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I Forge Iron

X-ray Fluorescence Analyzers


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These devices are expensive but also pretty complex. If your business depends on knowing composition, then $20-30k is a reasonable investment. The rest of us pay someone else to do the analysis.

The hand held units are very cool and work well within the limits of the technique (see caveats below). They are not only used in scrap yards but for identification of lead in houses and tableware, and even quick clinical measurement of heavy metal poisoning in patients.

The following is probably everything that you did not want to know about X-ray fluorescence. :-)

How it works:

  • The unit contains an xray source, typically a vacuum tube that bombards a copper (Cu) foil with accelerated electrons to produce Cu x-rays primarily at 8.048 KeV.
  • The Cu xrays strike the surface you are analyzing.
  • The xrays are absorbed and knock out electrons from atoms in the sample.
  • Outer orbital electrons of an atom collapse to fill any inner orbital vacancies created.
  • Energy is given off that is the difference between the outer and inner orbital energies for that atom. This is specific to the element and orbitals excited.
  • This energy is typically in the xray region, thus this is xray fluorescence. Xrays are absorbed and xrays re-emitted at a different energy.
  • Some of the xrays emitted strike a detector in the analyzer. The detector is usually a doped silicon chip that produces a current pulse proportional to the energy of the xray that strikes it.
  • The current pulses are collected and sorted by size to identify what energy xrays struck the detector.
  • The xray energies observed are characteristic of the elements present so you can identify an element.
  • The number of xrays observed for an element is proportional to the amount of that element present so you can determine concentration of the element.

  • The xrays produced will only escape from (typically) a few microns below the surface of most metals. The rest are absorbed within the sample. Thus this is a surface analysis technique. If you have a plated surface, for example, it may give only the composition of the plating. At the same time, the technique can be used to measure the thickness of thin films by measuring the absorption of the underlying metal signal by the surface layer.
  • Lower energy xrays are absorbed more than higher energy xrays as you might guess. Low atomic number metals like Al and Mg produce lower energy xrays which are absorbed more, produce a weaker signal and therefore take longer to analyze. Quantitative measurements also tend to be less accurate for low atomic number elements due to weaker signal and larger absorption correction factors used in the analysis.
  • Very low atomic number elements like boron, carbon and oxygen cannot be easily measured with hand held instruments since their xrays are significantly absorbed even in air. For carbon and oxygen, surface contamination makes quantitative measurement a challenge even in a vacuum. Don't expect to measure any element below Mg in the periodic table using a hand held instrument.
  • There are very significant interferences in the xray spectra for elements we are interested in. For example, you can sensitively measure phosporus in iron but not in aluminum due to the overlaps in their xray spectra.
  • Note that these units are mainly designed for sorting, not accurate quantitative analysis of trace elements. They can give you accurate measurement of the major components in an alloy but not the kind of trace quantitation you would expect from a lab analysis.
  • You can get false positives due to analysis artefacts that require some deeper understanding of the technique so interpret trace readings with caution.

If you want a more detailed and graphical description, the linked presentation is excellent, licensed under the Creative Commons and therefore free for your use.

FYI, I used to do xray analysis and wrote xray analysis data correction software for a living. That took me to the dark side in IT. ...now I'm a blacksmith.
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  • 2 weeks later...

Get to use one quite often to identify material in and around the power plant I work at. They work well if you do your part and clean the sample as it will give you false readings from any coating or scale on the object being tested. I generally test more than area to ensure the results. Ours will give you ASTM designation, Trade Name, and/or material composition.

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