[Ifeffit] Transmission EXAFS sample

Jatinkumar Rana jatinkumar.rana at helmholtz-berlin.de
Sun Nov 21 04:44:25 CST 2010


On 20.11.2010 19:00, ifeffit-request at millenia.cars.aps.anl.gov wrote:
> Send Ifeffit mailing list submissions to
> 	ifeffit at 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-request at millenia.cars.aps.anl.gov
>
> You can reach the person managing the list at
> 	ifeffit-owner at 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<dr.scott.calvin at gmail.com>
> To: XAFS Analysis using Ifeffit<ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] Transmission EXAFS sample
> Message-ID:<DEC4127B-B536-4A85-BED3-2799D5B596C4 at gmail.edu>
> 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 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 at millenia.cars.aps.anl.gov
>> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>>      
>
>
> ------------------------------
>
> Message: 2
> Date: Fri, 19 Nov 2010 23:21:43 -0500
> From: "Frenkel, Anatoly"<frenkel at bnl.gov>
> To: "XAFS Analysis using Ifeffit"<ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] Transmission EXAFS sample
> Message-ID:<69bc35f1-e92c-426c-8786-e2a5c881fae8 at blur>
> Content-Type: text/plain; charset="us-ascii"; Format="Flowed";
> 	DelSp="Yes"
>
> 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<dr.scott.calvin at gmail.com>
> To: XAFS Analysis using Ifeffit<ifeffit at millenia.cars.aps.anl.gov>
> Sent: Fri, Nov 19, 2010 18:30:37 GMT+00:00
> Subject: Re: [Ifeffit] Transmission EXAFS sample
>
>
> -------------- next part --------------
> An HTML attachment was scrubbed...
> URL:<http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20101119/67dc873b/attachment.html>
>
> ------------------------------
>
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>
>
> End of Ifeffit Digest, Vol 93, Issue 15
> ***************************************
>
>
>    
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

-- 

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 at 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





More information about the Ifeffit mailing list