Re: [Ifeffit] Transmission EXAFS sample
On 22.11.2010 19:00, ifeffit-requ...@millenia.cars.aps.anl.gov wrote: Send Ifeffit mailing list submissions to ifeffit@millenia.cars.aps.anl.gov To subscribe or unsubscribe via the World Wide Web, visit http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit or, via email, send a message with subject or body 'help' to ifeffit-requ...@millenia.cars.aps.anl.gov You can reach the person managing the list at ifeffit-ow...@millenia.cars.aps.anl.gov When replying, please edit your Subject line so it is more specific than "Re: Contents of Ifeffit digest..." Today's Topics: 1. Re: Transmission EXAFS sample (Scott Calvin) 2. Re: Transmission EXAFS sample (Scott Calvin) 3. maximum number of paths in Artemis (Yuan Ping) -- Message: 1 Date: Mon, 22 Nov 2010 07:13:42 -0800 From: Scott Calvin To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Transmission EXAFS sample Message-ID:<5b5040d9-f501-4ba0-8f24-38630b43b...@gmail.edu> Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes To my mind, when considering sample preparation the important thing is not so much the "right" thickness, as knowing the effects to guard against as the thickness deviates toward the thin or thick side. As transmission samples become thicker, the problem of "unwanted" photons becomes more severe. Those photons may be harmonics, photons scattered into the It detector, or photons from the tails of the resolution curve of the monochromator. As transmission samples become thinner, uniformity becomes more of an issue. If you play with the equations, you'll see that if your sample is a mixture of regions that have a thickness of 1.0 absorption lengths and regions that have a thickness of 2.0 absorption lengths, the spectrum is less distorted than if it is a mixture of 0.5 and 1.0 absorption lengths. So if a sample is on the thick side, it is particularly important to guard against harmonics in the beam and scattered photons. If it is on the thin side, it is particularly important to guard against nonuniformity. To put it another way, problems are synergistic. With a well- conditioned beam, a uniform sample, and linear detectors, the thickness almost doesn't matter (within reason)--at a modern beamline, a total absorption of even 0.05 or 4.0 will work. But as each of those conditions deviates from the ideal, distortions become much more severe. There's an old joke about someone on a diet going in to a fast food joint and asking for a double bacon cheeseburger, a large fries...and a diet Coke. In XAFS measurements, that attitude actually kind of works, because of the synergies I just discussed. Personally, I trust my ability to condition the beam and minimize scattering more than I trust my ability to make a uniform sample, so I lean a little toward the thicker side. --Scott Calvin Faculty at Sarah Lawrence College Currently on sabbatical at Stanford Synchrotron Radiation Laboratory On Nov 22, 2010, at 5:13 AM, Welter, Edmund wrote: Dear Jatin, the optimum mued of 2.x is not just derived by simple photon counting statistics. As Matt pointed out, for transmission measurements at a synchrotron beamline in conventional scanning mode this is seldom a matter. Nevertheless, one should avoid to measure subtle changes of absorption at the extreme ends, that is, transmission near 0 % or 100 %. In optical photometry this is described by the more or less famous "Ringbom plots" which describe the dependency of the accuracy of quantitative analysis by absorption measurements (usually but not necessarily in the UV/Vis) from the total absorption of the sample. This time the number is only near to 42, the optimum transmission is 36.8 % (mue = 1). So, to achieve the highest accuracy in the determination of small Delta c (c = concentration) you should try to measure samples with transmissions near to this value (actually the minimum is broad and transmissions between 0.2 and 0.7 are ok). In our case, we are not interested in the concentration of the absorber, but we are also interested in (very) small changes of the transmission resp. absorption in our samples. Or, using Bouger, Lambert Beer's law, in our case mue (-ln(I1/I0) is a function of the absorption coefficient (mue0). The concentration of the absorber and the thickness (d) of the sample are constant. -ln(I1/I0) = mue0 * c * d But then: If the optimum is a mue between 0.35 and 1.6 why are we all measuring successfully (ok, more or less ;-) using samples having a mue between 2 and 3? ...and 0.35 seems desperately small to me! Maybe sample homogeneity is an issue? Cheers, Edmund Welter -- Message: 2 Date: Mon, 22 Nov 2010 07:16:06 -0800 From: Scott Calvin To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Transmission EXAF
Re: [Ifeffit] Transmission EXAFS sample
