)
The (STEM) rasters a focused incident probe across a specimen that (as with the TEM) has been thinned to facilitate detection of electrons scattered through the specimen.
The high resolution of the (TEM) is thus possible in (STEM).
The focusing action (and aberrations) occur before the electrons hit the specimen in the (STEM), but afterward in the (TEM).
The (STEM) use of SEM-like beam rastering simplifies annular dark-field imaging, and other analytical techniques, but also means that image data is acquired in serial rather than in parallel fashion.
Often (TEM) can be equipped with the scanning option and then it can function both as (TEM and STEM).
5-Low-Voltage Electron Microscope (LVEM)
The Low-Voltage Electron Microscope (LVEM) is a combination of (SEM, TEM and STEM) in one instrument, which operates at relatively low electron accelerating voltage of 5 kV.
Low voltage reduces the specimen damage by the incident electrons and increases image contrast that is especially important for biological specimens.
Resolutions of a few nm are possible in (TEM, SEM and STEM) modes.
Applications of the (EM)
1-Protein localization
2-Electron tomography
3-Cellular tomography
4-Microscopy
5-Toxicology
6-Biological production and viral load monitoring
7-Particle analysis
8-Structural biology
9- 3D tissue imaging
10-Virology
L.A. SURA SALAH