Fluoresence data are collected to identify elements of interest in the sample. Typically those are elements which can be used for phasing but they also include elements which may be bound to the protein as a complex.
Collecting an X-ray fluorescence spectrum (MCA) gives a broad indication of what is in the sample, while an edge scan provides a detailed view of single element in the sample. An MCA spectrum will contain peaks at specific energies that correlate to individual elements in the sample. Elements are excited by the incident X-ray beam and the emitted light is measured. For this to work, the incident X-ray beam must be at least 1500 eV higher than the emission energy of the elements of interest.
To collect an edge scan:
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