[Ifeffit] Linear Combination Fitting - More Questions

[Dean Hesterberg]

Lisa,

It can be difficult if not misleading to apply linear combination  
fitting of a large number of standards to an XAS data set.  My advice  
is to augment LCF with independent data that provides support for  
likely chemical species in your sample.  Use this supporting  
information to select appropriate standards up front, and to judge  
the likelihood of a given fit.

For example, total elemental analysis of your sample in combination  
with chemical stoichiometry and mass balance can indicate whether it  
is possible to have a given proportion of a particular chemical  
species in your sample.  For example, if your sample contains only  
enough Fe to yield 20% of a given Fe-oxide mineral, then an LCF fit  
giving 60% of that mineral is not accurate, even if the fit is  
exceptionally better than other fits based on a goodness-of-fit  
parameter.

Additionally, you might try fitting across different energy ranges of  
your spectrum to isolate characteristic features of particular  
standards to better judge the most suitable standards.


Dean

DEAN HESTERBERG
Professor
Dept. of Soil Science
College of Agriculture and Life Sciences
Box 7619
3235 Williams Hall
NC State University
Raleigh, NC 27695-7619

voice: (919) 513-3035
fax: (919) 515-2167
email: dean_hesterberg at ncsu.edu

On Jul 17, 2008, at 8:53 AM, Gudrun Lisa Bovenkamp wrote:

> Hi everyone,
>
> I just learned that before Athena there was no such program to  
> check on
> all combinations, so it was not that confusing - or just more work?
> I think this feature of Athena is very good tool, but when I really do
> not know what is in my sample how can it help?
> I try to answer it myself and someone can correct me:
> Certainly not all of my, say 10 reference spektra, are that similar  
> that
> I can get several LCFs with the same say R-factor. So, I will get some
> clue to narrow the components. But still there can be, for example:
> Standard-combi 1, 2, 3 equal to 1, 4, 6.
> So,
>   my analysis gives two solutions and one must be wrong, but both  
> can be
> wrong. How can I tell?
>
> Can a PCA help me in this case?
>
> The results change significantly when I change the normalization.  
> How do
> I know what is the 'correct' adjustment?  - Yes, I read the Athena
> turorials, but I got no answers from there.
>
> I searched quite a while to find a complete documentation of the LCF
> formula used in Athena, but I could not find it.
>
> Many thanks,
> Lisa
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