Nina, Dominik, The issues Dominik raises are relevant for measuring and fitting experimental data, but they shouldn't affect your test of the LCF function using a simulated combination of spectra. Bruce's suggestion that the data normalization could be a problem also should not affect the LCF test - as long as you didn't change the normalization for the fitting standards compared to the spectra you used to make the simulated combination. Jeremy
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Dominik Samuelis Sent: Monday, August 15, 2011 8:53 AM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Athena: problems with LCF
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
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 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
On 15.08.2011 11:35, Nina Siebers wrote: the LCF of 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@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/
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