Hi Edmund, Thanks for that post... I'd never seen the UV-vis literature on this and didn't know what a Ringbom plot was. A google search led to Ramirez-Munoz 1967 (doi:10.1016/0026-265X(67)90042-2) on Atomic-Absorption Photometry, which shows a very nice result that significant distortions don't really appear until outside 20 to 80% total absorption. The focus seems to be on total absorption, which makes sense as the most important term. But for XAFS, it's not that unusual to have fairly large total absorption due to window and sample cell materials (say, diamonds in a diamond anvil cell), but still have decent data that has an edge step between 0.2 and 1.5.Perhaps that means that counting statistics really never matter for transmission XAFS, and the reason to not go above an edge step of 3 or 4 is spatial inhomogeneity in the sample and beam, and harmonics. --Matt On Mon, Nov 22, 2010 at 7:13 AM, Welter, Edmund wrote: > Dear Jatin, > > the optimum mued of 2.x is not just derived by simple photon counting > statistics. As Matt pointed out, for transmission measurements at a > synchrotron beamline in conventional scanning mode this is seldom a matter. > Nevertheless, one should avoid to measure subtle changes of absorption at > the extreme ends, that is, transmission near 0 % or 100 %. In optical > photometry this is described by the more or less famous "Ringbom plots" > which describe the dependency of the accuracy of quantitative analysis by > absorption measurements (usually but not necessarily in the UV/Vis) from the > total absorption of the sample. > > This time the number is only near to 42, the optimum transmission is 36.8 % > (mue = 1). So, to achieve the highest accuracy in the determination of small > Delta c (c = concentration) you should try to measure samples with > transmissions near to this value (actually the minimum is broad and > transmissions between 0.2 and 0.7 are ok). In our case, we are not > interested in the concentration of the absorber, but we are also interested > in (very) small changes of the transmission resp. absorption in our samples. > Or, using Bouger, Lambert Beer's law, in our case mue (-ln(I1/I0) is a > function of the absorption coefficient (mue0). The concentration of the > absorber and the thickness (d) of the sample are constant. > > -ln(I1/I0) = mue0 * c * d > > But then: If the optimum is a mue between 0.35 and 1.6 why are we all > measuring successfully (ok, more or less ;-) using samples having a mue > between 2 and 3? ...and 0.35 seems desperately small to me! Maybe sample > homogeneity is an issue? > > Cheers, > Edmund Welter > > > > > > > ___ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > > ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] Transmission EXAFS sample
On Nov 22, 2010, at 7:09 AM, Jatinkumar Rana wrote: Hi Scott, Sorry for mixing up the things. For the case, when i have very limited amount of sample that i can not cover 1sq.cm area, you, Matt and others have given very very clear explanation about possible solutions and the probable effects on data quality. I am really very thankful to all of you for sharing your experience and expertise. My last post was with reference to the case when i have enough powders (i.e., reference oxide compounds). It is just to be ensured that i am doing things 100% exactly in a same way it has to be done. With best regards, Jatin -- Jatinkumar Rana Yes, Jatin, the procedure you described is fine. There is no "right" way to make samples, although there are many wrong ways. --Scott Calvin Faculty at Sarah Lawrence College Currently on sabbatical at Stanford Synchrotron Radiation Laboratory___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] Transmission EXAFS sample
To my mind, when considering sample preparation the important thing is not so much the "right" thickness, as knowing the effects to guard against as the thickness deviates toward the thin or thick side. As transmission samples become thicker, the problem of "unwanted" photons becomes more severe. Those photons may be harmonics, photons scattered into the It detector, or photons from the tails of the resolution curve of the monochromator. As transmission samples become thinner, uniformity becomes more of an issue. If you play with the equations, you'll see that if your sample is a mixture of regions that have a thickness of 1.0 absorption lengths and regions that have a thickness of 2.0 absorption lengths, the spectrum is less distorted than if it is a mixture of 0.5 and 1.0 absorption lengths. So if a sample is on the thick side, it is particularly important to guard against harmonics in the beam and scattered photons. If it is on the thin side, it is particularly important to guard against nonuniformity. To put it another way, problems are synergistic. With a well- conditioned beam, a uniform sample, and linear detectors, the thickness almost doesn't matter (within reason)--at a modern beamline, a total absorption of even 0.05 or 4.0 will work. But as each of those conditions deviates