The Limits of EPMA Accuracy

[Probeman]

Using the new plot option of relative percent variance (i.e., accuracy) in Probe for EPMA from the Output | Plot Standard and Unknown XY plots menu dialog:

https://smf.probesoftware.com/index.php?topic=40.msg14388#msg14388

we can quickly evaluate our secondary standard accuracy. Here for Mg Ka in MgAl2O4 using MgO as a primary standard in the FIGMAS mount from Will Nachlas:



All within 1% relative accuracy. But since we're plotting relative percent accuracy, we can plot more than a single compound, here Ti Ka in SrTiO3 and RbTiOPO4 using TiO2 as a primary standard:



And here for Fe ka in a multitude of compounds using Fe metal as the primary standard for the sulfides, and magnetite as a primary standard for the oxides, silicates and glasses:



Note that only the natural chromite is significantly outside the 1% variance accuracy limits.

[John Donovan]

In order to utilize the accuracy checks shown in the above plots, one must run their secondary standards as standard samples. Therefore, some of you have asked: how do I run my standards more than once?

Well first, to re-run ones standard samples a second time, just check the "Acquire Standard samples (again)" checkbox.  Even if you don't have an unknown sample selected, the standards will run a second time.

To run your standard even more replicates, just create a random unknown sample to run along with your standards and check the Re-Standardization Interval checkbox and enter a short interval (in hours) as seen here:



Say every hour or so. That's easy, right?

[Probeman]

With the new plotting feature in the Output | Output Standard and Unknown XY Plots menu dialog, we can plot relative accuracy for multiple elements:



We can see generally ~1% or better relative accuracy for Si, Mg and Fe. In addition we can plot the absolute wt% differences from the published values of the glasses here for Fe Ka:



Most of these Fe analyses are off by only a few hundred PPM!

[John Donovan]

In case anyone missed this post over the July 4th weekend, I wanted to call attention to this post:

https://smf.probesoftware.com/index.php?topic=8.msg14405#msg14405

For best EPMA accuracy, please set your defaults in the [software] section of your Probewin.ini file for the following items to these values:

[software]
DeadtimeCorrectionType=4    ; 1=traditional, 2=high precision formula, 3=Super Precision, 4=Logarithmic, 5=Exponential
MACTypeFlag=6               ; default MAC file (1 = LINEMU, 2 = CITZMU, 3 = McMaster, 4 = MAC30, 5 = MACJTA, 6 = FFAST
DefaultZAFType=11           ; default zaf algorithm 1=armprz, 2=pdrzaf, ... 9=papfull, 10=papsimp, 11=DAM

[John Donovan]

Andrew and I presented our poster yesterday at M&M 2026: "Breaking the EPMA1% Accuracy Barrier".

I've attached it below (login to see attachments). I am happy to discuss details with all, as some of the material is quite unintuitive. Feel free to call or email me.

[Probeman]

When testing instrument accuracy you will want to utilize standards that you are very confident they are the composition they claim to be.  That is generally not the case for natural minerals. 

Some natural hematite and magnetite (and pyrite) standards may be pure and stoichiometric enough, but maybe not.

That is why my accuracy testing was generally performed with high purity synthetic minerals such as MgO, Al2O3, MgAl2O4, SiO2, SrTiO3, TiO2, YIG, Fe2SiO4 and Mg2SiO4 (and a natural magnetite and pyrite that have been checked for purity and stoichiometry).

https://smf.probesoftware.com/index.php?topic=1831.msg14108#msg14108

https://smf.probesoftware.com/index.php?topic=1831.msg14131#msg14131

What one would NOT want to do is to attempt to analyze Mg, Al or Si Ka in oxides or silicates using the pure metal because of the large peak shift between the metal and the oxide.

This Ka peak shift decreases as the atomic number increases, and is quite small by the time one reaches Fe. But depending on the quality of the LiF crystal, a peak shift in Fe Ka can be detected when performing high precision quant:

https://smf.probesoftware.com/index.php?topic=1423.msg14418#msg14418

Thus, it is best practice use Fe metal as a primary standard for Fe in alloys and sulfides, and utilize an oxide primary standard for oxides and silicates.