Scanning Electron Microscopy


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Scanning Electron Microscope Image of Human Blood http://images.townnews.com/nctimes.com/content/articles/2007/06/10/health/17_40_576_7_07.jpg


Light Microscope
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A scanning electron microscope functions similar to a traditional light microscope. In a compound light microscope, light illuminates a sample. The light waves bounce off the sample, are focused through a series of lenses and are detected by the human eye, where the brain translates the light into an image. The light microsope's magnification is limited by the warelength of visible light. Visible light has a wavelength of several hundred nanometers and therefore, can not distinguish between objects separated by a shorter distance.

==Electron Microscope ----
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An electron microscope uses electrons instead of photons to probe the surface of a sample. According to the de Broglie hypothesis, all moving matter has both wave and particle properties.
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The wavelength associated with a particle is equal to Plank's constant divided by its momentum



Electrons have a smaller wavelength than visible light and therefore, can distinguish between two points a mere 1 to 5 nanometers apart. First, a sample must be dehydrated, coated with a thin layer of a conducting material and placed in a vacuum tube. Electrons are fired from a gun at the top of the microscope. The beam of electrons is focused like visible light. Electrons would be easily deflected by a glass lens, so lenses of magnetic field focus the beam. The beam is moved across the surface of the sample. As the electrons hit the surface, several phenomena occur.

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Electron Microscopy http://www.fofweb.com/Science/default.asp?ItemID=WE40

Secondary Electrons


These are electrons emitted from the sample due to inelastic collisions between the incident beam and the electrons in surface molecules. As the angle between the incident beam and the surface approaches perpendicular more secondary electrons are scattered. These electrons are detected by a scintillator, a material that emits light when bombarded by ionizing radiation, i.e. electrons. The more electrons the brighter the image. This luminescence is amplified by a photomultiplier and displayed on a computer. In this way, the topography or surface features of the sample can be seen.
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Top: Back-Scattered Electrons Bottom: Secondary Electrons http://en.wikipedia.org/wiki/File:SEM_SE_vs_BE_Zr_Al.png

==Back-Scattered Electrons ----
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These are electron from the incident beam reflected back by elastic collisions with the surface molecules. Larger atoms, or atoms with a higher atomic number will deflect more electrons than smaller, low atomic number atoms. More electrons produce a brighter image on the scintillating detector. By analyzing these electrons the composition of a sample can be determined.

==X-Ray Analysis ----
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When secondary electrons are emitted from lower energy atomic orbitals, higher energy electrons drop down to fill the empty spot. When this occurs the electron emits energy in the form of electromagnetic radiation in the X-Ray range of the spectrum. These wave energies are unique to each element, allowing the sample's composition to be more accurately determined.
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==Application to Forensic Science ----
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Scanning electron microscopy techniques have become increasingly used in the forensic laboratory. The SEM has been used to compare and identify the physical and chemical characteristics of trace evidence, such as gunshot residue, dust, fibers, and paint. SEM analysis has the advantage that the sample is not destroyed like in most chemical analysis. Many microscopes can now analyze hydrated, uncoated sampes by adjusting the internal pressure to dissipate charge that would normally build up on the surface.

A study by Bartelink, Weirsema, and Demaree, published in the Journal of Forensic Sciences found that scanning electron microscope analysis of dead victims can aid in identifying murder weapons. Statistically significant differences were found the widths of the entry wounds of several different knives. The application of SEM can help forensic scientists narrow the field of possible murder weapons in homicide investigations.

Sources

Bell, Suzanne. "scanning electron microscope (SEM)." Encyclopedia of Forensic Science, Revised Edition. New York: Facts On File, Inc., 2008. Science Online. Facts On File, Inc. http://www.fofweb.com/activelink2.asp?ItemID=WE40&SID=5&iPin= EFSRE0525&SingleRecord=True (accessed December 15, 2008).

"Scanning Electron Microscope." Wikipedia, the free encyclopedia. 11 December 2008. 15 December 2008 <http://en.wikipedia.org/wiki/Scanning_electron_microscope>.

"Scanning Electron Microscopy." World of Forensic Science. Eds. K. Lerner and Brenda Lerner. Vol. 2. Detroit: Gale, 2005. 598-599. 2 vols. Gale Virtual Reference Library. Gale. Watchung Hills Reg H.S.. 15 Dec. 2008
http://find.galegroup.com/gvrl/infomark.do? &contentSet=EBKS &type=retrieve &tabID=T001 &prodId=GVRL &docId=CX3448300497 &source=gale &userGroupName=watchunghrhs &version=1.0.


Bartelink et al. "Quantitative analysis of sharp-force trauma: an application of scanning electron microscopy in forensic anthropology." Journal of Forensic Sciences. 1 November 2001. 15 December 2008 <http://www.astm.org/JOURNALS/FORENSIC/PAGES/JFS4661288.htm>.