from the ideal, distortions become much more severe. There's an old joke about someone on a diet going in to a fast food joint and asking for a double bacon cheeseburger, a large fries...and a diet Coke. In XAFS measurements, that attitude actually kind of works, because of the synergies I just discussed. Personally, I trust my ability to condition the beam and minimize scattering more than I trust my ability to make a uniform sample, so I lean a little toward the thicker side. --Scott Calvin Faculty at Sarah Lawrence College Currently on sabbatical at Stanford Synchrotron Radiation Laboratory On Nov 22, 2010, at 5:13 AM, Welter, Edmund wrote: Dear Jatin, the optimum mued of 2.x is not just derived by simple photon counting statistics. As Matt pointed out, for transmission measurements at a synchrotron beamline in conventional scanning mode this is seldom a matter. Nevertheless, one should avoid to measure subtle changes of absorption at the extreme ends, that is, transmission near 0 % or 100 %. In optical photometry this is described by the more or less famous "Ringbom plots" which describe the dependency of the accuracy of quantitative analysis by absorption measurements (usually but not necessarily in the UV/Vis) from the total absorption of the sample. This time the number is only near to 42, the optimum transmission is 36.8 % (mue = 1). So, to achieve the highest accuracy in the determination of small Delta c (c = concentration) you should try to measure samples with transmissions near to this value (actually the minimum is broad and transmissions between 0.2 and 0.7 are ok). In our case, we are not interested in the concentration of the absorber, but we are also interested in (very) small changes of the transmission resp. absorption in our samples. Or, using Bouger, Lambert Beer's law, in our case mue (-ln(I1/I0) is a function of the absorption coefficient (mue0). The concentration of the absorber and the thickness (d) of the sample are constant. -ln(I1/I0) = mue0 * c * d But then: If the optimum is a mue between 0.35 and 1.6 why are we all measuring successfully (ok, more or less ;-) using samples having a mue between 2 and 3? ...and 0.35 seems desperately small to me! Maybe sample homogeneity is an issue? Cheers, Edmund Welter ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] Transmission EXAFS sample
On 21.11.2010 19:00, ifeffit-requ...@millenia.cars.aps.anl.gov wrote: Send Ifeffit mailing list submissions to ifeffit@millenia.cars.aps.anl.gov To subscribe or unsubscribe via the World Wide Web, visit http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit or, via email, send a message with subject or body 'help' to ifeffit-requ...@millenia.cars.aps.anl.gov You can reach the person managing the list at ifeffit-ow...@millenia.cars.aps.anl.gov When replying, please edit your Subject line so it is more specific than "Re: Contents of Ifeffit digest..." Today's Topics: 1. Re: Transmission EXAFS sample (Jatinkumar Rana) 2. Re: Transmission EXAFS sample (Scott Calvin) -- Message: 1 Date: Sun, 21 Nov 2010 11:44:25 +0100 From: Jatinkumar Rana To:ifeffit@millenia.cars.aps.anl.gov Subject: Re: [Ifeffit] Transmission EXAFS sample Message-ID:<4ce8f809.3040...@helmholtz-berlin.de> Content-Type: text/plain; charset=ISO-8859-1; format=flowed On 20.11.2010 19:00,ifeffit-requ...@millenia.cars.aps.anl.gov wrote: Send Ifeffit mailing list submissions to ifeffit@millenia.cars.aps.anl.gov To subscribe or unsubscribe via the World Wide Web, visit http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit or, via email, send a message with subject or body 'help' to ifeffit-requ...@millenia.cars.aps.anl.gov You can reach the person managing the list at ifeffit-ow...@millenia.cars.aps.anl.gov When replying, please edit your Subject line so it is more specific than "Re: Contents of Ifeffit digest..." Today's Topics: 1. Re: Transmission EXAFS sample (Scott Calvin) 2. Re: Transmission EXAFS sample (Frenkel, Anatoly) -- Message: 1 Date: Fri, 19 Nov 2010 10:30:37 -0800 From: Scott Calvin To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Transmission EXAFS sample Message-ID: Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes Hi Jatin, Matt covered most of what I would say, but I'll add a few comments of my own. I'm not sure how you arrived at the conclusion that you have only a few percent of what you need--you must be assuming a sample area somehow. I have frequently made transmission measurements on samples where I only had a few milligrams available. Generally, I did it by spreading it on a layer of tape as well as I could and then folding the tape over and over again--sometimes to make as many as 16 layers. (Of course, that many layers is not advisable if you're below 6 keV or so, as the absorption of the tape itself would kill the signal). Even if there are lots of pinholes because you can't cover the tape effectively, 16 layers from folding will make them cancel out fairly well. I can then narrow the beam a bit to match the size of my sample. Flux isn't really the issue here, so I don't even need a focussed beamline--I can just narrow the slits. Two other tips: 1) Realize that even with a tiny amount of sample that much of it won't end up on the tape. The process of brushing on tape is designed to separate the small grains from the big ones, with only the small ones ending up on tape. Allow that to happen! 