[Ifeffit] Stoichiometry from EXAFS data

[Scott Calvin]

Hi Peter,

I've done this as well, and compared to reliable methods (e.g. ICP).  
I'd be skeptical of 1%. It's generally quite difficult to determine  
edge steps to that accuracy. Assuming you're using Athena to determine  
the edge step, find the most extreme pre- and post-edge lines that  
seem acceptable and note the range of edge steps. That will yield an  
uncertainty range.

If you have strong features at the white line and just past it, I'd be  
surprised if you can do much better than 10%. If features in that  
region are small, such as you might have in an intermetallic alloy,  
then you might get down to the sub-5% range.

While I think that determining the edge step is likely the major  
source of error, you also have to be aware of the usual suspects in  
XANES analysis, such as the presence of harmonics in transmission or  
self-absorption in fluorescence. Testing for linearity with tricks  
like putting sheets of aluminum foil before I0 can help detect some  
(but not all) of those kinds of issues.

--Scott Calvin
Sarah Lawrence College

On Aug 2, 2010, at 4:03 AM, Peter Zalden wrote:

> Dear Feff users,
>
> lately, we measured a sample containing Sb and Te at EXAFS beamline  
> CEMO, Hasylab and are wondering whether one can determine the  
> stoichiometry from the height of the different K edges´ steps, if  
> one normalizes the values on the edge steps of the elements (cf. http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html) 
> . The absorption gases and the specimen were not changed for the  
> different K edges.
> Of course, I have already tried doing so and from statistical  
> reproducibility and from the resulting values compared to the  
> expected ones I would estimate an error of this method of about 1%.  
> A source of error that I could imagine originates from the different  
> beam position at different energies combined with a slight  
> inhomogenity in the pressed sample powder. Now my question is: Are  
> there any other sources of error that I should take into accout? Is  
> there any reference on this method from a more experienced user that  
> I could cite?
>
> Kind regards,
> Peter