[Ifeffit] Multiple phases in samples

[Scott Calvin]

Hi Dien Li,

Create a FEFF calculation and for each phase.

Weight the SO2 of the paths for each calculation by the proportion of that phase that is present; that is often done with a guessed parameter.

For example,

Set SO2_U = 0.85
Guess frac_goethite = 0.3
Guess frac_organic = 0.3
Def frac_uraninite = 1 - frac_goethtite - frac_organic

Then, for the paths associated with U on goethite, enter in the SO2 field the expression SO2_U * frac_goethite, and similarly for the other phases.

While that's how you do it in theory, these kinds of fits often end up unstable; Ifeffit may find fits that work well statistically but are utter nonsense; e.g., they might have negative fractions. The more you can constrain or restrain the fit, the less likely you are to have that happen.

In fact, if you have more than one sample in this class, you might want to try PCA first, including target transforms, to start getting a sense of what's present, and then switch to Artemis once the possibilities have been narrowed down.

--Scott Calvin
Sarah Lawrence College

On Jul 7, 2014, at 1:20 PM, dien.li at srs.gov<mailto:dien.li at srs.gov> wrote:

Hi, All

"Assuming" there are multiple phases in one sample, saying, U on goethite, U on organic matter, and uraninite, how to fit the EXAFS data using Artemis? Thanks.



Dien Li, Ph.D.
Environmental Sciences and Biotechnology
Savannah River National Laboratory
Aiken, South Carolina, 29808
Tel: 803-725-7520
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