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@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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