[Ifeffit] Transmission EXAFS sample

[Scott Calvin]

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
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