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Topic: help with energy dispersive spectra (EDS Spectra)  (Read 7841 times)

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Offline JamesP

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help with energy dispersive spectra (EDS Spectra)
« on: February 25, 2008, 04:48:35 PM »
i have a task which asks me to name 4 minerals based on their spectra

mineral 1 gave a spectra for Cu only
mineral 2 gave a spectra for Pb only
Mineral 3 gave a spectra for Fe only
Mineral 4 gave a spectra for Si only

This is based on describing a copper ore in terms of its mineralogy. The general topic if Scanning electron microscopy. How can i know then minerals just based on their spectra?
Thanks

Offline Alpha-Omega

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Re: help with energy dispersive spectra (EDS Spectra)
« Reply #1 on: March 26, 2008, 11:37:47 PM »
i have looked at your question a few times.  Trying to make sure I understand what you are asking.

SEM/EDS is a combined analytical technique. 

SEM generates high energy electrons and focuses them on a specimen. The
electron beam is scanned over the surface of the specimen in a motion similar to
a television camera to produce a rasterized digital image.

SEM permits the observation of materials in macro and submicron ranges.
The instrument is capable of generating three-dimensional images for analysis of
topographic features


EDS is an analytical technique which utilizes x-rays that are emitted from the specimen
when bombarded by the electron beam to identify the elemental composition of
a specimen.


SEM can be used in conjunction with EDS and the analyst can perform an elemental analysis on microscopic sections of the material or contaminants that may be present.

In scanning electron microscopy, (SEM) an electron beam is scanned across a sample's surface. When the electrons strike the sample, a variety of signals are generated, and it is the detection of specific signals which produces an image or a sample's elemental composition. The three signals which provide the greatest amount of information in SEM are the secondary electrons, backscattered electrons, and X-rays.

Secondary electrons are emitted from the atoms occupying the top surface and produce a readily interpretable image of the surface. The contrast in the image is determined by the sample morphology. A high resolution image can be obtained because of the small diameter of the primary electron beam.

Backscattered electrons are primary beam electrons which are 'reflected' from atoms in the solid. The contrast in the image produced is determined by the atomic number of the elements in the sample. The image will therefore show the distribution of different chemical phases in the sample. Because these electrons are emitted from a depth in the sample, the resolution in the image is not as good as for secondary electrons.

Interaction of the primary beam with atoms in the sample causes shell transitions which result in the emission of an X-ray. The emitted X-ray has an energy characteristic of the parent element. Detection and measurement of the energy permits elemental analysis (Energy Dispersive X-ray Spectroscopy or EDS). EDS can provide rapid qualitative, or with adequate standards, quantitative analysis of elemental composition with a sampling depth of 1-2 microns. X-rays may also be used to form maps or line profiles, showing the elemental distribution in a sample surface.


Instrument Capabilities:

Secondary electron imaging of topographic features with magnification up to 100,000X and spatial resolution better than 50 Å

Backscattered electron imaging of chemical phase difference

Qualitative and quantitative elemental analysis with EDS

X-ray imaging: elemental line scans and maps


Limitations:

Limitations

Limited detection of elements below Na in the periodic table

No detection of elements below C in the periodic table

X-ray detection limit ~ 0.1% depending on the element

Samples must be compatible with vacuum (no fluids)


This 2 articles will give you and idae of what the combined technique will accomplish and how metallic elements can be identified.

http://www.herguth.com/technical/SEM.pdf

http://academic.bowdoin.edu/courses/f04/chem210/pdf/C210_EXP_2_SEM.pdf




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