[Ifeffit] XAFS detection limit

Jo Melville melville at mit.edu
Sun Feb 9 13:14:48 CST 2020

This is such a fantastic answer! Thank you for sharing your expertise.

On Sun, Feb 9, 2020, 9:50 AM Matt Newville <newville at cars.uchicago.edu>

> Hi Christian,
> On Thu, Feb 6, 2020, 8:40 PM Christian Wittee Lopes <chriswittee at gmail.com>
> wrote:
>> 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
> very challenging.
> Hope that helps,
> --Matt
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