Re: [Ifeffit] Linear combination fitting

Hello Dominik, You explaination about LCF fitting is very clear. Thanks. Best wishes, Mengqiang Zhu ----------------------- Mengqiang Zhu Ph.D Candidate Environmental Soil Chemistry Department of Plant and Soil Sciences University of Delaware 152 Townsend Hall Newark, DE 19716 http://ag.udel.edu/soilchem/zhu.html ---- Original message ----

Date: Wed, 08 Apr 2009 23:20:32 +0200 From: ifeffit-bounces@millenia.cars.aps.anl.gov (on behalf of Dominik Samuelis ) Subject: Re: [Ifeffit] Linear combination fitting To: XAFS Analysis using Ifeffit

Dear Mengqiang,

...

"If the component sum is close to 1, it means that no significant component is missed in the LCF fitting."

I'd say that this statement is not generally true.

It basically goes back to another question: "Does the XANES portion (if fitting norm(E)) or the EXAFS portion (if fitting chi(k)) of a standard carry enough information for unambigously relating a single chemical compound to a unique spectrum, that additionally is linearly independent of all combinations of other possible spectra".

Chemical space is huge. This means, that there will be a case where two or more compounds map to very similar spectra (thus breaking the unambigiousity). There will also be a case where the weighted sum of spectra of compounds A+B+C... equal compound X (thus breaking linear independence). Both cases invalidate the statement.

A way of tackling this problem is by adding other information. Usually, when doing a LCF, one is working in a very limited composition space. If you have knowledge about the components of a chemical system, and maybe even about the phase diagrams of this system, you can be much more confident with selecting appropriate standards for the LCF.

After recording the standards for this system, you could then test for linear independence and for ambigiousity by trying to fit one standard with linear combinations of the other standards. For this, the combinatorial fitting tool of Athena is quite nice :-) If none of your standard spectra can be described by linear combinations of the other standards, you can be quite sure about the unique and independent mapping beween chemical compound and spectrum. In this case, I would also feel that after fitting an unknown compound from the same composition space, a residual close to unity most likely means that you've missed no important contribution.

Cheers, dominik

Quoting mzhu@udel.edu:

...

Hello Dominik, Based on your reply about LCF analysis, is the following statement true?

"If the component sum is close to 1, it means that no significant component is missed in the LCF fitting."

If it's true, could you please provide some reference articles for that? Thanks.

Best wishes,

Mengqiang Zhu

----------------------- Mengqiang Zhu Ph.D Candidate Environmental Soil Chemistry Department of Plant and Soil Sciences University of Delaware 152 Townsend Hall Newark, DE 19716 http://ag.udel.edu/soilchem/zhu.html

---- Original message ----

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Date: Tue, 07 Apr 2009 22:17:21 +0200 From: ifeffit-bounces@millenia.cars.aps.anl.gov (on behalf of Dominik Samuelis ) Subject: Re: [Ifeffit] Linear combination fitting To: XAFS Analysis using Ifeffit

Dear Guvenc,

Shelly and Scott have already replied to this issue, and I fully agree with their points. Nevertheless, I'd like to add some points, that generally apply to linear component fits.

The huge discrepancy between chi(k) and norm(E) fit very likely is an indication that your data is not sufficiently described by your standards. It might be that you're either using a standard component that is not present in your system, or that you have not only two, but three or more components in your system.

There is a number of ways to check which one could be the case. Very often it helps to fit the system without the restriction that the weights of the standards must sum to one. That way, you effectively allow for the determination of a residual component during the fit. If your system is sufficiently described by the standards, the weights of the standards should sum to a constant value close to unity. The difference to unity basically gives the residual.

If you have a non-negligible (or even concentration-dependent) residual, PCA might help you to determine how many standard components actually are required to fit the system. Be aware, though, that the interpretation of PCA results is somewhat complicated. If PCA shows that you need 4 linearly independent quantities to fully describe your system, it doesn't necessarily mean that your system has 4 components. Even temperature differences between scans might manifest as an additional "component".

Cheers, dominik

Quoting Guvenc Akgul :

...

Dear all,

I have done Ti doped ZnO EXAFS data analysis but I guess it is getting mixture structure at high concentration for example 55% Ti doped ZnO.Also I done linear combination fit , but at fitting space norm(E),ZnO weight is 0.779 and ZnTiO3 is 0.221 then I changed fitting space is chi(k) ZnO weight is getting 25.2 and ZnTiO3 is getting 74.8 and it is not same like at fitting space norm(E) why?

regards

guven _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

-- 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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-- 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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