At the recent QMA 2019 in Minneapolis it just so happened that Mike Jercinovic and I both showed up at "The Beacon" restaurant for breakfast at 7 AM, as we were both staying at "The Graduate". We continued to have breakfast together every morning at 7 AM the rest of the week, and of course the conversation ranged over many subjects while he had his omelet and I had my eggs "over easy".
And of course we talked about trace elements. Let me just say it: Mike "gets" trace element EPMA analysis. For one thing he's a really smart guy and for another, he's thought about trace elements for a long time, and finally he's done a tremendous amount of work on trace elements, in some very difficult materials. In short, he's an expert on the subject. So forgive me, but I'm going to paraphrase Mike on this topic, but I think it's a fairly accurate representation of his feelings on the subject. And he can chime in on where he agrees or disagrees.
Mike said to me, over those eggs, that when we're measuring major elements, there's no question that we need to have accurate matrix corrections and well characterized standards, that ideally are *roughly* matrix matched to deal with all those nasty details such as peak shifting, beam sensitivity, etc. (i.e., oxide and silicate standards for analyzing oxides and silicates, sulfide and sulfosalt standards for analyzing sulfides and sulfosalts, etc.). Simply because the accuracy of major element concentrations are affected by these choices. Background measurements and background models? Not so important for characterizing major element concentrations. It's simply a question of P/B.
Trace elements are a totally different beast. When we get down to concentration levels approaching zero, the thing that dominates our trace element accuracy is the accuracy of the background measurement (and possibly spectral interferences, if any are present). Basically the effects of accuracy from our primary standard on our trace elements are "in the noise". That is to say, a few percent error in our primary standard intensities or matrix corrections, is generally going to be significantly smaller that the precision of our trace element measurements.
But accuracy errors in our background characterization are additive, not multiplicative. A 500 PPM accuracy error in our background measurement is a 500 PPM accuracy error in our trace element accuracy. At the 500 PPM level, that is a 100% accuracy error! How accurate are your backgrounds at 0.05 wt% levels?
Of course for minor elements it's a bit of both ends, that is the accuracy of our standards, matrix corrections and background characterization. Depending of course on the elements, samples and matrix physics.
So, for trace elements at least, lets stop focusing on standards and matrix corrections and place the emphasis on accurate measurements of the background. And how can we accurately determine our background at say, those 0.05 wt% levels? By trying to measure zero! Ideally a blank standard that matrix matches our unknown, contains a zero concentration of the elements of interest, and of course is measured
as an unknown, under the same conditions as our unknowns. As Mike would say "try measuring nothing, because if you can't measure nothing, you can't measure anything!"
But here's another way of looking at it: EPMA sensitivity is limited, compared to many other techniques (SIMS, LA-ICPMS, etc.), to around, let's be generous, say ~1 PPM. But what if we turned that limitation of sensitivity into an advantage in accuracy?
Say we have a standard blank that has less than 1 PPM of the element of interest (as determined by SIMS or LA-ICPMS). As far as the electron microprobe is concerned, that is a big fat zero concentration. And guess what? We can use this standard to determine our trace element accuracy (background at zero concentration)
to within the precision of our measurement. If we measure this blank standard (as an unknown) and we get zero wt% +/- 3 PPM, then our trace element accuracy is +/- 3 PPM. And you can take that to the bank.
You might not agree with everything I've said so far, but I think Mike J. does. And if you don't agree, I think that simply means that you haven't thought enough about trace elements. I'm sorry.
Now here is where I'm going to get a little weird and rant a bit. I'm seeing a lot of time and resources spent on synthesizing "doped" trace element standards. But it's really difficult to obtain "doped" trace concentrations that are one, homogeneous, and two, accurately characterized. Assuming one can synthesize something that is perfectly homogeneous (and how do we know that really?), how do we know if it's actually 95 PPM or 105 PPM, or 110 PPM?
But do you know what is perfectly homogeneous, and perfectly accurate (as far as EPMA is concerned)? A standard blank that has less than 1 PPM of the element of interest. If one is measuring Ti in quartz, then any synthetic quartz that has less than 1 PPM of Ti, is perfectly homogeneous and perfectly accurate,
to within the precision of our EPMA measurement (e.g., Kartsen Goemann and Sandrin Feig utilize a SpectroSil SiO2 glass with 50 PPB of Ti with excellent results).
So, if you're trying to synthesize doped standards at the 100 PPM, or 200 PPM, or 500 PPM or whatever trace element level, please stop. And instead please synthesize matrix matched standards with *zero* concentrations of the elements of interest. I think we've got synthetic quartz covered, but we could use all sorts of other synthetic standards, for example, synthetic TiO2, ZrSiO4, Fe2SiO4, Mg2SiO4, FeS2, synthetic plag, synthetic k-spar, synthetic sphalerite, etc., etc., etc.
And please produce these materials up to kilogram quantities. Yes, that might require collaborating with laser and optical material companies or other university or national labs. But this way, maybe, just maybe, every damn lab on the planet (including SEM labs!) can have 1 gram of the same standard material and finally, maybe then, we can all be "on the same page" when it comes to standard accuracy! Then we can retire San Carlos olivine and Amelia albite (fiascos) once and for all! Frankly it's embarassing what we have for EPMA standards today.
I'll close with an apology to Mike J. in advance, but I just had to get this off my chest! OK, that's Probeman's rant for the week. Have a great weekend.
