Differences between fluorescence and transmission of the same sample
Dr. Ravel, Thanks so much for the link and the advice! I appreciate it greatly. I will advise my lab mates as such and may have to only use the fluorescence data if that is indeed the case. I also included the file where the lower temperature is included and you might come to the same conclusion. The samples that I ran were pre-sieved, and the ones included in the .prj file aren't so that should be pursued prior to running the experiment. If they crush the particle and sieve the sample, I think that we can be more certain that this is not the case. This was very helpful! Andrew
What can be done in this case, is not to use either flourescence or transmission data, - because both are bad by the reasons Bruce explained - but use corrections, if you cannot change your experimental geometry (different concentration of the sample or different thickness or different reactor cell). What we do in these cases, if we must keep original thick sample for fluorescence measurements, we make the same sample prepared for transmission on a tape or as a pellet, that gives proper thickness to be free of any leakage or thickness effects that reduce oscillation intensity or introduce noise due to poor statistics. Then we compare EXAFS oscillations plotted together for the two samples: one measured in fluorescence in the cell you want to use in your in situ experiment, and the other meausred in tranmission in ideal conditions. The former will have reduced intensity. You can manually find scaling factor that is needed to match two intensities. Based on many early papers (e.g., Kim, Stern and Heald, Physical Review B, Thickness effect in EXAFS data or something like that), this scaling factor is approximately constant. Once you found it, then you can take all your in situ data in flourescence and later on, during data processing and analysis, correct all the EXAFS data by scaling them up with the same factor. Don't change the sample geometry midway, and do not apply similar correction to recover XANES. Other strategies should be used for XANES corrections. Anatoly ________________________________ From: ifeffit-bounces@millenia.cars.aps.anl.gov on behalf of Andrew Campos Sent: Wed 5/5/2010 11:24 AM To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] Differences between fluorescence and transmission of thesame sample Dr. Ravel, Thanks so much for the link and the advice! I appreciate it greatly. I will advise my lab mates as such and may have to only use the fluorescence data if that is indeed the case. I also included the file where the lower temperature is included and you might come to the same conclusion. The samples that I ran were pre-sieved, and the ones included in the .prj file aren't so that should be pursued prior to running the experiment. If they crush the particle and sieve the sample, I think that we can be more certain that this is not the case. This was very helpful! Andrew
On Wednesday 05 May 2010 11:24:16 am Andrew Campos wrote:
Thanks so much for the link and the advice! I appreciate it greatly. I will advise my lab mates as such and may have to only use the fluorescence data if that is indeed the case.
I also included the file where the lower temperature is included and you might come to the same conclusion. The samples that I ran were pre-sieved, and the ones included in the .prj file aren't so that should be pursued prior to running the experiment. If they crush the particle and sieve the sample, I think that we can be more certain that this is not the case. This was very helpful!
I would actually come to a different conclusion about your low tempertaure data. Those data look like they suffer from self-absorption (or over-absorption, if you prefer). See http://xafs.org/Experiment/OverAbsorption That is, the sample measured in the low T mesaurement appears to be too concentrated for the fluorescence measurement, however the transmission data in that case looks just fine. As Anatoly said, you could correct the fluorescence data after the fact. Given that you have the corresponding transmission data, this correction can be done accurately by comparison. I am not clear why the low and high temperature data behave differently, but then I am not clear quite what is being measured in each case. As for the high temperature data -- it is hard to stress highly enough the importance of good sample preparation. 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/
Andrew, You have the edge step > 90,000 for fluorescence. You may want to check your data columns and see you have read in proper If/I0 (not ln If/I0 or something else). Regards, Joo ________________________ JOO H. KANG, Ph.D. Analytical Sciences The Dow Chemical Company 1897 Building - Office E77 Midland, MI 48667 USA Phone: (989) 638-3862 Fax: (989) 638-6443 E-mail: jhkang@dow.com -----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Andrew Campos Sent: Wednesday, May 05, 2010 11:24 AM To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] Differences between fluorescence and transmission of thesame sample Dr. Ravel, Thanks so much for the link and the advice! I appreciate it greatly. I will advise my lab mates as such and may have to only use the fluorescence data if that is indeed the case. I also included the file where the lower temperature is included and you might come to the same conclusion. The samples that I ran were pre-sieved, and the ones included in the .prj file aren't so that should be pursued prior to running the experiment. If they crush the particle and sieve the sample, I think that we can be more certain that this is not the case. This was very helpful! Andrew
On Wednesday 05 May 2010 02:25:45 pm Kang, Joo (JH) wrote:
You have the edge step > 90,000 for fluorescence. You may want to check your data columns and see you have read in proper If/I0 (not ln If/I0 or something else).
Use Control-b or Group -> "About current group" to check how the data were imported. 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/
participants (4)
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Andrew Campos -
Bruce Ravel -
Frenkel, Anatoly -
Kang, Joo (JH)