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