Intensity measurements

Once a suitable crystal has been obtained, a molecular structure investigation requires measurement of the intensities of as many Bragg reflections as possible. In this chapter, some of the options that must be decided by the experimenter will be considered, and some of the criteria used to assess the accuracy and completeness of the data will be presented. The experimenter has to make a number of strategic decisions in collecting the crystal intensity data. These include: • What X-ray source should be used? • What X-ray detector should be used? • Under what conditions should the crystal be maintained? • How long should each crystal be exposed? • What data collection technique will be used? • What resolution limit should be applied? The choice of source and detector will depend largely on what is available, but the major decision is whether to use facilities in the home laboratory or whether to use a synchrotron at a central facility. The energy released by absorption of X-rays in a crystal inevitably damages it. The process of radiation damage increases crystal disorder and reduces the intensity of scattering. The experimenter will ultimately have to abandon data collection from the damaged and disordered crystal. Under ideal experimental conditions, all the useful diffraction data can be obtained from a crystal long before radiation damage takes its toll, and radiation damage does not create a practical problem. At the other end of the scale, it may be necessary to combine the measurements from many crystals in order to obtain a complete set of diffracted intensities. There is no definite criterion to decide when a crystal is so badly damaged that it must be discarded. But if the measurements are going to be of highest quality, any observable change is bad news. The most serious effects occur in the part of the diffraction pattern at the highest observed resolution, where the observed intensities of the Bragg reflections will be altered most rapidly. The first observable effect of radiation damage is usually a reduction of high angle intensities due to increased disorder.