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Hi Ellery,
Two things- I note that the image of the analytical results I posted 
previously did not come through the last email to the list server, so 
I'm sending it as an attachment in case that works better.

Second, something very interesting. I originally calculated the H2O 
results for the Withers glasses two ways. One just using water by 
difference from 100% (not measuring oxygen) and also by measuring 
oxygen to calculate the H2O from excess measured oxygen. Just as you 
did. When I posted the earlier analysis (now attached), I noticed 
that it was still in the mode where H2O was being calculated by 
stoichiometry to excess oxygen and included in the matrix correction.

So then when I wanted to show you the effect on SiO2 from not 
including H2O, I simply unchecked the stoichiometry to excess oxygen 
option and reported the change in SiO2 concentration due to the 
change in the matrix correction. That wasn't quite right. Because I 
was still measuring oxygen (and including it in the matrix 
correction), the concentration change in SiO2 I reported to you was 
only due to the lack of hydrogen in the matrix!

I would never have thought that 0.59% hydrogen could cause such a 
large matrix effect in the SiO2 concentration. I'm going to double 
check this but here is a preliminary summary:

                         SiO2

without H2O             70.025
with excess O           70 22    (but without the 0.59% H)
with H2O                70.57

Note that in the last step I calculated H2O two different ways just 
to double check. First using the hydrogen by stoichiometry to excess 
measured oxygen calculation and also by calculating oxygen by 
stoichiometry and adding 0.59% hydrogen to the matrix as a fixed 
concentration. The SiO2 results are almost exactly the same for both methods.

I don't quite believe it but it appears that the matrix effect on 
SiO2 of 0.59% hydrogen actually a larger effect than the 4.9% excess 
oxygen (for this composition anyway).

I just checked again- it seems to be. The Z correction on Si ka for 
no hydrogen goes from 1.2194 to 1.2254 with 0.59% hydrogen. That's a 
0.48% change in the Si ka intensity. Weird.
john

John J. Donovan
Director, MicroAnalytical Facility
CAMCOR, University of Oregon

(541) 346-4632 (office)
(541) 346-4655 (lab)

www.epmalab.uoregon.edu