On Tuesday 10 August 2010 01:54:01 pm Elsa E. Sileo wrote:

I have some measurements at the As edge of As(V) adsorbed onto pure goethite (alfa-FeOOH), Mn partially substituted Goethite (alfa-Mn,Fe.OOH), Al substituted goethite (alfa-Al,FeOOH) and Mn and Al goethite (alfa-Al,Mn,FeOOH) The EXAFS signals change from one spectrum to another.

I have refined the distances As-Fe on pure goethite, but when I try to refine Mn-FeOOH or Al,FeOOH, the analysis gets more difficult because As forms different complexes with Al and Fe. Unfortunately for me is almost impossible to have the pure phases alfa-MnOOH or alfa-AlOOH, and measured the As adsorbed.

How can I analyze the spectra and have some knowledge about the As-surface complexes formed?

Hi Elsa, The general problem of analysis of impure phases is one that many peple have used Feff and Ifeffit on. Among the folks who regularly post here on the mailing list, both Shelly Kelly and Scott Calvin have published on analysis of mixed phase systems and have discussed their strategies for doing so. Of course, neither of them has worked on quite the same system as you have, but the concepts in their papers are readily transferable. The basic idea is that, although you may not have pure phase MnOOH or AlOOH *samples*, you can certainly imagine what the structure of those might be and make a Feff calculation on that imagined structure. That will give you a good guess as to what the contribution to the XAS data of As in one of the other possible environments looks like. In you fit, you then take paths from the additional Feff calculations that you can use to model the parts of the data that come from As in the other environments. Of course, this can be a difficult -- you need to figure out a sensible way to parameterize amplitudes, delta R values, and sigma^2 values for these additional paths. And you must do so while being mindful of the limits on the information content of your data. Do you have a copy of Shelly's recent review article? Kelly, S. D. ; Hesterberg, D. ; Ravel, B. Analysis of soils and minerals using X-ray absorption spectroscopy. In Methods of soil analysis, Part 5: Mineralogical methods; Ulery, A. L. ; Drees, L. R. , Eds.; Soil Science Society of America: Madison, WI, 2008; pp 367− 463. Shelly can probably send you a copy if your library does not have this book. The section starting on page 451 under the heading "Determining a Second Shell Atom Type within Sediments" is quite relevant to your problem. I'd recommend reading that (and, frankly, the rest of the article) quite carefully. HTH, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/