[Ifeffit] XAFS detection limit

Sun Feb 9 16:43:10 CST 2020

You might also take a look at Steve Heald's article in JSR:

https://dx.doi.org/10.1107%2FS1600577515001320

"Strategies and limitations for fluorescence detection of XAFS at high 
flux beamlines"

-R.

On 2020-02-09 8:00 a.m., Christian Wittee Lopes wrote:
> Hi Chris and Matt,
>
> Thank you for the complete information, this will help a lot. It is 
> always good to have such experienced people contributing to someone 
> else's growth.
>
> Kind regards,
>
> Christian
>
> Em dom., 9 de fev. de 2020 às 11:50, Matt Newville 
> <newville at cars.uchicago.edu <mailto:newville at cars.uchicago.edu>> escreveu:
>
>     Hi Christian,
>
>     On Thu, Feb 6, 2020, 8:40 PM Christian Wittee Lopes
>     <chriswittee at gmail.com <mailto: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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>
>
> -- 
> *Christian Wittee Lopes*
>
> /Postdoctoral Researcher/
> Institute of Physics, Universidade Federal do Rio Grande do Sul (UFRGS)
> Phone: +55 54 992430264
>
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