Re: [Ifeffit] How to judge if the coordination numbers are different?

Matthew, Thank you so much for your reply. I definitely will try what you suggested. Could you please explain to me why an increased chi-square can be used to argue for a true difference. "To check the robustness of the differences, you could try doing your co-refinement with just some of the data, for instance the 1-day 300 plus 55-day 1200 with none of the others." I will try what you suggested above. I just do not understand how to use to results to test the robustness of the difference. Could you please explain to me a little more? Shall I try fitting two data sets, instead of all of them, in the same way, and see if the chi-square changes? Again, thank you so much for your insightful message. Fiona

My intuitive reaction is that these differences are unconvincing, but that itself is unconvincing, being based on gut feel. I assume that the numbers in () are error bars? Anyway, you can try fitting the whole set with common, concentration-dependent CNs (6 variables) or age-dependent CNs (also 6) or just one common pair of CNs (2 variables) by slaving some CNs to others. This is easy to do in Artemis. It can then be argued that if the reduced sum-square ("chi-square") goes up on applying these constraints, then you've got real differences. However, it may be that the same effect can be obtained by fixing the CNs and letting the ss's float in the same way as you've done with the CNs. To check the robustness of the differences, you could try doing your co-refinement with just some of the data, for instance the 1-day 300 plus 55-day 1200 with none of the others.

A possible experimental control, if you can get more beamtime, would be to age a 1200mmol sample, then dilute it with more peat to a concentration of 300mmol, and age some more, while at the same time aging a 300mmol sample. If these two came out indistinguishable, then either the concentration effect is due to a reversible reaction or is some strange artifact of concentration. Since you're working right at the margins of what the technique can do, you have to consider possibilities like that. mam ----- Original Message ----- From: To: "XAFS Analysis using Ifeffit" Sent: Wednesday, November 11, 2009 3:01 PM Subject: [Ifeffit] How to judge if the coordination numbers are different?

Dear XAS community members, I am studying the structural change of ferrihydrite as it ages with peat, which is one kind of organic matter. Totally I have three systems. In these systems, ferrihydrite/peat ratios are 300, 600, 1200 mmol Fe/kg peat. Smaller number means more peat in the background with ferrihydrite. I have collected EXAFS data for these three systems that have aged for different times. The magnitude portion of the Fourier transformed data look very much the same within each system, with small variation in the amplitude. I included two Fe-Fe paths (R=3.075 and 3.376 Å) to fit the second shell. The coordination numbers of these paths seem to increase for each system as it ages. Following is some results: 300 mmol/kg 1-day 7-day Fe-Fe1 1.6 (0.2) 1.7 (0.2) Fe-Fe2 1.7 (0.4) 1.8 (0.4) 600 mmol/kg 1-day 7-day 55-day Fe-Fe1 1.8 (0.3) 1.9 (0.3) 1.9 (0.4) Fe-Fe2 2.0 (0.3) 1.9 (0.3) 2.1 (0.5) 1200 mmol/kg 1-day 7-day 55-day Fe-Fe1 1.8 (0.3) 1.9 (0.3) 2.1 (0.4) Fe-Fe2 2.0 (0.3) 2.0 (0.2) 2.0 (0.5) I was told that fitting results of coordination number have big error. When I fit these data, I put all data sets together. These data share a lot of common variables such as coordination number of first shell Fe-O path. The bottom line is that the total number of variables are much smaller than the independent points, which increases the accuracy of fitting. The fitting results coordination numbers are different. However, I do not know if the difference is real difference, or they are the same within error, or statistically they are just the same. Could anybody share your knowledge with me? Regards. Fiona Kizewski North Carolina State Univerity _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

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