Choosing wavelengths for MAD data collection
Inspect your EXAFS scan
Once you have determined the actual scattering factors f' and f''
in the energy range of interest for your sample crystal
(see theory vs experiment),
you need to choose exactly which x-ray wavelengths to use for
MAD data collection.
Schematic of experimental values for f' and f'' as a function of
X-ray energy in keV = 12.398/ in Å
How many wavelengths to use?
For a single type of anomalous scattering atom, i.e. a MAD experiment
at a single absorption edge, you need a minimum of 2 wavelengths
(see MAD phasing tutorial). It is better to
be have more data points so that the set of simultaneous equations
for MAD phasing is over-determined, so 3 wavelengths are better than 2.
Beyond that it is a question of diminishing returns; 4 is better than 3,
but may not be worth the extra data collection time for the slight
What should they be?
- The largest signal will come from choosing the wavelength with
in the figure above).
- The second wavelength is usually chosen to have maximal |f'|
in the figure above). Note that
very close together, requiring great precision in setting up the
apparatus which controls wavelength during data collection.
- Additional wavelengths
chosen at points remote from the absorption edge. The available
signal increasing slowly as the distance from the first two
wavelengths increases. However the diffraction conditions
(crystal absorption and diffracting power, diffraction geometry, etc)
become more disparate as the distance increases. The choice usually
comes down to the practical limitations imposed by the particular
beamline apparatus being used. Typically
between 100eV and 1000eV from the absorption edge.
X-ray Anomalous Scattering
MAD phasing tutorial
Ethan A Merritt ©1996-2001/ firstname.lastname@example.org /
Biomolecular Structure Center at UW