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. [figure]

Schematic of experimental values for f' and f'' as a function of x-ray energy

X-ray energy in keV = 12.398/lambda 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 improvement.

What should they be?

back to X-ray Anomalous Scattering MAD phasing tutorial
Ethan A Merritt ©1996-2001/ / Biomolecular Structure Center at UW