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 < newvi...@cars.uchicago.edu> escreveu: > Hi Christian, > > On Thu, Feb 6, 2020, 8:40 PM Christian Wittee Lopes <chriswit...@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 > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit > -- *Christian Wittee Lopes* *Postdoctoral Researcher* Institute of Physics, Universidade Federal do Rio Grande do Sul (UFRGS) Phone: +55 54 992430264
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit