Hi all, although there is still a ton left for me to learn about DTSA-II, EDS detectors, and x-ray microanalysis in general, I think that my personal experiment to turn my humble little JEOL 6490LV with an OI X-Max 50mm2 into a probe-lite has been fairly successful. I have attached a spreadsheet of measurements I've made on a variety of geological standards that have served my JEOL 8200 well over the years. Everything is quantified with measured O, even though I'd probably not do that for a real analysis unless there was some pressing reason. Livetimes are marked. All were collected by beam deflection, and most from free-hand rectangular box areas of wildly varying dimensions but usually >10 microns on a side. You'll have to log in to see and download the two Excel files, I think.
A few observations I'd like throw out:
- The standardization I made 32 days ago is still producing excellent accuracy.
- My EDS is FAR more forgiving of beam deflection than the WDS spectrometers. That makes sense but I have not quite tested it as well as I'd like. So far the evidence I have seen suggests that I can deflect the beam >200 microns from stage center without suffering any degradation in quant results.
- Single-line uncertainty has severely diminishing returns at around 40 seconds of collection time at 1nA, which is about a 20-23% dead time for my detector at process time 4 and
roughly 18k cps input rate.
- When I propagate single-line uncertainty on a garnet measurement through the garnet-biotite thermometers found in a handy spreadsheet on Dave Waters webpage (
https://www.earth.ox.ac.uk/~davewa/pt/th_tools.html), you end up with a 2-sigma error of 2-10 degrees C from >60s EDS spectra, which is WAY smaller than the disagreement between the thermometers themselves. I don't do much petrology so I can't quite judge the utility of that, but it sure seems precise enough to use that particular thermometer.
- The biggest problem I think I have is current drift. The W-filament in this LV instrument gets treated very poorly compared to the pampered existence of the 8200's W-filament, and I don't always succeed in preventing a bit of current drift while collecting spectra. Some of the linear trends with time you see in the data is likely due to current drift.
Please have a look and see what you think. I think that standardized EDS really has the potential to make a lot of the data produced by microprobes available to anyone with SEM+EDS, a way to measure their beam current, and the ability to borrow standards once every few weeks/months.
Best,
Phil