Dear Mailing list, I am a relatively new user in terms of XAS data handling and I encountered a strange problem during LCF in Athena. I am working on LCF-combo fittings for Mn minerals as described in Manceau et al. 2012*. Fitting f.ex. a cryptomelane sample of myself with the open source monovalent manganese mineral standards of Manceau et al. 2012 in a fit range of -20 to +30 k, gives the following result shown below (I´ll attach the Athena project): " LCF fit of Cryptomelane_T_20K_ecd as flattened mu(E) from 6536.79 to 6586.79 Fit included 100 data points and 6 variables, and approximately 41.333 measurements Weights sum to 1: no Weights forced between 0 and 1: no Overall e0 shift used: yes Noise added to data: 0 R-factor = 0.0003537 Chi-square = 0.00815 Reduced chi-square = 0.0000858 . standard weight e0 ======================================================== . Manganosite.dat 0.042 (0.006) 1.631 (0.044) . Groutite.dat 0.030 (0.029) 1.631 (0.044) . Ca2Mn3O8.dat 0.288 (0.026) 1.631 (0.044) . Ramsdellite.dat 0.584 (0.022) 1.631 (0.044) . Mn2O3.dat 0.044 (0.025) 1.631 (0.044) . sum ........... 0.988 " Note that the box “all standards share an E0” is checked before clicking on “Fit this group”. Optically, the fit itself “looks” good compared to the data, yet, it is somewhat shifted on the x-axis compared to the data. I was wondering why, as this was not the first sample where this was the case. When I now uncheck the “All standards share an E0” box, leave everything else just the way it is and just click “Fit this group” again, the fit suddenly is shifted perfectly on my data. The strange thing is: Nothing else in the output-results changes – weights, R-factor, chi-square etc. just stay the same. Only slight changes in the numbers in brackets after the reported weights. The "new" result is: " LCF fit of Cryptomelane_Mikon_T_20K_ecd_reb-a-m as flattened mu(E) from 6536.79 to 6586.79 Fit included 100 data points and 5 variables, and approximately 41.333 measurements Weights sum to 1: no Weights forced between 0 and 1: no Overall e0 shift used: no Noise added to data: 0 R-factor = 0.0003537 Chi-square = 0.00815 Reduced chi-square = 0.0000849 . standard weight e0 ======================================================== . Mn2O3.dat 0.044 (0.024) 1.631 (0.000) . Manganosite.dat 0.042 (0.006) 1.631 (0.000) . Ca2Mn3O8.dat 0.288 (0.024) 1.631 (0.000) . Ramsdellite.dat 0.584 (0.019) 1.631 (0.000) . Groutite.dat 0.030 (0.026) 1.631 (0.000) . sum ........... 0.988 " Can anybody tell me, what is actually happening? I exported both data to excel to replot the fits to exclude a simple plot-bug, but the data behind show that this is real. So, I am changing the data behind the plot, but actually not the output data … what is going on here? You find the project attached – I hope someone can tell me, how to cope with that! Thank you very much, Teresa *Manceau, A., Marcus, M. A., & Grangeon, S. (2012). Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy. American Mineralogist, 97(5-6), 816-827. -- Teresa Zahoransky Soil Mineralogy Gottfried Wilhelm Leibniz Universität Hannover Institute of Mineralogy Callinstr. 3, Room 325 D-30167 Hannover, Germany Phone: +49 (0)511 762-8058 Email: t.zahoransky@mineralogie.uni-hannover.de
Hello Teresa,
I'll throw my meager two cents in to say that I'm not sure why it is happening that way, but I can replicate what you are seeing. It seems like it might be a problem with the "All standards share an E0" box because when I directly input those first results (with the 1.631 E0 shift) into the weight and E0 boxes, and click "Plot data and sum," it lines up very nicely. But then even when I run the "Fit all combinations" with that "All standards share an E0" box checked, every output is shifted and I think you would be fine understanding that it is working that way and then just going back again and putting the numbers in directly and "plot data and sum" if you want the better looking plot for your dataset.
I usually like to "Fit all combinations" to get the full set of possibilities but depending on the number of standards you allow it to use and how many you're checking against to begin with, this can get a bit processor-intensive sometimes and working through the quick fitting first to eliminate some options works well in my opinion.
-Derek
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Derek Micheal Peloquin, Ph.D.
ORISE Postdoctoral Research Participant
US Environmental Protection Agency
Office of Research and Development
Center for Environmental Solutions and Emergency Response
Land Remediation Technology Division
Contaminated Sites and Sediments Branch
5995 Center Hill Ave
Cincinnati, OH 45224
Office: (513) 487-2670
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From: Ifeffit
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Peloquin, Derek -
t.zahoransky＠mineralogie.uni-hannover.de