Questions on EXAFS
I have a couple of questions on the use of EXAFS: 1. How sensitive is EXAFS/XANES in determining ~ couple of ppm (up to 100ppm) of halogen content in solid content? 2. Also, are there lab-scale EXAFS/XANES equipments available towards this? Anyone working on lab-scale equipments? Thanks Raja
Raja,
On Sun, Mar 16, 2008 at 3:07 PM, Rajasekaran S
I have a couple of questions on the use of EXAFS:
1. How sensitive is EXAFS/XANES in determining ~ couple of ppm (up to 100ppm) of halogen content in solid content?
It depends, somewhat on what you mean. Probably the simplest answer is: EXAFS/XANES is terrible at telling the halogen content in "solid content". EXAFS/XANES studies one selected element, ignoring those that are not directly bound to the element selected. That it, it would be pretty much useless at telling a Cl/Br ratio. You may want to look into X-ray Fluorescence analysis, which can non-destructively determine elemental abundances. On the other hand, EXAFS/XANES can successfully measure the bonding of Cl and Br when they are at 100ppm or so (perhaps less) in solid content. In my experience, EXAFS/XANES on elements at concentrations of a few ppm is very challenging, though it does depend what the rest of the system is.
2. Also, are there lab-scale EXAFS/XANES equipments available towards this? Anyone working on lab-scale equipments?
There are lab-based EXAFS/XANES systems. I believe Rigaku sells machines for laboratory EXAFS. They may be the only company to do so. Hope that helps, --Matt
Greetings all, I am an APS user and a graduate of XAFS Summer School - class of 2007 :) We have been doing EXAFS analysis on Ge[2]Sb[2]Te[x] thin films. I noticed that at the Ge edge in particular, the k-weight value used in the background removal cannot equal 3, or else the background function no longer follows the data and oscillates wildly and/or it shoots up at the edge energy dominating the plot window. Values of 1 and 2 work fine. Even more puzzling is that this problem is inherent for the Ge edge of data taken over numerous runs, and for various samples (GST-224, GST-225, 226, 227). Now if it is that the Ge-edge data is noisier than our Sb and Te edge data at higher k (I don't think this has been true), then in my opinion we should notice this at the Sb and Te edge data. So why the Ge-edge data? I want to point out that there are options that we have used, namely simply using a k-weight of 2, or specifying a spline clamp at high k that is "slight" or "weak". I just would like to know what lies at the root of this problem. If further information is required, I would be happy to send you one of my project files showing some original scans collected for the Ge edge, as well as the merged Ge edge file. Thank you very much, -- Joseph Washington Research Assistant Department of Physics North Carolina State University 431 Riddick Hall Raleigh, NC 27695-8202 email: jswashin@ncsu.edu
Hi Joseph: On Mon, 7 Jul 2008, Joseph Washington wrote:
Greetings all, I am an APS user and a graduate of XAFS Summer School - class of 2007 :) We have been doing EXAFS analysis on Ge[2]Sb[2]Te[x] thin films. I noticed that at the Ge edge in particular, the k-weight value used in the background removal cannot equal 3, or else the background function no longer follows the data and oscillates wildly and/or it shoots up at the edge energy dominating the plot window. Values of 1 and 2 work fine. Even more puzzling is that this problem is inherent for the Ge edge of data taken over numerous runs, and for various samples (GST-224, GST-225, 226, 227).
COuld you tell us why you would like to use a k-weight of 3 in the background removal process? I find that 1 (or 2 sometimes) is sufficient to get a good result. I just checked on a recent data set and I agree that k-weight 1 nd 2 give nearly identical results but that k-weight 3 for the same parameters is slightly different. It would help a lot if you oculd send a project file for us to look at. Carlo -- Carlo U. Segre -- Professor of Physics Associate Dean for Special Projects, Graduate College Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://www.iit.edu/~segre segre@debian.org
Hi Carlo, Since much of our data is located at high k values, it would be an improvement to be using a high k weight in the background removal. Whether or not this will be a vast improvement might be questionable. My main concern is why this occurs for all our Ge edge data across the board, and if this is indicative of an underlying issue in data collection at the beamline or preprocessing. Please see an attached project file containing some scans and a merge. I appreciate the assistance, Joseph Carlo Segre wrote:
Hi Joseph:
On Mon, 7 Jul 2008, Joseph Washington wrote:
Greetings all, I am an APS user and a graduate of XAFS Summer School - class of 2007 :) We have been doing EXAFS analysis on Ge[2]Sb[2]Te[x] thin films. I noticed that at the Ge edge in particular, the k-weight value used in the background removal cannot equal 3, or else the background function no longer follows the data and oscillates wildly and/or it shoots up at the edge energy dominating the plot window. Values of 1 and 2 work fine. Even more puzzling is that this problem is inherent for the Ge edge of data taken over numerous runs, and for various samples (GST-224, GST-225, 226, 227).
