Electron Microscopes Flashcards
Electrons can be focussed using electromagnets in a vacuum.
Why wouldn’t an electron microscope work effectively without vacuum conditions?
Electrons are absorbed and deflected by air molecules
This would create distorted images
What are Artefacts?
Artefacts aredamage caused in specimen preparationand can be confused with specimen ultrastructure.
Many artifacts are a result of mechanical or chemical action during sample preparation and some artifacts are due to irradiation by the electron beam during examination of the specimen in the microscope.
[Artefacts are things that result from the way the specimen is prepared. Artefacts may appear on the finished photomicrograph but are not part of the natural specimen. It is therefore not always easy to be sure that what we see on a photomicrograph really exists in that form.]
Why do light microscopes have a poor resolution?
Light microscopes have poor resolution as a result of the relatively long wavelength of light.
State 2 advantages of using the electron microscope.
- The electron beam has a very short wavelength and the microscope can therefore resolve objects well - it has a high resolving power.
- As electrons are negatively charged the beam can be focused using electromagnets
Why do a near vacuum need to be created for an electron microscope to work?
(This is a disadvantage)
The best modern electron microscopes can resolve objects that are just 0.1 nm apart - 2000 times better than a light microscope.
Because electrons are absorbed or deflected by the molecules in air, a near-vacuum has to be created within the chamber of an electron microscope in order for it to work effectively.
What are the 2 types of electron microscope?
⚫ the transmission electron microscope (TEM)
⚫ the scanning electron microscope (SEM).
The transmission electron microscope
How does the TEM work?
The TEM consists of an electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet.
In a TEM, the beam passes through a thin section of the specimen.
Parts of this specimen absorb electrons and therefore appear dark. Other parts of the specimen allow the electrons to pass through and so appear bright.
An image is produced on a screen and this can be photographed to give a photomicrograph.
The resolving power of the TEM is 0.1 nm.
However this cannot always be achieved in practice because:
⚫ difficulties preparing the specimen limit the resolution that can be achieved
⚫ a higher energy electron beam is required and this may destroy the specimen.
What are the main limitations of the TEM?
The main limitations of the TEM are:
- The whole system must be in a vacuum and therefore living specimens cannot be observed.
- A complex ‘staining’ process is required and even then the image is not in colour.
- The specimen must be extremely thin.
- The image may contain artefacts.
Why is the result from a TEM a 2D image?
In the TEM the specimens must be extremely thin to allow electrons to penetrate.
The result is therefore a flat, 2-D image.
How can we partly get over the issue of 2D result in TEM?
We can partly get over this by taking a series of sections through a specimen.
We can then build up a 3-D image of the specimen by
looking at the series of photomicrographs produced.
However, this is a slow and complicated process.
One way in which this problem has been overcome is the development of the SEM.
What is SEM, and how is it different to TEM?
The SEM directs a beam of electrons on to the surface of the specimen from above, rather than penetrating it from below.
The beam is then passed back and forth across a portion of the specimen in a regular pattern.
The electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface.
What is a limitation present in TEM but not SEM?
All the limitations of the TEM also apply to the SEM,
except that specimens need not be extremely thin as electrons do not penetrate.
How can we build up a 3D image from SEM?
Is the resolution lower or higher than TEM?
We can build up a 3-D image by computer analysis of the pattern of scattered electrons and secondary electrons produced.
The basic SEM has a lower resolving power than a TEM, around 20 nm, but is still ten times better than a light microscope.
