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June 2008

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Ellery Frahm <[log in to unmask]>
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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

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