|
Hi everyone,
Thank you for your reply.
I understand the difference between peak fitting and linear combination
fitting. It is like, If I don't know anything about my sample, peak fitting
should be used. If I know the components of my sample, I can use linear
combination. That is why I use peak fitting first, because I know very
little about my sample. However, what I am thinking is that from the
results of peak fitting, I know there may be some types of sulphur in my
sample, and based on the peak positions, I can chose the standards for
linear combination fitting.
I can't use linear combination fitting in the first place, because I don't
exactly know what sulphur forms in my sample, and I may miss an important
species if I chose the wrong standards. However, I still want to try linear
combination after peak fitting, because I think the linear combination
fitting may be more accurate than peak fitting. It uses the whole spectrum
of the standard, while the peak fitting neglectes the effect of small
features on the fitting. Does it make sense?
My samples are fluid coke and their activation products. They have reduced
and oxidized sulphur species, more likely in organic forms. I have 26
standards covering the sulphur oxidation states from elemental sulphur to
sulphate. Most of them are organic sulphur.
Any more suggestions? Thank you!
Best regards,
Jenny
--- On Fri, 10/24/08, Bruce Ravel <bravel@bnl.gov>
wrote:
From: Bruce Ravel <bravel@bnl.gov>
Subject: Re: [Ifeffit] Fit XANES spectra using Athena
To: "XAFS Analysis using Ifeffit"
<ifeffit@millenia.cars.aps.anl.gov>
Received: Friday, October 24, 2008, 9:06 AM
On Friday 24 October 2008 00:59:21 Jenny Cai wrote:
> Hello everyone,
>
> Sorry for bothering you again.
>
> I am using least-squares peak fit and linear combination fit to analyze my
> samples. I have spent tons of time on it, and it really makes me crazy.
Why
> can't I get consistent results from these two methods?
>
> Please see the attached file. Both of these methods work well
individually,
> but linear combination fit always need more peaks than peak fit to get an
> 'ok' fitting. Should I stick on one method for all my samples, no
matter
> what results the other one gives? It is confusing me so much. Could anyone
> help me out?
Jenny,
I think you are comparing apples and oranges. I certainly think you
are looking for some correspondance between peak fitting and linear
combination fitting that
probably does not exist.
When doing linear combination fitting, you are making the implicit
assumption that your data can be described in that way. Suppose that
you go to the chemical cabinet and grab jars of 5 stable, non-reactive
chemicals. Scoop out a spatula-full of each and mix them together
very well in ajar. Then scoop out a bit of the mixture, spread it on
a piece of tape, and measure some XAS. In that case, we certainly
expect LCF to work well. The measured spectrum should be a linear
combination of the spectra from each of the 5 original materials in
proportion that has something to do with how many absorber atoms were
in each scoop.
Now suppose that you take a scoop of soil from a swamp. You might
expect that your metal atom is distributed among oxide and sulfide
species. You measure some xas on the swamp soil and on a library of
standards. You may find that you can do LCF using some
mixture of
oxides and sulfide, or you might not. That may mean that you
neglected to consider an important standard, a carbide, for example.
*Or* it might mean that the oxides and sulfides in the real sample
include some kind of exotic organic species that isn't quite like the
standards you have available. In the latter case, your LCF fit
results will be approximate in the best scenario.
So, you don't say anything about what sulfur species you are using as
standards, nor do you say anything about what the sample is. Should
you expect a perfect fit from some number of standards? Do you know
for a fact that your standard is simply a linear combination of
standards. That is, should you expect the LCF to tell you what the
components *are* or should you expect the LCF to tell what the sample
resembles?
As for the peak fitting, what do each of the line shapes mean
physically? Chemically? No XAS data
is described by a single
Gaussian. Neither is any XAS data described by a single arc-tangent.
So why should the number of lineshapes required to generate a
mathematical function that resembles your data be related to the
number of species you require in a linear combination fit? You seem
to making yourself crazy based on the assumption that there is some
magical relationship between the six lineshapes you used in the peak
fitting and the number of standards you use in the linear combination
fitting.
It seems to me that you would make yourself a lot less crazy if you
didn't cling to this unsubstantiated assumption.
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/
_______________________________________________
Ifeffit mailing list
Ifeffit@millenia.cars.aps.anl.gov
http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit |
