|
|
Questions?
|
Have questions about electron microprobe analysis? Want to know more about our facility? Send us an e-mail or visit our Contact Us page.
|
|
|
|
|
EDS Qualitative and Semi-Quantitative
|
| |
|
|
Our electron microprobe is outfitted with an energy-dispersive spectrometry (EDS) system, which we use primarily for qualitative identification of elemental abundances. Unlike wavelength-dispersive spectrometry (WDS), the EDS system does not "tune in" specific X-rays. Instead, a solid-state detector collects and counts all of the emitted X-rays at once, and it divides the energy spectrum into different "channels" or ranges. We can collect a spectrum of X-ray energies from 0 to 10 keV and display it on the computer screen, as shown above. Peaks will show up on the spectrum, corresponding to energies of elements present in a sample.
The EDS system can be used for quantitative analysis (one simply counts the X-rays received in the channels that correspond with a peak of interest). For many combinations of elements, however, the EDS system is less desirable than WDS because corrections must be made for overlapping peaks and the background noise is much higher, which limits sensitivity. Still, if one wants to determine quickly what elements are present in a sample, EDS is the way to go!
|
|
|
|
|
WDS Semi-Quantitative and Quantitative
|
| |
|
|
Our microprobe also has a wavelength-dispersive spectrometry (WDS) system; it is outfitted with five WDS spectrometers and eight different crystal types. Wavelength-dispersive spectrometers are "tuned" to the characteristic X-ray of interest for analysis. The "tuning" is done by scattering of X-rays from a crystal positioned between a sample and the detector. By changing the angle of incidence of the X-rays, the crystal will constructively diffract X-rays of specific wavelengths. As a result, both the crystal and the detector move to accommodate the different incident angles. In addition, different crystals are used to cover the entire X-ray spectrum: lithium fluoride (LIF), pentaerythritol (PET), thallium acid pthalate (TAP), and artificial layered dispersive element (LDE) crystals are the most commonly used.
WDS analysis results in a spectral resolution and sensitivity an order of magnitude better than is possible with EDS analysis; the detection limits of WDS ordinarily varies between 300 and 30 parts per million (ppm). Also, in comparison to EDS, WDS offers more accurate quantitative analyses, particularly for light elements, and better resolution of overlapping X-rays peaks for improved element identification and quantification.
|
|
|
|
|
WDS Line Analysis
|
| |
|
|
The WD spectrometers can be "tuned" to specific elements as the electron beam is scanned from one point to another on the surface of a specimen. The result is a line profile for each element. Above are line profiles for nickel, chromium, silicon, and manganese in a specimen of cast iron. One can see that the silicon and nickel are associated, as are the chromium and manganese. Line analyses can then be calibrated with quantitative analyses.
|
|
|
|
|
WDS Map and Image Analysis
|
| |
|
|
Our microprobe is also capable of different types of imaging: secondary electron (SE) images, backscattered electron (BSE) images, and characteristic X-ray maps. Images in each of these imaging modes can be acquired, processed, and stored digitally and be displayed in either grayscale or false color. Check out our Image Gallery!
For SE imaging, the electron microprobe functions as an SEM, providing topographic information about a sample. The spatial resolution for SE imaging is approximately 100 to 200 nm, depending on the accelerating voltage, beam current, and other operating conditions. Common applications include studies of grain morphology, precipitates on a mineral surface, microfossils (for instance, ancient diatoms), and other materials too small for visible light microscopy.
BSE images show atomic number differences within a sample. A certain fraction of the electrons in the beam are scattered "backward" out of the sample as a result of interactions with its nuclei, and these "backscattered" electrons can be used to form images. The number of electrons are backscattered increases with increasing mean atomic number of the material. In a BSE image, the brighter the area, the heavier the mean atomic mass of that material.
In X-ray maps (also called element maps), the element distributions are shown in maps that represent concentrations as colors, very much like a weather radar map. The spatial resolution of X-ray maps is approximately 1 micron. Such mapping can delineate sub-micron particles or can be carried out across surfaces up to 90mm on a side.
|
|
|
|
Content published on this Web Site is copyright the University of Minnesota Regents, the Electron Microprobe Laboratory, and/or the laboratory's users. Some content (particularly analyses and images of specimens) represents the intellectual property of laboratory users. Reproduction or distribution without permission is prohibited. Site content is available for educational and informational uses only, provided that the content is unmodified and that permission is granted by the author and/or the laboratory manager.
|
