EPMA question:
We were measuring Cassiterite samples on our Jeol FEG-EPMA (15kV, 15nA, focussed beam) - using a cassiterite primary Std to standardize Sn. On the unknowns (also cassiterite), we got very high totals of 107-108%... Any ideas what might cause this?
The Nb-Ta oxides we analysed on the same routine gave good results.
Porous primary cassiterite std?
Bad polishing/carbon coating of the Std holder?
Something to do with the matrix corrections (ZAF)?
Suggestions/tips much appreciated.
Hi Anouk,
It is conceivable that the local high temperature and low pressure promote partial reduction of cassiterite, i.e. loss of oxygen so that part of the Sn(4+) is reduced to Sn(2+), which of course would increase the apparent Sn content. This kind of process is much less likely with Nb or Ta because their highest valence (5+) oxides are much harder to reduce than SnO2.
And as you suggest, it is possible that differences in the physical state of the sample vs. standard would make one easier to reduce than the other (e.g. porosity).
However, since I have no practical experience with EPMA, it would be useful to get advice from an expert.
Have you got an idea of the error of the measurement? In general with EDS the errors on the elemental compositions could be high..
Since you do not "standardize" oxigen, I recommend to pay close attention to the reply of Emanuel Cooper . Disclaimer: I've never worked with EPMA as a geologist.
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