I meant fit the standards to the unknown, and I also noticed that Bruce had
already replied to the same question below.
Anatoly
From: ifeffit-boun...@millenia.cars.aps.anl.gov on behalf of Frenkel, Anatoly
Sent: Sun 5/15/2011 2:34 PM
To: XAFS Analysis using Ifeffit
Subject: RE: [Ifeffit] Athena (Question about LC fitting)
It is really important to have the data for the unknown sample and for
standards correctly aligned in energy. That means that all of them should be
measured in an experiment with the same sample located in the reference
position between the transmitted and reference beam detectors. The absorption
coefficients of all the reference data should be aligned together in energy.
That will ensure that the shifts between the actual data are real and caused by
the oxidation state differences. It is sometimes called alignment in absolute
energy but it simply means that the shifts are real.
When this is taken care of, you can align the standards to the unknown using
LCF, and do not float the energy origin corrections (delta E0) of any of the
samples used in LCF (either data or standards) since they were already set
correctly by experimental conditions.
Anatoly
From: ifeffit-boun...@millenia.cars.aps.anl.gov on behalf of Yu-Ting Liu
Sent: Sun 5/15/2011 1:45 PM
To: ifeffit@millenia.cars.aps.anl.gov
Subject: [Ifeffit] Athena (Question about LC fitting)
Hi,
Related to this issue, I have a question about whether alignment is
necessary for LCF when the oxidation state among sample and standards
is different. I used to align spectra of samples and standards before
LCF while handling P. However, I got confused about Se. For Se
speciation, I prepared six Se species with oxidation state of -2
(FeSe), 0 (elemental Se), 0 (seleno-cystine), 0 (seleno-methionine), 4
(selenite), and 6 (selenate) to fit samples collected from sediments
and biofilms. I am wondering whether the shift of spectra due to the
alignment would make standards mis-fit spectral features of samples.
Thank you so much for the help!
Yu-Ting Liu
Yu-Ting (Lusia) Liu
Postdoctoral Associate
Duke University
Civil and Environmental Engineering
Durham, NC, 27708-0287
---
Belay,
I am CCing my response to the Ifeffit Mailing List, which is the
appropriate venue for questions about the use fo the software. I
encourage you to subscribe to and use the mailing list.
On Tuesday, May 10, 2011 08:50:10 am you wrote:
Dear Dr. Ravel,
I am doing my PhD in title with identication of Fe-phases in cement
hydrates. I am using Athena for LC fitting. I use EXAFS spectra for the
fittings. My question is before LC fitting of EXAFS spectra , do I need to
calibrate the energy? I have normlized and alligned the spectra very well.
The problem is data calibration, when I calibrate the spectra, the energy
shifts and it destroys the fitting. Can you please give a clue why this
happens or do I make a mistake on the procedure? Thanks so much for your
cooperation.
Alignment puts a group of spectra on the same relative energy scale.
It is often necessary to do an explicit alignment when working at a
beamline with an unstable and unencoded monochromator, as might be
found at some older beamlines.
Calibration intends to put a single spectrum on the correct absolute
energy scale. That is, if you know what point in a spectrum
represents a particular absolute energy, then calibration does as I
said.
LCF requires that the ensemble of data and standards all be aligned --
i.e. put on the same relative energy axis. Once that is done, you can
proceed with LCF analysis.
LCF does not require that the data be calibrated, only that they be
aligned. That is, LCF requires that all the data+standards be on the
same relative energy axis, but does not require that that energy axis
be calibrated. The reason that is so is because LCF fitting is an
abstract, numerical function that treats one data set (the data) as
a linear combination of two or more other data sets (the standards).
No part of that mathematical process requires knowledge of the species
of the absorber atom. Indeed, LCF does not require that the x-axis
represent energy, only that it be the same for all data+standards.
That said, the interpretation of your data might be easier to consider
and easier to present in publcation if your data is calibrated. For
instance, the Xray Data Booklet tells us that copper metal has an edge
at 8979 eV. Calibration is the process of transforming your data such
that its edge does, in fact, happen at 8979 eV. In Athena, using the
calibration dialog both does an energy shift and sets the E0 value to
8979.
If you want to calibrate your data as well as aligning your data, I
find it less confusing to do the calibration first. I believe that
the procedures can be commutative, but calibrating after aligning