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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