Transmission EXAFS sample
Dear all, I have very basic question about the sample preparation for EXAFS. It is well understood and proven by several researchers that the optimum amount of sample (per unit area) required for EXAFS is determined by the fact that the total absorbtion of the entire sample above the absorption edge (of interest) should be between 2 to 2.5, more precisely, it should be 2.3. The reason which forces me to put this question to a mailing list is that, i treat my sample through a process which yields only few percentage of the total amount of sample required for EXAFS. I would prefer to measure EXAFS in transmission mode with samples prepared on several layers of Kapton tape which are bound together to ensure pinhole free sample. My questions are : -- Can my sample be only few percentage of the "actual amount" (i.e. calculated based on above fact) required, and still i can perform transmission EXAFS ? How would this affect my data ? (I guess, it will be heavily dominated by noise) -- What if, i have required amount of sample but since material's density is so high that it yields only small volume of powder (for a given weight), that it can not be covered up on multiple layers of Kapton tape to ensure pinhole-free sample ? I look forward to any comments or suggestions as this would help me improvise the quality of my data. With best regards, Jatin -- Jatinkumar Rana Helmholtz-Zentrum Berlin für Materialien und Energie GmbH Institut für angewandte Materialforschung FI-1 Mikrostruktur Hahn-Meitner-Platz 1 D-14109 Berlin Germany Tel: +49 30 8062-43217 Fax: +49 30 8062-43059 eMail: jatinkumar.rana@helmholtz-berlin.de **************************************************************************************** Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V. Vorsitzender des Aufsichtsrats: Prof. Dr. Dr. h.c. mult. Joachim Treusch Stellvertretende Vorsitzende: Dr. Beatrix Vierkorn-Rudolph Geschäftsführer: Prof. Dr. Anke Rita Kaysser-Pyzalla, Prof. Dr. Dr. h.c. Wolfgang Eberhardt, Dr. Ulrich Breuer Sitz der Gesellschaft: Berlin Handelsregister: AG Charlottenburg, 89 HRB 5583
Hi Jatin from my modest experience the problem comes with the noise from detectors (typically from ionisation chambers- before and after sample). When the sample is too thick than signal/noise ratio becomes smaller, that means you are limited in k space. I have measured liquid samples with edge jump ~0.1 (in ln scale) and this allowed me to observe changes in the first coordination shell, but nothing more. If you measure you can also try to rotate sample 45 deg. answering for your second question - usually I mix sample with cellulose, or with other powders and make a pellet. with such small signal you can also think about TFY (e.g. PIPS diode) - it gives sometimes in such situation better signal/noise ratio cheers kicaj W dniu 10-11-19 10:55, Jatinkumar Rana pisze:
Dear all,
I have very basic question about the sample preparation for EXAFS. It is well understood and proven by several researchers that the optimum amount of sample (per unit area) required for EXAFS is determined by the fact that the total absorbtion of the entire sample above the absorption edge (of interest) should be between 2 to 2.5, more precisely, it should be 2.3.
The reason which forces me to put this question to a mailing list is that, i treat my sample through a process which yields only few percentage of the total amount of sample required for EXAFS. I would prefer to measure EXAFS in transmission mode with samples prepared on several layers of Kapton tape which are bound together to ensure pinhole free sample.
My questions are :
-- Can my sample be only few percentage of the "actual amount" (i.e. calculated based on above fact) required, and still i can perform transmission EXAFS ? How would this affect my data ? (I guess, it will be heavily dominated by noise)
-- What if, i have required amount of sample but since material's density is so high that it yields only small volume of powder (for a given weight), that it can not be covered up on multiple layers of Kapton tape to ensure pinhole-free sample ?
I look forward to any comments or suggestions as this would help me improvise the quality of my data.
With best regards, Jatin
Dear Jatin, The idea that the optimum absorption length (mu*t) for transmission experiments is 2.3 assumes that the errors in the measurement are due to counting statistics of the x-rays. For any synchrotron experiment, the number of x-rays in the transmission chamber is high enough that the noise from counting statistics is rarely significant. This means that using a value of 2.3 is really not that important. The more important issues are a) having a uniform sample. b) not having (mu*t) so high that higher-order harmonics dominate the transmission measurement. For transmission measurements, it's difficult to overstate the importance of a uniform sample. For an ideal thickness, I would say that the better rules of thumb than mu*t = 2.3 are to aim for an edge step of 0.1 to 1.0, and a total absorption less than 3.0. If you only have enough material for an edge step as low as 0.02 (as you imply), then measuring in fluorescence or electron emission is probably a better choice. Such a sample won't be severely affected by "self-absorption" (or "over absorption" to use the term this mailing list prefers) in the fluorescence measurement. I would recommend simultaneously measuring transmission and florescence for such a sample. My concern about a very thin sample is uniformity. Specifically, is the grain size really well below mu/0.02 so that a collection of particles can give a uniform thickness? Since you didn't give any details of the system, it's hard to guess. Is it feasible to pack that material into a smaller area so that the thickness is increased and use a smaller x-ray beam?
-- Can my sample be only few percentage of the "actual amount" (i.e. calculated based on above fact) required, and still i can perform transmission EXAFS ? How would this affect my data ? (I guess, it will be heavily dominated by noise)
I would guess that a sample with mu*t of 0.02 would be dominated by pinholes.
-- What if, i have required amount of sample but since material's density is so high that it yields only small volume of powder (for a given weight), that it can not be covered up on multiple layers of Kapton tape to ensure pinhole-free sample ?
If you cannot get the grain size small enough to have many overlapping grains in the sample, the sample won't be uniform enough for good transmission data. The techniques of using multiple layers of mixing with a low-Z binder don't solve this problem. These do help to make a uniform collection of overlapping grains, but don't make the grains smaller. I would recommend trying to increase the thickness at the expense of cross-sectional area, and/or measuring in both transmission and fluorescence. Hope that helps, --Matt
participants (3)
-
"Dr. Dariusz A. Zając" -
Jatinkumar Rana -
Matt Newville