[Ifeffit] Athena: problems with LCF

[Kropf, Arthur Jeremy]

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 at millenia.cars.aps.anl.gov 
> [mailto:ifeffit-bounces at 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
> 
> 
> 
> 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/
> 
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