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Old 6th December 2010, 11:36 AM   #229
The Almond
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Originally Posted by ElMondoHummus View Post
Question for Sunstealer, The Almond, or anyone else who knows analytical chemistry:

I had once read an excuse made in passing by Jones (I think...) somewhere (don't recall where) that a couple of the reasons he elected to use X-EDS was 1. Because he didn't want the samples to be destroyed in the test (laughable, I know, given the DSC test they performed), and 2. Because he didn't have a whole lot of material to begin with. So the questions I have about this are probably obvious, but here they are anyway:
  1. Is it the case where running an X-EDS analysis on a sample doesn't consume it? I'd imagine not, but it's been so long since I took chemistry (and I never, ever did any X-ray spectroscopy in my labs) that I simply don't know.
  2. How much material would be needed to conduct an X-EDS analysis?
  3. Are there any tests which clearly show bonding and do not consume the material?
If anyone asks where I read this: Please don't, because I don't remember, and I'm not even certain it was Steven Jones who said that; I only remember the statement being in context and conjunction with the Bentham paper. However, the questions above are still valid ones that can lead to informative answers.
I'll take a stab at your questions. They have non-trivial answers, but I think there are a few interesting ideas.
1) It depends on how you're doing the X-ray microanalysis. Generally speaking, SEM-EDX is non destructive. At low accelerating voltages (< 25 keV) and low probe currents (< 20 nA), the damage to the surface from the incident electron beam is very small. However, with a modern electron microprobe, it's very easy to melt large chunks of your sample and visibly disturb the surface. You get the same X-rays in the end, but you can't beat the sense of accomplishment you get from destroying a small chunk of a very valuable mineral. Of course, if you're doing X-ray fluorescence (irradiation with a photon beam instead of an electron beam), the sample is very rarely disturbed.
  • Most people tend to think of SEM-EDX as destructive because of mounting and sample size requirements. To get good, reproducible data, you need to mount a portion of the sample in epoxy, polish it, clean it and then coat it with carbon. That process will consume some of the sample, and may make it unusable for other techniques.
2) Since SEM-EDX is a surface analysis technique, you only need an incredibly small amount of material to get compositional information. I did some work early on with gold nanoparticles measuring only 10 nm in diameter. With an appropriately tuned system, I was able to get reliable, reproducible information on their composition. Of course, you'll also need some mighty small tweezers to be able to mount the little particles.

3) X-ray diffraction would very clearly show the bonding without destroying the sample. Again, though, preparing the sample for XRD would likely require grinding, and possibly mixing in an internal standard like LaB6. That would make the sample unusable for other techniques, but not "destroyed". Of course, other labs could take the prepared sample and run a replicate test very easily.
I hope that answers your questions. It's worth noting that, with a modicum of experience doing compositional analysis, it's pretty easy to see where Harrit et al went wrong. Harrit, Jones, Farrer and Basile may be competent researchers in other fields, but they've utterly failed in their analysis of dust and in their understanding of X-ray analysis. They failed because they assumed that they already knew everything, and none of them had the humility to spend 2 hours at the library looking up the relevant methods and information.
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