BL4-2 Contact |
Data Processing Basics
The first step in data processing is to normalize the scattering curves to the incident beam intensity integrated over the x-ray exposure time. This will correct for small flux fluctuations during the measurements.
2. Sample transmission correction
A sample cell containing a protein solution typically transmits less than 50% of the incident photons due to absorption by water and the window materials. The transmission of a typical protein solution up to about a few percent (w/v) is practically the same as that of the corresponding solvent solution in the same sample cell. As long as the same cell is used it is not necessary to correct for sample transmission. The transmission measurement requires both the incident beam flux monitor and another flux monitor immediately down stream of the sample. We routinely use a beam stop in which an x-ray photodiode is incorporated for beam transmission measurements. However, it is usually better to remove the second flux monitor for the scattering measurements in order to eliminate the air gap which will reduce the background scattering.
3. Dead time correction
The x-ray scattering data is multiplied by a coefficient which depends on the total count rate in case of gas chamber detectors. Accurate correction to ~100,000 cps is typical.
As discussed above, a series of scattering curves is usually recorded to check for time-dependent changes (notably the potential radiation-induced aggregation) in static measurements. Having confirmed the absence of time-dependent changes, one normally averages the series of scattering curves in order to improve the data statistics. Error propagation must be taken into account.
5. Background subtraction
A solvent scattering curve should be processed in the same way as the protein scattering curves, then subtracted channel-by-channel from the corresponding protein scattering curves.
6. Converting detector channel number to Q(h) or s
The detector channel or pixel number must be converted to something physically meaningful for data interpretation. Normally two different quantities are used:
Q = h = 4psinq/l, where 2q is a scattering angle and l is the wavelength either in Å or nm.
s = 2sinq/l. This quantity is equivalent to “inverse D spacing” or “resolution” in crystallography.
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