[Ifeffit] Transmission EXAFS sample

[Matt Newville]

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 <edmund.welter at desy.de> 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
>
>
>
>
>
>
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