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* Hi Ellery,
I'm sure you are correct about the lack of Na loss (especially since you were only counting 10 seconds) but I will just mention that on the Withers glasses I measured I saw an over 100% correction in Na intensity over time (about half the intensity was lost over a admittedly longer period of 60 secs) with column conditions of 15 keV, 10 nA and a 20 um beam. (So a somewhat more focused beam than you are using.)

Just to be sure, do you see an increase in the Si concentration when the H2O is included in the matrix correction? Note in my analysis below the SiO2 is 70.57 with H2O in the matrix and only 70.22 without H2O. Also you can see the 107% positive correction for Na loss (the line labeled VOL%) and in addition, a -0.7% correction for Si GAIN and also a 2.9% gain for oxygen loss (over 120 seconds). Maybe this last issue is related to your low totals?

Finally, did you perform an area peak factor correction for the oxygen measurement relative to your oxygen standard? In my case I used MgO as the oxygen standard which yields an APF correction of 1.031 (3.1%) relative to my unknown composition.

I guess the other question is what oxygen mass absorption coefficient did you use? I used    

      O       ka      Si  8.7900e+03    Bastin  (1992)

This could make a difference too since the matrix is mostly silicon (besides oxygen). I'm guessing that you will find the low totals being due to a number of small effects rather than a single large effect.
john

Emacs!


At 08:39 AM 6/11/2008, you wrote:

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*

Hi John,

The first question is: are you performing the intensity at zero time 
correction for Na loss? This glass (even in 10 seconds) will lose 
some Na relative to "omphacite (albite)". Obviously if you have lost 
Na then the H20 from 100% calculation will be high.

The only treatment of Na so far has been charting the counts on the 
peak over time using his conditions to show that there is no loss of 
counts over a relevant period of time, at least 30 seconds.  I'll have 
him double-check his measured vs expected Na totals based on the 
experiment.  I will also check if there is enough Na in the glasses to 
possibly explain totals at least 5% low.

The other consideration is that the H2O needs to be added to the 
matrix correction to adjust the SiO2 concentration (all the elements 
are affected by the addition of H2O to the matrix, but SiO2 being 
the largest concentration requires this the most). This is because 
Si ka is more absorbed by oxygen than by itself. In these 
composition glasses one will typically see an increase in the total 
(or decrease in the H20 by difference) of about 1% absolute.  These 
corrections should be performed in the matrix correction loop for 
best accuracy (as opposed to an Excel spreadsheet).

Yes, we are definitely adding H2O to the matrix correction, not just 
adding it later in Excel by difference.

I'll send you a short note I wrote on these water by difference 
correction issues off-line that also includes a discussion on a new 
"blank" correction for accuracy I've developed for traces that 
amazingly can also be used for calibration of major concentrations 
oxygen.

Thanks -- I'll definitely check that out as well as pass it along to 
our researcher.

Best,
Ellery