[Ifeffit] Athena: problems with LCF

[Dominik Samuelis]

Dear Nina,

in addition to what Stefan Mangold has mentioned (energy range of your 
spectra too small for proper normalization, for both pre-edge and 
post-edge), there is another (maybe) critical issue in your data. Your 
"A" spectrum has an edge step value of \Delta µx=0.03, while your "C" 
spectrum has \Delta µx=2.92. In other words, your "A" and "B" standards 
are extremely dilute, and your "C" and "D" standards are almost too 
concentrated. There is almost 2 orders of magnitude difference in Cd 
concentration between A and C.

In principle, normalization of the spectra should take care of the 
largely different concentration of Cd in your reference samples. This 
assumes that Beer-Lambert law holds, i.e. µx linearly depends on the 
concentration.

In reality however, there is a number of effects that lead to a 
non-linear relation between concentration and µx ("thick sample 
effects", e.g. largely different harmonics content, pinholes for 
transmission experiments, self-absorption for fluorescence). Grant 
Bunker shows all the relevant effects in this set of slides: 
http://gbxafs.iit.edu/training/XAFS_sample_prep.pdf

Maybe you can tell us a little more about your experiment, i.e. sample 
preparation, XAS experimental setup, and data treatment? Maybe we can 
then come up with some more advice.

Best regards,
Dominik



On 15.08.2011 11:35, Nina Siebers wrote:
> Dear All,
>
> I acquired Cd L3-edge spectra of some binary and ternary mixtures in
> varying proportions and for the individual components. The mixtures were
> created on Cd-mass basis. Then, I tried to fit the reference spectra to
> the spectra of the mixtures using linear combination fitting of Athena
> to get their abundance. However, the results were disappointing despite
> all spectra were carefully energy calibrated and normalized, so I
> decided to create simple mathematical binary and ternary mixtures by
> summing up the spectra of the individual reference spectra. After that I
> did an edge-step normalization in excel and imported the normalized
> calculated mixtures into Athena. Then, I tried the fitting again to
> exclude mixing-failures and check sensitivity of LCF with the idealized
> spectra. Even though the results of the LCF of the mathematical mixtures
> were better compared to the real mixtures, LCF was also not able to
> reliable deconvolute these spectra into the individual reference spectra.
>
> Does anybody have an explanation for that? It would be nice if somebody
> could give me information about the mathematical fitting algorithm
> implemented in Athena.
>
> Attached is a data file of three mixtures (two ternary and one binary
> mixture) including the mathematical mixture created in excel (named
> calculated at the end). Mixing ratios are named 1to1to1 (meaning 1:1:1
> of the components in the same order). For the 1:1:1 ternary mathematical
> mixture the deconvolution was very good, but the others need improvement.
>
> I hope I made my problem clear this time.
>
> Thanks a lot!
> Wishes,
> Nina
>
>
>
>
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-- 
Dr. Dominik Samuelis
d.samuelis at fkf.mpg.de
Max-Planck-Institut für Festkörperforschung
Max Planck Institute for Solid State Research
Heisenbergstr. 1
70569 Stuttgart
Germany
Phone +49-711-689-1769
Fax   +49-711-689-1722
Web   http://www.fkf.mpg.de/maier/