The Principles and Limitations of Electron Microscopy.

Natasha Prior K10

An Electron Microscope is a type of microscope that makes use of a beam of electrons rather than visible light. Which is due to the fact that the wavelength of electrons is much smaller than the wavelength of visible light, an Electron Microscope not only gives a high magnification but it also has high resolution. This means that details can be seen clearly. An Electron Microscope is very similar to a Light Microscope in the ways in which it works but, instead of using glass lenses to focus a beam of light, it uses magnets to focus a beam of electrons. Electrons are very small, so they are scattered if they hit molecules in the air due to this they must travel through a vacuum. Specimens that are to be examined must be cut into very thin sections, and these must be treated so that they can be examined in these conditions. The variety of chemical processes that are carried out in this preparation may change the appearance considerably. Features which have been introduced in this way are known as artefacts and care needs to be taken in interpreting electron micrographs because of the possible presence of artefacts.

An examination of a specimen using an Electron Microscope can yield the following information:

Topography: The surface features of an object or “how it looks”, it’s texture; direct relation between these feature and materials properties, e.g, hardness, reflectivity…etc.

Morphology: The shape and size of the particles making up the object; direct relation between these structures and materials properties e.g, ductility, strength, reactivity…etc.

Composition: the elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties e.g, melting point, reactivity, hardness…etc.

Crystallographic Information: how the atoms are arranged in the object; direct relation between these arrangements and materials properties e.g, conductivity, electrical properties, strength…etc.

Electron Microscopes were developed due to the limitations of Light Microscopes. A good light microscope ...

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Morphology: The shape and size of the particles making up the object; direct relation between these structures and materials properties e.g, ductility, strength, reactivity…etc.

Crystallographic Information: how the atoms are arranged in the object; direct relation between these arrangements and materials properties e.g, conductivity, electrical properties, strength…etc.

Electron Microscopes were developed due to the limitations of Light Microscopes. A good light microscope can only magnify an object effectively about 1500 times. The Electron Microscope can give clear pictures that are magnified 500,000 times. It is important to appreciate how important this can be in practice: with the Electron microscope an object the size of a pinhead can be enlarged to the point at which it has a diameter of well over a kilometre; a cell with a diameter of 10 micrometres finishes up with a diameter of 5 metres.

Light Microscopes are limited by the physics of light to 500x or 1000x magnification and a resolution of 0.2 micrometers. In the early 1930’s this theoretical limit had been reached and there was a scientific desire to see the fine details of the interior structures of organelles e.g, Mitochondria, nucleus…etc. This required 10,000x plus magnification, which was just not possible, using light microscopes.

The transmission electron microscope (TEM) was the first type of Electron Microscope to be developed and is patterned exactly on the Light Transmission Microscope except that a focused beam of electrons is used instead of light to “look through” the specimen. It operates very much like the brightfield light microscope does, however, in the TEM the “light” is actually a beam of electrons, produced by energizing a tungsten filament with high voltage, so that the electrons come off it. The condenser lens focuses the beam and maintains it in a stable position.

The first Scanning Electron Microscope (SEM) debuted in 1942 with the first commercial instruments around 1965. Its late development was due to

the electronics involved in “scanning” the beam of electrons across the sample. The SEM is used for examining surface structure, they are all constructed with a specimen stage that permits the object under scan to be rotated, tilted and traversed from side to side and back and forth. Unlike a conventional microscope, it is possible to view almost all parts of the specimen by twisting it around in various ways to get a different vantage point. This allows the observer to examine any point on a surface, so long as a straight line can be drawn between that point and the observer, whose virtual position is at the top of the microscope, looking down at the specimen (Roberts et al1993).

The two types of Electron Microscope have some similarities and some differences. They are similar in that they both examine specimens under vacuum but they differ in how the images are produced. The TEM, as the name implies, actually relies on the transmission of the beam through the specimen: the direct physical interposition of the specimen into the beam is necessary to produce the image. The SEM, on the other hand, can examine specimens much too thick for the beam to pass through and its image is produced electronically.

The Light Microscope uses visible light with an average wavelength of 550nm. This light is focused onto the specimen by a condensing lens. The resolving power of the microscope depends on the wavelength of light used – the shorter the wavelength, the greater the resolution produced. Since the wavelength of light cannot be altered, all light microscopes have a limited resolution of between 200 and 25nm. Many cell structures are smaller than this and this limitation led to the development of the electron microscope, which is much more powerful. Images of organelle structures can be viewed at a higher resolution, so they appear to be much clearer and more visible. It is because the powerful beams of electrons are being forced through the small gaps at the edge of the specimen and the details are made clearer as the electron passes through it.

Two principal considerations in Electron Microscopy are that air must not be present inside the microscope, i.e. the specimen must be mounted in a vacuum. Electrons would be scattered or absorbed by air and so no image could be formed. The second principle is that obviously beams of electrons would equally be absorbed by lenses. They are usually focused by magnetic lenses, which deflect the beam in magnetic fields. Just as in a light microscope we have a condenser and an objective but we cannot view the image with the eye and so there is a third “lens” called a “projecting eyepiece”, which produces a real image which can be focused on to a fluorescent screen. (Bonney et al 1982).

The Electron Microscope has been very useful since it was invented as it allows us to venture into a specimen in much more detail. Even though Light Microscopes are good for using in school, an Electron Microscope is much more effective due to it having a much higher resolution and magnification.

References:

Brian H.Bonney, 1982. Cell biology level II. BECTEC books.

Michael Roberts, Michael Reiss and Grace Monger, 1993. Biology, Principles and Processes, Thomas nelson and Sons limited.

Peer Reviews

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26th of Jul 2012

Quality of writing

At times the layout of this essay is confusing, there is section in which the candidate has suddenly decided to use headings and give information in a bullet point style which is something I would not recommend for this type of work. You should be writing in continual prose as this is the accepted way write a scientific essay. However the candidate does uses appropriate technical terms and there no issues with spelling or grammar.

Level of analysis

The candidate has written about the differences between optical and electron microscopes which would be fine if the aim of the essay was to compare the two, however the essay title implies that the purpose of this essay is to discuss the electron microscope, well it is acceptable to mention the optical microscope the candidate wastes time discussing comparisons when it simply isn’t necessary, as it actually detracts from their work rather than adds to it. However I do recommend that you take the time to research your topic, as including additional information shows your interest and dedication and can make your essay more enjoyable to read, just make sure that the information you include is relevant. In this case, it would acceptable to include information on the history of the microscope and how the electron microscope was first developed or perhaps you could give examples in which the electron microscope has aided scientific discoveries. Furthermore this essay ends without any sort of conclusion, this is a common mistake but one that you should avoid. It is necessary to conclude your work as this gives you an opportunity to leave the reader with a good impression. You should summarise your key points and ideas and state why these are important and you should also try to write a personal reflection as your teacher/examiner is interested in what you have to say and this can help draw your essay to an appropriate close.

Response to question

The candidate goes straight into the essay without any kind of introduction, this is not a sensible idea. You should always take the time to properly introduce your essay, as this is your chance to grab the readers attention and get them interested in what you have to say. You can engage the reader by mentioning a fact or quote that you find interesting, though this will only work if it is relevant to the topic you’re discussing. An introduction should clearly state what you plan to discuss so that the premise of your essay is clear and you should briefly introduce your topic, in this case, you should describe what an electron microscope is and what is used for.

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