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XRF Analysis of British coinage through slabs - interesting results
jmlanzaf
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Really fascinating study - at least to me. I know it is darkside, but it does speak to the general accuracy (or lack of) for XRF analysis through NGC and PCGS slabs:
https://britnumsoc.blog/wp-content/uploads/2022/07/269-xrf-slabs-oddie-blog-001.pdf
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WOW! shocking to see a huge read of tungsten for a gold coin thru a slab
thx for sharing - agree v fascinating
Yes. It does seem that for the elements you KNOW are there, the ratios appear to be correct in many cases - if you exclude the "fake" materials. But it is really odd to me that you could get such a whopping ghost signal for Tungsten.
As I mentioned on another thread. A friend and I shot a platinum coin (999) through the slab and it showed 95% silver and almost no platinum. Knowing how XRF works, I don't understand the ghost signals. The plastic shouldn't be cshifting the fluorescence by any simple mechanism I can think of.
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I wonder if polarization is having an effect on all this - there is super strong birefringence in plastic
I don't own an XRF unit but based on most folks I've seen using one they frequently mention they don't trust them 100% on even bare gold since the depth of the reading is only a few microns. On thick gold jewelry they cut into the piece. On a coin which you don't want to cut all the various testing is done (weight, diameter, thickness, specific gravity and electromagnetic resistivity (Sigma machine)) and other features of the coin are used to verify it is genuine.
The plastic will mess with the XRF in slabs. Other plastics don’t always. Not sure why but I’ve used XRF on thousands of coins and many slabs. Differs per slab era as well.
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What I believe is happening is that the plastic in the slab modifies the X-ray signal reaching the detector both from the original signal going through the slab to the coin and then from the resulting return signal emanating from the coin and passing through the plastic in the slab a second time. This would likely be from energy-dependent attenuation and scattering. The XRF device then has to interpret the returning signal via its own internal software and that can can truly weird final results so that the measured spectrum of an uncorrected XRF software algorithm can give an inaccurate composition.
Please note the last of these references is a technical paper written in-house by Thermo-Fisher Scientific and should be thought of as a quasi-advertisement for the product and its usage. They state that a handheld XRF will give reliable readings through slab plastics, but then only demonstrate data for use through extremely thin films, which to me means they had no reliable data to prove the instrument is accurate through slab plastics.
https://pmc.ncbi.nlm.nih.gov/articles/PMC8207333/?utm_
https://link.springer.com/chapter/10.1007/978-3-031-58277-6_12?utm_
https://thermofisher.com/blog/metals/?p=10941&utm_
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That's consistent, for I would have said ±15% accuracy. The plastic and air gap will have great influence on the XRF test.
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There's something about the XRF process that makes tungsten (and selenium) a false reading for gold. I saw similar results in my writeup at https://www.so-calleddollars.com/xrf_results.html
Yeah, this is my understanding of them as well. The biggest down side for me is the depth of testing. 😎
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To me, the question would be if the errors/interference from the slab was consistent from slab to slab, and thus could be calibrated against. If so, then for someone who routinely tested coins through slabs, that XRF could be re-calibrated with that background shift in mind.
However, I suspect that not only would each "brand" of slab be different, but each brand would also be internally inconsistent, thus rendering any attempt at re-calibration almost-useless.
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The short answer based on that article is "No, the interference is not consistent from slab to slab".
The longer answer is that the interference may be directionally consistent. For example, if I saw a yellow object where the XRF results reported selenium and tungsten, I'd be pretty confident that the yellow came from gold rather than brass. For anything that needs actual percentages, XRF through a slab does not give reliable data.
The two page 'abstract' has only peripheral information. Without a technical, peer reviewed paper, really hard to see any viable results. Of course sending XRF through a plastic barrier between it and the medium one wishes to anallyze is bound to produce erroneous results. We have known that for decades and have never done that in my groups 50 or so papers using such data - we don't put anything between the XRF and the medium we were analyzing (rock powders), and have alot of internal standards as checks. So no surprises here..........
Reality is that for accurate results you want to measure materials without any medium between the XRF rays and the material one is analyzing. Anything else is likely in create errors and uncertainties that can't be corrected for in many cases............