The fact that PCA tells between known Ni-N and Ni-O complexes by adding one more component when the wrong complex is added (N to O or vice versa) does not mean that there is specific fingerprinting sensitivity to N or O. One needs to demonstrate first that all Ni-N pairs have similar distances and disorder, regardless of their chemical surrounding, and that the same is true for Ni-O. Only in that case your PCA results indeed mean that the Ni sorbed biofilm have both types of bonds. Otherwise you cannot prove that it is Ni-N plus Ni-O vs. a situation where there are two unique types of Ni-O that have not been matched to your Ni-O standard and may thus fool you in thinking that one of them is due to Ni-N instead. Thus, before you conclude that, you should verify whether various Ni-N and Ni-O complexes have same pair properties. From your description it appears not to be the case since you have two components (not one) for your oxygen ligands and nitrogen ligands.
Anatoly
----- Original Message -----
From: ifeffit-bounces@millenia.cars.aps.anl.gov
Date: Fri, 19 Dec 2008 16:34:36 -0500 From: ifeffit-bounces@millenia.cars.aps.anl.gov (on behalf of "Frenkel, Anatoly"
) Subject: Re: [Ifeffit] How to identify N and O atoms in the first coordinate To: "XAFS Analysis using Ifeffit" , Dear Mengqiang,
To the best of my knowledge, in most cases Nitrogen cannot be discerned from Oxygen as a nearest neighbor since their scattering properties are similar. Thus, if you want to solve an "inverse" problem ("Which type of neighbors do I have") you cannot do it. However, if you believe you have some knowledge about the type of metal-oxygen bonding: either M=O or a M-O or a M-OH or a M-OH2 (the first one is the shortest and the strongest, the others may have similar interatomic distances but different bond strengths), you can advance somewhat.
If you have a certain number of distinct models you want to compare the data against, it can be done via a linear combination fitting, or by direct algebraic extraction of any given type of the pair contribution from the data, as described, for example here:
E. Poverenov, I. Efremenko, A. I. Frenkel, Y. Ben-David, L. J. W. Shimon, G. Leitus, J. M. L. Marin, D. Milstein. A terminal Pt(IV)-oxo complex bearing no stabilizing electron withdrawing ligands and exhibiting diverse reactivity. Nature 255, 1093-1096, 2008.
See supplementary info to that article for specific details. Regards, Anatoly
________________________________
From: ifeffit-bounces@millenia.cars.aps.anl.gov on behalf of mzhu@udel.edu Sent: Fri 12/19/2008 4:20 PM To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] How to identify N and O atoms in the first coordinate
Hello all, Merry Christmas! I have some samples of heavy metal sorption on bacterial biofilm. I want to figure out what elements are coordinated with the heavy metal in the first coordinate. Since the oxygen and nitrogen have similar scattering factors, how can I differetiate them? Using shell-by-shell fitting or linear combination fitting with standards? 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 _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
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