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-boun...@millenia.cars.aps.anl.gov > [mailto:ifeffit-boun...@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 > > > > > > > > > > _______________________________________________ > > Ifeffit mailing list > > Ifeffit@millenia.cars.aps.anl.gov > > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > > > -- > Dr. Dominik Samuelis > d.samue...@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/ > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit