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Electron Microscopy - Coursework Example

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Electron Microscopy
la. Briefly describe how the main lens aberrations arise in electron microscopy.

Electron microscopy is a technique in microscopy that offers higher magni-fication, higher resolution and greater depth of field in viewing objects. When the main lens aberrations arise in electron microscopy1 the high quality of resolution, light focusing to the point of object and depth of field is not achieved. Lens aberrations can exist in a form of chromatic aberrations. This exist when a range of wavelengths is instantly present in the light (for example in 'white' light) and occur because a single lens will enable light to be diverted by an amount depending on the wavelength. Therefore, a lens will have varying focal lengths for light of diverse wavelengths. Spherical aberration also occur in the Electron Microscopes when electrons passing through the side of the lens are refracted greater than those passing along the axis.2 (Lam, 2009); while Diffractive aberrations are brought about by the deviations from geometrical optics caused by the wave nature of light.3 Consequently, the result of each aberration is to deform the image of each point in the object in a specific way, leading to a total loss of quality and resolution in the image.1
b. Where appropriate include in your answer how the aberrations are influence
by aperture size.
Wherever light passes into an aperture, diffraction occurs in order that

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a parallel light beam is converted into a series of cones that are seen as circles. For light of a certain wavelength, the central spot's diameter is inversely proportional
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1. Beanland, R., Goodhew, P. J. and Humphreys, F. J. 'Prelims', Electron Microscopy and Analysis, 1:1, 0 – 19.
2. Lam, R,L. Electron Microscopy, Institute of Technology Press, Wayward, 2009.


to the aperture's diameter from which the diffraction is existing. Thus, the smaller the aperture is, the larger is the circle's (airy disc) central spot. Very small apertures can be used in order to create the Airy disc clearly visible, however, the same effects occur at the point of relatively larger apertures of the microscope. To make the image of every point as small as possible, aperture should be as large as is feasible.3 The only way of minimizing monochromatic aberrations is to restrain the electrons to paths that is very near the optical axis, by applying a small objective aperture. Spherical aberration can also be reduced if small aperture is use; although can make the diffraction-limited resolution worse.

c. From the answer provided in lb comment on the best aperture size (small, medium, large) for spherical, chromatic, diffraction and astigmatism.
High quality image formation in an optical system appears when all of the rays
start at a single object point cross to a single image point, or congruently, when the geometrical wavefront in the space of image has a spherical shape that is centered on the image point. 4
The background intensity generated by spherical aberration can be adjusted by limiting the angular aperture of its objective lens. To do this, the objective aperture has to be set at the back focal plane of its objective lens. This way, aperture works to decrease the background intensity brought about by the spherical aberration and therefore, increases the contrast of the image points. When the size of its objective aperture is reduced, the more scattered electrons are stopped, thus contrast
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3. J.C.H. Spence, Y.M. Huang, O. Sankey, Acta Met. 41, 1993, 2815.

4, Zach, J., and Haider, M. Optik 99, 1995, 112.

improves. The Objective aperture sizes range within 25 and 75 µm.; so, the smaller the aperture, the greater would be the aperture contrast, since more scattered electrons will be prevented from reaching the image.5
Diffraction is due to the wave nature of electrons and aperture size of final lens. The only means to reduce diffraction problems is by way of increasing the final aperture size. Astigmatism on the other hand, results from magnetic lenses which do not have perfect symmetry and the stigmator is one way of correcting the asymmetry in the final lens. 1

2. Explain the design features of aberration corrected electron microscopes. What
improvements in resolution can potentially be achieved with these corrected
microscopes?
Aberration-corrected electron microscopes has been developed with two parallel but converging paths. It has electron-optical column design, which is built, and tested using the 100 kV VG cold field emission (CFEG) gun as the electron source, With it is a 200 kV CFEG that is being developed separately. The column has been created for 200 kV operation. There is also a possibility for operation at 100 kV and also lower voltages that involves several advantages, like reduced knock-on damage as far as light element materials are concerned. 7
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5. Cosslett, V. E. Fifty Years of Instrumental Development of the Electron Microscope. Advance Optics Electron Microscope, 1987, 10:215-267.

6. 1. Beanland, R., Goodhew, P. J. and Humphreys, F. J. 'Prelims', Electron Microscopy and Analysis, 1:1, 0 – 19.

7. Haider, M., et. al. Transmission Electron Microscopes (TEMs), Scanning Transmission electron microscopes (STEMs)1998, 5-7.

3. Explain how tomographic images are achieved in a TEM. What special instrumental constraints are necessary to be able to obtain high resolution tomographic images?
Transmission electron tomography is a contemporarym increasingly familiar three-dimensional electron microscopy approach which can provide new insights iin the structure of subcellular components. The apparatus has tomography that fills the gap between optical microscopy and high resolution structural methods (the nuclear magnetic resonance or X-ray diffraction). Two main limitations were identified from electron tomography: the missing wedge and also the low signal-to-noise ratio of the images.
It also utilizes tomographic tilt series, needed to develop efficient as well as robust algorithms for the projection alignment in the absence of markers (beads) and a more improved reconstruction algorithms. The display components, interpretation, and 3-D tomographic communication results pose entirely new challenges.



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8. Messaoudil, C., Boudier, T., Sorzano, C.O., and Sergio, M. TomoJ: Tomography Software for Three-Dimensional Reconstruction in Transmission Electron Microscopy. BMC Bioinformatics 2007, 8:288

9. National Institute of General Medical Sciences. TEM. Accessed March 26, 2011 from http://www.nigms.nih.gov/nigms.nih.gov/Templates/CommonPage.aspx?NRMODE=Published&NRNODEGUID={67F97070-ECED-48C3-8A9D-28CF8C2214BD}&NRORIGINALURL=%2fNews%2fReports%2felectron_microscopy.htm&NRCACHEHINT=Guest#image

Summary

This essay discusses electron microscopy, that is a technique in microscopy that offers higher magnification and greater depth of field in viewing objects. When the main lens aberrations arise in electron microscopy the high quality of resolution, light focusing and depth of field is not achieved…
Electron Microscopy
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