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

[Matthew]

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: <jrkizews at ncsu.edu>
To: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
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
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