[Ifeffit] Transmission EXAFS sample

Mon Nov 22 09:08:40 CST 2010

On 21.11.2010 19:00, ifeffit-request at millenia.cars.aps.anl.gov wrote:
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> Today's Topics:
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>     1. Re: Transmission EXAFS sample (Jatinkumar Rana)
>     2. Re: Transmission EXAFS sample (Scott Calvin)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Sun, 21 Nov 2010 11:44:25 +0100
> From: Jatinkumar Rana<jatinkumar.rana at helmholtz-berlin.de>
> To:ifeffit at millenia.cars.aps.anl.gov
> Subject: Re: [Ifeffit] Transmission EXAFS sample
> Message-ID:<4CE8F809.3040208 at helmholtz-berlin.de>
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> On 20.11.2010 19:00,ifeffit-request at millenia.cars.aps.anl.gov  wrote:
>    
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>> or, via email, send a message with subject or body 'help' to
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>> 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>
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>>
>> 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";
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>>
>> 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
>>
>>
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>> End of Ifeffit Digest, Vol 93, Issue 15
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>>
>>      
> 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 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

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
Institut für angewandte Materialforschung
FI-1 Mikrostruktur

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Tel: +49 30 8062-43217
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