Use the scan.
And yes, don't fry your crystal doing it. The best way to do this is put
the fluorescence detector as close to the sample as you can and then
optimize the count rate by attenuating the beam. However, some beamlines
are set up to hit your crystal with full beam and then back off the
fluorescence detector to the other side of the room where it can bask in
the warm sunny glow of your precious crystal. On beamlines like this,
you must use a sacrificial crystal to do your absorption scan.
Obviously, I am not a fan of the latter method, but it really does
depend on what beamline you are going to use. Ask your beamline
scientist about this!
Note: technically, a "MAD scan" is not a fluorescence scan, it is an
absorption scan. You are measuring absorption and using fluorescence as
a tally. A fluorescence scan would have the fluorescent photon energy on
the x-axis.
Unfortunately, methods of doing absorption scans are not the only thing
that differs from beamline to beamline and wavelength calibration is
notoriously difficult to do with an accuracy of five decimal places (1
eV in 12 keV). Personally, I calibrate on copper metal, since it is very
stable and well understood, and the absolute Bragg angle I see from
silicon powder gives me a consistent wavelength. Some beamlines
calibrate on SeMet, but the problem with SeMet is that the actual energy
of the "edge" is NOT the canonical "Se edge" found in the literature (it
is about 1 eV higher). SeMet is also relatively toxic, expensive and
tends to oxidize, shifting the peak. Probably the best example of the
shifting edge of Se I know of is figure 1 in Sarret et. al. (2005)
Applied and Environmental Microbiology, 71(5), 2331–2337. This paper
contains spectra for nine Se-containing reference compound, a
substantial fraction of the total that are known.
Because of all this, I have been pushing the use of dandruff shampoo as
an internationally recognized, inexpensive and particularly rad-hard
selenium reference. Selsun Blue(R) (and the "extra strength" variety of
Head & Shoulders(R)) is 1% selenium sulfide, and gives a nice clear
spectrum with a sharp white line. What remains is to calibrate this
stuff against some NIST-traceable Standard Reference Material.
So, my advice is to ask your beamline scientist, and don't forget to
pack shampoo.
-James Holton
MAD Scientist
Jerry McCully wrote:
Dear All:
Next week we are going to try some seleno-Met labeled crystals.
We checked the literature to try to find out the peak wavelength that
has been used for SAD or MAD data collection. But they are slightly
different ( may be 50 ev) in different papers.
I guess this is due to the discrepancy between the fluorescence
scanning and the theoretical vaules of f' and f''.
When we collect the data, which wavelength should we use? Should we
trust the scanning results?
Thanks a lot for your comments.
All the best,
Jerry McCully
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