[Ifeffit] P K-edge EXAFS

[matthew marcus]

It's not impossible, but difficult for a couple of reasons:
1.	Not too many beamlines can do it.  It's hard for soft X-ray beamlines 
and soft for hard X-ray beamlines.  Air absorption is significant.  If a 
Si DCM is used, the Bragg angle has to be higher than many 
monochromators can reach.

2.	The spectrum typically features a tall white line, which is very 
subject to overabsorption in concentrated samples.  The small absorption 
depths make it hard to do transmission without running into 
thickness/hole effects.  Harmonic contamination can be an issue.

3.	If you work in fluorescence, you have the low fluorescence yield to 
contend with, as well as absorption in a detector window of typical 
thickness.  The energy resolution of a typical SDD is insufficient to 
cleanly separate the fluorescence from elastic.

That's not to say that it can't be done.  It has.  It's just a bit more 
difficult than, for instance, Fe.

As to substituting As for P, that seems dangerous.  For one thing, the 
atoms are different sizes.  You may recall that sometime back there was 
a flap over some bacteria that supposedly had been transitioned from 
using P to using As for life functions.  This turned out to be bogus.  I 
gather that ATAs doesn't work the same as ATP, for instance.  It's not 
an isomorphous replacement.
	mam

On 9/13/2023 12:03 PM, Marcelo Eduardo Alves wrote:
> Hi there,
> 
> I begin by examining phosphorus adsorption by goethite, hematite, and 
> gibbsite.
> 
> Why is it difficult, if not impossible, to obtain EXAFS spectra at the 
> P-K edge that can be properly used, i.e., spectra with a high 
> signal-to-noise ratio, especially for dilute samples?
> 
> Would it be possible to use arsenic as a phosphorus proxy to get 
> conclusions from As K-edge EXAFS data that could be extended to P?
> 
> Thank you.
> 
> 
> 
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