Hi all
I am hoping that someone can give me some advice about analysing Troilite. What I thought should be very straight forward is turning me crazy. To give you some background I have a user who is attempting to analyse some troilite (amongst other sulfides which are coming out fine), and we are continuously overestimating the Fe content getting ~64.5% Fe (Stoichiometric troilite is Fe: 63.53% S: 36.47%). We are pretty much spot on with our S measurements. Beam conditions used are 20kV/20nA with a range of beam sizes from focussed to 10um. I am assuming 20nA is OK to use, perhaps I should be reducing it? I turned on TDI's at one point and had a look at the Fe signal and it is pretty rock solid.
Initially we were getting much too low Fe contents (~61%) until I discovered that for whatever reason (Fe-S vs Fe-S2 bonding?) compared to the Fe standard/s I peaked on (Astimex marcasite, P&H Chalcopyrite) there was quite a large peak shift in Fe towards lower values in the troilite. I checked the peak position for Fe in my 3 standards and they are within a few sintheta units of each other, but quite different to that in the troilite sample (~10 units). Following this discovery at the start of every troilite analysis I was peakscanning for Fe (and S) and was hoping this would fix my low Fe problem, but has seemed to now push it too far the other way with high Fe.
As such I am getting totals of ~101-101.5% with Fe anywhere from 64.5-65% no matter which of my limited FeS standards I use (Astimex marcasite, Astimex Pentlandite, P&H Chalcopyrite). I have had a literature scour and there are obviously a number of papers that have analysed troilite via EPMA, and its interesting to note is that in a lot of them Fe is always underestimated by ~1wt% with no explanation as to why (peakshift?).
Does anyone have any experience successfully analysing troilite and ideas of what I am doing wrong here. I am suspecting a standardisation problem but cant say for sure. Does a troilite standard exist? I know of the meteorite Canyon Diablo of which the troilite is used as a Sulfur isotope standard. Not sure if they are homogeneous in Fe/S though.
Cheers