Hi Scott, thanks a lot for your quick response! I found your suggestion very helpful and tried to change the edge step to both extremes by tuning the fitting range for the pre- and post-edge lines in Athena. Due to the very flat structure in the XANES range (cf. attachement), I could modify the value for the edge step by 3% total, which corresponds to an error of +/-2%. One could possibly discuss if this value represents the one-sigma or maybe the two-sigma interval, but the error is nicely small anyway. In the last campaign, we measured a sample of Sb_2Te_1 in two different annealing conditions and from those different data sets (as concerns the EXAFS range), I determined the stoichiometries: Sb_2.06Te_0.94 and exactly the same for the second sample. Therefore, a sub-5% error seems reasonable to assume for these semi-metallic systems. Concerning the influence of higher harmonics: The beam was usually detuned to 70% intensity of the main reflection so that this should not have a strong influence, since the amount of detuning was not changed for both edges. Best regards, Peter Am 02.08.2010 12:52, schrieb 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
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