2) You can sometimes get a second piece of tape to have some sample on it by putting it sticky side down on your mortar and peeling it back. A thin layer of dust from the sample will stick to the tape, and give you a little more absorption and a bit more of a uniform distribution. If you stack that with the primary piece of tape and then fold a few times, you may end up in pretty good shape, as long as you're not operating at a low enough energy so that all the layers of tape are a problem.. This procedure doesn't give me the best data I've ever seen, but it's often not bad. --Scott Calvin Sarah Lawrence College On Nov 19, 2010, at 8:13 AM, Matt Newville wrote: 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
Re: [Ifeffit] Transmission EXAFS sample
Dear Jatin, the optimum mued of 2.x is not just derived by simple photon counting statistics. As Matt pointed out, for transmission measurements at a synchrotron beamline in conventional scanning mode this is seldom a matter. Nevertheless, one should avoid to measure subtle changes of absorption at the extreme ends, that is, transmission near 0 % or 100 %. In optical photometry this is described by the more or less famous "Ringbom plots" which describe the dependency of the accuracy of quantitative analysis by absorption measurements (usually but not necessarily in the UV/Vis) from the total absorption of the sample. This time the number is only near to 42, the optimum transmission is 36.8 % (mue = 1). So, to achieve the highest accuracy in the determination of small Delta c (c = concentration) you should try to measure samples with transmissions near to this value (actually the minimum is broad and transmissions between 0.2 and 0.7 are ok). In our case, we are not interested in the concentration of the absorber, but we are also interested in (very) small changes of the transmission resp. absorption in our samples. Or, using Bouger, Lambert Beer's law, in our case mue (-ln(I1/I0) is a function of the absorption coefficient (mue0). The concentration of the absorber and the thickness (d) of the sample are constant. -ln(I1/I0) = mue0 * c * d But then: If the optimum is a mue between 0.35 and 1.6 why are we all measuring successfully (ok, more or less ;-) using samples having a mue between 2 and 3? ...and 0.35 seems desperately small to me! Maybe sample homogeneity is an issue? Cheers, Edmund Welter ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] Transmission EXAFS sample
On Nov 21, 2010, at 2:45 AM, Jatinkumar Rana wrote: Hi Scott, Yes I have assumed the sample cross section area to be 1 sq. cm. and then calculated the amount of sample required for that. What i planned is following : I would calculate the amount of sample required for 1sq.cm area, take that amount of sample and make it very fine paste using mortar and pestle, and then apply it uniformly on a piece of kapton tape. Then fold the tape over and over again in such a way that final bunch of tapes will yield to 1 sq.cm. area containing the required amount of sample. Will it be the right approach ?? OR I can take randomly few milligrams of powder (i.e. not strictly as per calculation) and make a several uniform layers of tape ?? With best regards, Jatin -- Hi Jatin, I'm not sure I understand. If you have enough sample for the 1 square centimeter target, then there shouldn't be a problem, right? I was assuming from your initial question that you weren't going to have enough sample to do that. --Scott Calvin Faculty at Sarah Lawrence College Currently on sabbatical at Stanford Synchrotron Radiation Laboratory___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] Transmission EXAFS sample
On 20.11.2010 19:00, ifeffit-requ...@millenia.cars.aps.anl.gov wrote: Send Ifeffit mailing list submissions to ifeffit@millenia.cars.aps.anl.gov To subscribe or unsubscribe via the World Wide Web, visit http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit or, via email, send a message with subject or body 'help' to ifeffit-requ...@millenia.cars.aps.anl.gov You can reach the person managing the list at ifeffit-ow...