ELECTRON MICROSCOPE - DIFFERENCE BETWEEN SEM & TEM

 ELECTRON MICROSCOPE

·       The Electron microscope was developed by Ernst Ruska and Max Knoll in 1931 and was in use in many laboratories by the early 1940s.

·       Objects smaller than about 0.2 µm, such as viruses or the internal structures of cells, must be examined with an Electron microscope.

·       Generally, Electron microscopes magnify objects 10,000× to 100,000×, though millions of times magnification with good resolution is possible.  

·       In Electron microscopy, a beam of electrons is used instead of light. Like light, free electrons travel in waves.

·       The Resolving power of the electron microscope is far greater than that of the other microscopes.  

·       Instead of using Glass lenses, an Electron microscope uses Electromagnetic lenses to focus a beam of electrons onto a Specimen.

·       Electron microscopes provide detailed views of the smallest bacteria, viruses, internal cellular structures, biopsy samples, metals, crystals, large molecules and large atoms.

·       Cellular structures that can be seen only by using electron microscopy are referred to as a cell’s Ultrastructure. Ultrastructural details cannot be made visible by Light microscopy because they are too small to be resolved.

·       Images produced by Electron microscopes are always black and white, but they may be colored artificially to accentuate certain details.

·       Photographs taken on any Microscope are called Micrographs. Photographs taken on an Electron microscope are called Electron micrographs.

Parts of Electron Microscope

a) Electron Gun

·       Heated Tungsten filament, which generates Electrons.

b) Electromagnetic Lenses

·       Condenser lens – Condenser lens focuses the Electron beam on the Specimen. A Second Condenser lens forms the Electrons into a Thin tight beam.

·       Objective lens - Electron beam coming out of the Specimen passes down the second of Magnetic coils called the Objective lens, which has high power and forms the intermediate magnified image.

·       Projector or Ocular lens – It is the third set of Magnetic lenses which produce the final further Magnified image.

 

c) Specimen Holder

·       The specimen holder is an extremely thin film of carbon or collodion held by a metal grid.

d) Image viewing and Recording system

·       The final image is Projected on a Fluorescent screen.

·       Below the Fluorescent screen is a Camera for recording the Image.

Advantages of Electron Microscope

·       Very high magnification

·       Incredibly high resolution

·       Material rarely distorted by preparation

·       It is possible to investigate a greater depth of field

·       Diverse applications

Limitations of Electron Microscope

·       Electron microscopes are much more expensive than Light microscopes.

·       Electron microscopes also take up much more space and require additional rooms for preparation of Specimens and for processing of Photographs.

·       Requires highly skilled persons to operate.

·       The live specimen cannot be observed.

·       As the penetration power of the electron beam is very low, the object should be ultra-thin. For this, the specimen is dried and cut into ultra-thin sections before observation.

As the Electron microscope works in a vacuum, the specimen should be completely dry.

Types of Electron Microscope

·       There are two types of Electron microscopes: (i) Scanning Electron Microscope (SEM) and (ii) Transmission Electron Microscope (TEM).

Difference between SEM and TEM

S. No	Scanning Electron Microscope	Transmission Electron Microscope
1	Low Resolution.	High Resolution.
2	Resolving power is 10 nm	Resolving power is 0.1 nm
3	Electron beam scans over the surface of the sample	Electron beam pass through the sample
4	Maximum magnification is 1 to 2 million times.	Maximum magnification is 50 million times.
5	Voltage requirement is high (50 to 30000 volts)	Voltage requirement is low
6	Specimen preparation is easy	Specimen preparation needs skilled persons and very thin specimen (100 nm) is required.
7	Aluminium stubs are used for specimen mounting	Thin films on Copper grid are used for specimen mounting
8	Specimen fixation is completed by Aldehydes and Osmium tetroxide.	Specimen fixation is completed by two methods, (i) Chemical fixation of specimen and (ii) Cryofixation fixation of the specimen.
9	Specimen is coated with a conductive material to prevent the charge buildup on specimen surface	No coating is required for Specimen.
10	Specimen contrast formed by electron adsorption	Specimen contrast formed by electron scattering
11	Electron beam scans over the surface of the sample and create an image of the specimen.	Electron beam passes through the sample and creates an image of the specimen.
12	For Image formation, Electrons are captured and counted by Detectors and image will appear on PC screen.	Formed image will appear on a Fluorescent screen or PC screen.
13	Produces 3D Black and White images	Produces 2D Black and White images
14	Shows only the surface morphology of the specimens, macromolecular aggregates and tissues.	Shows multiple characteristics of objects such as Ultrastructure of cell, Crystallization, Viruses and Tissues
15	Less expensive than TEM	More Expensive than SEM
16	Requires less time when compared to TEM	Long process and requires more time when compared to SEM
17	Relatively safe to human use	Relatively detrimental to human health