Using the "Flank" method in Probe for EPMA

[John Donovan]

Some of you may be familiar with the so called "Flank" method for the determination of Fe2+/Fe3+ in some minerals by comparing subtle peak shifts in the region of the Fe La/Lb emission lines for different Fe oxidation states.

Höfer, Heidi E., and Gerhard P. Brey. "The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis." American Mineralogist 92.5-6 (2007): 873-885.

I myself have never attempted these measurements but I hear that they can be useful.  Anyway, Stuart Kearns, Ben Buse, Ery Hughes and I were chatting about some possible tweaks to PFE that might make these "Flank" determinations easier and the general consensus was that these measurements are so esoteric that there probably isn't much value in making changes to the software.

But I decided that one small change might be helpful and not just for "Flank" method determinations, and that is adding a "difference" plot option to the Plot! window as shown here:



Using the this new feature I plotted two samples from Ery's run and we obtain this plot where blue is the difference plot:



Maybe this will aid in the selection of these peak "Flanks"... though in fact Ery apparently averaged many of these plots together to get better counting statistics (too long of a beam exposure and one risks oxidizing the sample further).

But this feature may also be useful in other studies such as this plot comparing SiO2 and SiO:



Then by also checking the "Normalize Samples" option we obtain this:



These differences become even more clear when using a PET rather than a TAP crystal:



Maybe this "difference wavescan" option will be useful to some of you...