[Ifeffit] XAFS detection limit
newville at cars.uchicago.edu
Sun Feb 9 08:48:56 CST 2020
On Thu, Feb 6, 2020, 8:40 PM Christian Wittee Lopes <chriswittee at gmail.com>
> Dear all,
> Recently I was questioned about the EXAFS detection limit when describing
> different metal species in a bimetallic sample.
> By checking Pd and Cu K-edges, for example, I found Pd metal nanoparticles
> and CuO clusters, respectively. But additional techniques tell me I can
> have copper atoms in intimate contact with the Pd nanoparticles. What would
> be the minimum amount of these "single atoms" needed to be detected by
> EXAFS? is there a detection limit or it depends on several parameters?
As Chris Chantler says, there are a lot of things that can influence this,
so there really isn't one simple answer. Also, as Chris says, advances in
analytic methods have been (mostly) been improving the situation.
At my beamline, we often get asked questions about detection limits.
We're typically working in a different context than
nanoparticles/catalysts, but I think the basic ideas are about the same.
A good starting rule-of-thumb for absolute detection limits is 1 ppm by
atomic weight. You might be able to do better sometimes, but there are
situations where XANES at 10 ppm is very hard. For sure, a matrix of
light elements is much better than a matrix of heavy elements.
For very dilute samples, one will be using fluorescence XAFS measurements
with a solid-state detector or know very well why you are doing something
different. These solid-state detectors and electronics are fundamentally
limited to have energy resolutions of ~120 eV (often 250 eV) and maximum
total count rates of 5 MHz (often 0.5 MHz). Many beamlines use "a
handful" (2 to 16) parallel detectors, and some have up to 100 (but often
with each having a lower individual maximum count rate, and perhaps
less-than-ideal energy resolution).
With a count rate of a few MHz total and a sample with 1ppm of "element of
interest", the elastic and Compton scattering and/or fluorescence from
other elements will dominate that total count rate and the energy
resolution will give non-zero background in the fluorescence spectrum.
That means that even seeing a peak from 1 ppm of an element in an X-ray
fluorescence spectrum with a solid-state detector is challenging. Not
impossible, but definitely not routine.
For sure, adding more detectors or counting for a long time can help. But
those are linear in time and the number of detectors (and no beamline has
1000 parallel detectors). Low Z matrices like water, biological material,
carbon-rich materials are easier. Samples with nearby or overlapping
fluorescence lines are much harder. That is 10 ppm Zn in water: yes, 1
ppm Zn in water: maybe, 10 ppm Zn in CaCO3: maybe, 100 ppm Zn in Cu metal:
no. For sure, XANES at 1ppm is sometimes possible. Getting interpretable
XAFS would take a lot longer, perhaps days of counting.
Using filters and/or Bent Laue Analyzers in front of a solid-state (or
integrating) detector can sometimes help to eliminate the unwanted scatter
signals before they get to the solid-state detector. Using crystal
analyzers ("wavelength" vs "energy" dispersive fluorescence) can help -
they have lower backgrounds and are not limited by the total scatter - but
the solid angle for these tend to be small. Using crystal analyzer arrays
are probably really needed to get the best detection limits. A few
beamlines do regularly do HERFD analysis with arrays of crystal analyzers,
and many of the rest of us are trying to catch up. Still, I believe that
"1 ppm" is around the state of the art, if not "heroic".
All of that is for the detection limit of an atomic species. If you are
asking about detecting Cu in/on/with Pd nanoparticles with CuO also
present, the answer is far worse. Cu XAS measurements will be an average
of all Cu atoms in the illuminated volume -- you cannot avoid the CuO.
Seeing that 1% of the Cu atoms are bound to Pd and not to oxygen would be
Hope that helps,
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