It appears no one has responded to your last post here, but I will begin again with a quote from your post yesterday in the other topic:
Actually my target is simple. I try to understand why the measured content of Fe in Fe2O3 standards (Taylor and SPI) is 2wt% lower than it should be.
...
Fe standard - Fe metal in the same block with Fe2O3 (carbon coating is the same).
O standard - Fe2O3 or MgO at the same block.
Doesn't matter: oxygen measured or calculated, use or not Fe+2/+3 correction, width and shape BG (from detail WSs analysis).
FeKa is a strong line, so APFs for Fe wont affect.
MAC, APF for O change O. Of course Fe changes a bit as well but not so successfully as it needs.
The only point I thought was a wrong peaking. But also not, everything is good.
Certainly, my affords with searching the solution give me new knowledge but...
You and your colleagues many times recommended to look at results of calculations with different corrections. I did. But what should I see is not clear. For instance here are results for calculation with default MAC table (LINEMU Henke...) and FFAST table. Empirical MAC and APF values are not use in the calculations.
Let's start by assuming you are measuring Fe Ka using Fe metal as a primary standard and Fe2O3 as a secondary standard and calculating oxygen by stoichiometry (2:3). Then your only concern should be the measurement of Fe Ka.
The following should
not be concerns:
1. Peaking issues should not be an issue (little to no peak shifts for Fe Ka)
2. MAC issues should not be a concern as MACs are very small for Fe Ka in oxygen as shown here:
MAC value for Fe ka in O = 22.55 (LINEMU Henke (LBL, 1985) < 10KeV / CITZMU > 10KeV)
MAC value for Fe ka in O = 22.20 (CITZMU Heinrich (1966) and Henke and Ebisu (1974))
MAC value for Fe ka in O = 22.25 (MCMASTER McMaster (LLL, 1969) (modified by Rivers))
MAC value for Fe ka in O = 22.26 (MAC30 Heinrich (Fit to Goldstein tables, 1987))
MAC value for Fe ka in O = 22.25 (MACJTA Armstrong (FRAME equations, 1992))
MAC value for Fe ka in O = 20.85 (FFAST Chantler (NIST v 2.1, 2005))
MAC value for Fe ka in O = 22.00 (USERMAC User Defined MAC Table)
3. Carbon coating differences should not be an issue as long as the over voltage is reasonable (15 keV or higher).
4. Background corrections should not be an issue using the LiF Bragg crystal. However analyzing Fe Ka on a PET Bragg crystal could be very problematic for the background will be very curved at such a low sin theta.
5. Surface polish should not be an issue for an energetic line such as Fe Ka, but always worth making sure the samples are well polished.
However, these issues
could be problematic:
1. Your Fe metal standard could have surface oxidation, but if so this would tend to raise the concentration of Fe in your Fe2O3 secondary standard (the primary standard intensity is in the denominator of the k-ratio).
2. I note that you have only analyzed for Fe cations. Have you checked that your Fe2O3 standard from SPI/Taylor is actually 99.99% pure? Is it natural or synthetic? Common natural impurities are Si, Ti, Al, Mn, H2O. This alone could explain your observations.
Again, as I have said before, we need to identify, obtain and distribute at least two high purity synthetic minerals for each of the (at least to begin with, common) geological elements, on a global basis, so that we can actually begin to rigorously compare our results with each other. This problem is described here for those that have not seen it:
https://probesoftware.com/smf/index.php?topic=1415.0For example, Will Nachlas has documented that a so called 99.99% Rh metal standard in one of his commercial mounts actually has ~4 wt% Fe in it!

Quantitative analysis of oxygen as a major element is a whole separate endeavor, and here is a link to one of those discussions:
https://probesoftware.com/smf/index.php?topic=197.0But I suggest that we first figure out what is going on when you try to analyze Fe2O3 using Fe metal and calculating oxygen by stoichiometry. I would start by analyzing for the trace and minor elements, but maybe start by examining the Fe2O3 with EDS using a long count time to obtain sufficient sensitivity.