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June 2008

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Ellery Frahm <[log in to unmask]>
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*

Hi Eric,

> Did you standardize with the same beam size as was used on the  
> unknown glasses?

Yes, he did.

> I do not like using a 30 micron beam because X-rays generated  
> outside the area that is actually analyzed by the detector can enter  
> the analytical area and fluoresce additional X-rays that are not  
> accounted for in the F correction.  I would try a 10 micron beam and  
> run the analyses on the unknowns at 3 nA.

I'll suggest that.  We weren't to concerned about these issues with  
the beam diameter, though, because he is analyzing in the middle of a  
large melt sample at least 100 microns thick and at least as wide.   
But to check if this really is the problem, I'll have him add his  
capsule material in future tests.

> When you estimated H2O by difference from 100 %, did you include the  
> H2O in the ZAF program or did the program reduce the data not  
> "knowing" about the extra H2O?  The latter could lead to errors  
> particularly in Na-Si corrections due to the missing O and the  
> somewhat different average atomic number (depending in part on  
> whether the program calculates the Z with missing elements as having  
> an atomic number of zero).

At first, he didn't add the H2O to the correction, but we quickly  
added it to the ZAF correction and have had it that way ever since.

> Analysis of O is dependent on the C thickness because of the strong  
> absorption of O Ka by C.  If possible, the standards and unknowns  
> should be coated together.  The problem could be tested in part by  
> doubling the coating and repeating the analysis for O.

He tried different carbon coat thicknesses -- 50 and 200 microns and  
found similar results.

> As I understand it, the O analysis technique has been able to work  
> on rhyolite glasses.  Have other andesitic glasses given reasonable  
> results for H2O in the past?

I am not sure how often this group has analyzed hydrous andesitic  
glasses before -- I'll have to ask.

> I wonder if this particular sample is not adequately grounded.  What  
> does the beam current and the sample current look like during the  
> analysis, and is the sample current similar to that on andesite  
> standards?

I keep coming back to this explanation myself, actually.  The glass is  
surrounded by a metal capsule in a brass mount holder, and we've  
coated it twice now, so it really shouldn't be a problem.  Checking  
the sample current is a good idea, though -- I'll definitely have him  
try that.  I guess the next step would be getting out the carbon or  
silver paint.

> I have learned from Youxue Zhang and his students here over the  
> years that FTIR on real andesitic glasses is very challenging  
> because of the large background induced by Fe.  I wonder if there  
> could be serious errors in the FTIR data.

He's done the FTIR twice now, but you're right that it shouldn't be  
considered "truth" either.

Thanks,
Ellery

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