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December 2013

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From:
"Straszheim, Warren E [BIOTC]" <[log in to unmask]>
Reply To:
JEOL-Focused Probe Users List <[log in to unmask]>
Date:
Mon, 30 Dec 2013 20:48:24 +0000
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I've been operating an 8200 for Ames Laboratory for almost four years now. I've worked with SEM and EDS since about 1980 and trying to push it for all its worth.
Currently, I am having challenges analyzing FeGa compounds. Many are in the neighborhood of 15% Ga.

I have been using Fe and GaAs as standards and I have a sample of about 65 at% Ga that has FeGa3 and Fe3Ga4 phases present. The FeGa3 is supposed to be a line compound but the Fe3Ga4 has almost 2 at% width to the phase.

I've run my regular probe analyses and included the FeGa3 as an unknown to check. While the composition is supposed to be 79 wt% Ga and 21 wt% Fe, I measured 77% Ga and 23% Fe. I had to adjust the Fe standard down in intensity about 8% to get the proper composition. Then Fe came out less than 36% for the Fe3Ga4 phase where it is suppose to be 37.5% I'm not sure if that is due to error in matrix correction or the width of the phase diagram favoring the Ga-rich side. However, the Ga analyses came out pretty much on the mark.

I tried setting up the FeGa3 as a standard for both Fe and Ga. It gave the right results for FeGa3, of course. It also gave the right answer for Ga in Fe3Ga4, but it gave a low value for Fe like before. It also gave a value of 92% for my Fe std.

I have tested the same materials with an Oxford EDS system which used Fe and FeGa as a standard. It ends up giving a close answer for the two FeGa phases in my standard. Now I suppose I need to turn off normalization and see how the numbers come out.

I've calculated the ZAF factors for various compositions of Fe and Ga using the CalcZAF program.

I found a ZAF factor as low as 0.83 for 1% Fe in Ga. About 70% of the correction was due to the fluorescence effect. The other 30% was due to Z.
The ZAF factor was as high as 1.12 for 1% Ga in Fe. About 60% was due to Z and 40% due to absorption.

The ZAF factor was 0.88 for Fe and 1.03 for Ga at the FeGa3 composition. The probe gave a factor of 0.91 for Fe at that composition; thus there seems to be a 3% difference in correction between CalcZAF and the probe.

I know that it's best to have a standard in the neighborhood of the unknowns so that uncertainties in the matrix correction cancel out. That's not really an option for me here, but I am working on it. Still, shouldn't the matrix corrections work over a broader range? I wonder why the WDS analyses and can't match the apparent accuracy of the EDS results. Is there something about the probe matrix correction that isn't up to par for this particular mix of elements?

The bottom line is that the matrix correction appears to be significant and not all that good over a wide range of compositions, especially for the probe. The Oxford EDS systems may be using a different set of physical constants which may be more correct - at least for the Fe-Ga system.

Any ideas out there?

Warren

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