@millenia.cars.aps.anl.gov When replying, please edit your Subject line so it is more specific than "Re: Contents of Ifeffit digest..." Today's Topics: 1. Re: Transmission EXAFS sample (Scott Calvin) 2. Re: Transmission EXAFS sample (Frenkel, Anatoly) -- Message: 1 Date: Fri, 19 Nov 2010 10:30:37 -0800 From: Scott Calvin To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Transmission EXAFS sample Message-ID: Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes Hi Jatin, Matt covered most of what I would say, but I'll add a few comments of my own. I'm not sure how you arrived at the conclusion that you have only a few percent of what you need--you must be assuming a sample area somehow. I have frequently made transmission measurements on samples where I only had a few milligrams available. Generally, I did it by spreading it on a layer of tape as well as I could and then folding the tape over and over again--sometimes to make as many as 16 layers. (Of course, that many layers is not advisable if you're below 6 keV or so, as the absorption of the tape itself would kill the signal). Even if there are lots of pinholes because you can't cover the tape effectively, 16 layers from folding will make them cancel out fairly well. I can then narrow the beam a bit to match the size of my sample. Flux isn't really the issue here, so I don't even need a focussed beamline--I can just narrow the slits. Two other tips: 1) Realize that even with a tiny amount of sample that much of it won't end up on the tape. The process of brushing on tape is designed to separate the small grains from the big ones, with only the small ones ending up on tape. Allow that to happen! 2) You can sometimes get a second piece of tape to have some sample on it by putting it sticky side down on your mortar and peeling it back. A thin layer of dust from the sample will stick to the tape, and give you a little more absorption and a bit more of a uniform distribution. If you stack that with the primary piece of tape and then fold a few times, you may end up in pretty good shape, as long as you're not operating at a low enough energy so that all the layers of tape are a problem.. This procedure doesn't give me the best data I've ever seen, but it's often not bad. --Scott Calvin Sarah Lawrence College On Nov 19, 2010, at 8:13 AM, Matt Newville wrote: 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 b
Re: [Ifeffit] Transmission EXAFS sample
That's probably how they discovered graphene, by trying to make exafs sample. Anatoly Sent from my mobile phone, please forgive typos. -Original message- From: Scott Calvin To: XAFS Analysis using Ifeffit Sent: Fri, Nov 19, 2010 18:30:37 GMT+00:00 Subject: Re: [Ifeffit] Transmission EXAFS sample ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] Transmission EXAFS sample
Hi Jatin, Matt covered most of what I would say, but I'll add a few comments of my own. I'm not sure how you arrived at the conclusion that you have only a few percent of what you need--you must be assuming a sample area somehow. I have frequently made transmission measurements on samples where I only had a few milligrams available. Generally, I did it by spreading it on a layer of tape as well as I could and then folding the tape over and over again--sometimes to make as many as 16 layers. (Of course, that many layers is not advisable if you're below 6 keV or so, as the absorption of the tape itself would kill the signal). Even if there are lots of pinholes because you can't cover the tape effectively, 16 layers from folding will make them cancel out fairly well. I can then narrow the beam a bit to match the size of my sample. Flux isn't really the issue here, so I don't even need a focussed beamline--I can just narrow the slits. Two other tips: 1) Realize that even with a tiny amount of sample that much of it won't end up on the tape. The process of brushing on tape is designed to separate the small grains from the big ones, with only the small ones ending up on tape. Allow that to happen! 2) You can sometimes get a second piece of tape to have some sample on it by putting it sticky side down on your mortar and peeling it back. A thin layer of dust from the sample will stick to the tape, and give you a little more absorption and a bit more of a uniform distribution. If you stack that with the primary piece of tape and then fold a few times, you may end up in pretty good shape, as long as you're not operating at a low enough energy so that all the layers of tape are a problem.. This procedure doesn't give me the best data I've ever seen, but it's often not bad. --Scott Calvin Sarah Lawrence College On Nov 19, 2010, at 8:13 AM, Matt Newville wrote: 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 ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] Transmission EXAFS sample
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 ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] Transmission EXAFS sample
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 ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
[Ifeffit] 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.r...@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 ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