COuld you tell us why you would like to use a k-weight of 3 in the background removal process? I find that 1 (or 2 sometimes) is sufficient to get a good result. I just checked on a recent data set and I agree that k-weight 1 nd 2 give nearly identical results but that k-weight 3 for the same parameters is slightly different.
It would help a lot if you oculd send a project file for us to look at.
Carlo
-- Joseph Washington Research Assistant Department of Physics North Carolina State University 431 Riddick Hall Raleigh, NC 27695-8202 email: jswashin@ncsu.edu
Hi Joseph, I have a book chapter that explains how the different k-weighting affect the chi(k) data produced in the background removal. I'll send it to you directly, so as not to fill up everyone's mail box. Here is the reference for others that might be interested. Kelly, S.D., Hesterberg, D., and Ravel, B., in "Analysis of Soils and Minerals Using X-ray Absorption Spectroscopy", (A.L. Ulery and L.R. Drees, Eds.) p. 367. Soil Science Society of America, Madison, WI, USA, 2008. Shelly -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Joseph Washington Sent: Tuesday, July 08, 2008 6:03 AM To: ifeffit@millenia.cars.aps.anl.gov Subject: Re: [Ifeffit] Athena: k-weight=3 in background removal problem Hi Carlo, Since much of our data is located at high k values, it would be an improvement to be using a high k weight in the background removal. Whether or not this will be a vast improvement might be questionable. My main concern is why this occurs for all our Ge edge data across the board, and if this is indicative of an underlying issue in data collection at the beamline or preprocessing. Please see an attached project file containing some scans and a merge. I appreciate the assistance, Joseph Carlo Segre wrote:
Hi Joseph:
On Mon, 7 Jul 2008, Joseph Washington wrote:
Greetings all, I am an APS user and a graduate of XAFS Summer School - class of 2007
We have been doing EXAFS analysis on Ge[2]Sb[2]Te[x] thin films. I noticed that at the Ge edge in particular, the k-weight value used in the background removal cannot equal 3, or else the background function no longer follows the data and oscillates wildly and/or it shoots up at the edge energy dominating the plot window. Values of 1 and 2 work fine. Even more puzzling is that this problem is inherent for the Ge edge of data taken over numerous runs, and for various samples (GST-224, GST-225, 226, 227).
COuld you tell us why you would like to use a k-weight of 3 in the background removal process? I find that 1 (or 2 sometimes) is sufficient to get a good result. I just checked on a recent data set and I agree that k-weight 1 nd 2 give nearly identical results but
:) that
k-weight 3 for the same parameters is slightly different.
It would help a lot if you oculd send a project file for us to look at.
Carlo
-- Joseph Washington Research Assistant Department of Physics North Carolina State University 431 Riddick Hall Raleigh, NC 27695-8202 email: jswashin@ncsu.edu
Hi Joseph, The Fourier transform parameters for the background subtraction do not need to be the same for the analysis of chi(k). In fact, they really shouldn't be the same -- the goals for these FTs are very different., as the FT for background subtraction is only to determine a good background function and to identify the oscillations in chi(k), not to analyze them. Setting the k-weighting to 3 for the background subtraction effectively tells the program to not care about the background at low k. This can cause wild oscillations in the background at both low- and high-k, especially when using clamps of any kind. For background subtraction, the k-weight should be 1, though using 2 and sometimes 0 also work well in many situations. Furthermore kmin should be very close to 0 (1 Ang^-1, at the most, though there may be special situations), which is usually too low for most analysis of chi(k). Hope that helps, --Matt
participants (5)
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Carlo Segre -
Joseph Washington -
Kelly, Shelly D. -
Matt Newville -
Rajasekaran S