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ISa: Difference between revisions
→An estimate for the overall quality of an experimental setup
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As you can see from the formula, low values of a and b are ''good'' in the sense that a high upper limit of I/sigma(I) results. If e.g. the crystal is badly split or broken, or reflections are too close on the detector, or the data reduction is not good (wrong parameters), then the values of a and b are elevated. | As you can see from the formula, low values of a and b are ''good'' in the sense that a high upper limit of I/sigma(I) results. If e.g. the crystal is badly split or broken, or reflections are too close on the detector, or the data reduction is not good (wrong parameters), then the values of a and b are elevated. | ||
If your crystal is good (and no matter ''how'' good your crystal is!), then a and b will reflect the quality of the other components of the experimental setup (e.g. beamline stability). I have seen values around 20 for good crystals that still allowed my to solve a MAD structure, but that required high multiplicity of observations. On the bright side, I have also seen a value of 87.6 for Z. Dauter's 0.98A Proteinase K (2ID8) sulfur-SAD data from J. Holton's APS/22-ID beamline. | If your crystal is good (and no matter ''how'' good your crystal is!), then a and b will reflect the quality of the other components of the experimental setup (e.g. beamline stability). I have seen values around 20 for good crystals that still allowed my to solve a MAD structure, but that required high multiplicity of observations. Values around 30 allowed me to solve a sulfur-SAD structure at medium resolution (diffraction to 2.3 A, anomalous signal to 3 A). On the bright side, I have also seen a value of 87.6 for Z. Dauter's 0.98A Proteinase K (2ID8) sulfur-SAD data from J. Holton's APS/22-ID beamline. | ||