EDS Method Principle - GR Petrology

Modified on: Mon, 2 Nov, 2020 at 4:20 PM

GR Petrology uses EDS for elemental analysis. The principle used is: electrons from the electron gun in our scanning electron microscope are directed towards the sample.  Typically we cover about 2mm2 area to get a good average.  But we can do spots as small as 10µm to identify individual particles if desired.  The electrons hit atoms in the sample and those atoms emit X-rays with an energy more or less unique to the element of the atom.  Heavier elements emit a few different X-rays due to their having multiple electron orbitals.  

This beam is non-destructive, although it might make a slight mark on plastics.  We make a spectrum of all the X-rays detected, and the various peaks at different places on the spectrum indicate which elements are present.   I attached an example spectrum for your reference, you have seen these in our reports before.
The EDS can detect all elements from Carbon through Uranium on the periodic table down to a 0.01%weight detection limit (100ppm).  We can also detect boron, depending on which other elements in the sample might interfere. 
Other methods of analysis like XRF can’t detect any elements below Na sodium i.e. they miss the first and second rows of the periodic table.  XRF has low sensitivity (1,000ppm) for the lightest elements it detects, the sensitivity gets better and is down to a few tens of ppm once you get to iron and higher.  ICP is another (very cool) method, where the sample is heated to 6,000°C and the spectrum of light is recorded which indicates the elements.  Needless to say that destroys the sample.

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