Preparing a crystal for diffraction studies

FIGURE 7.7. Methods of mounting crystals, (a) A crystal mounted in a glass fiber, as used for a small-molecule crystal that does not decompose on exposure to air. (b) Diagram of the mounting of a crystal in a capillary tube, (c) and (d). A crystal of a chemically modified horse hemoglobin enclosed with mother liquor in a thin walled glass capillary. (Courtesy J. J. Stezowski). 

The crystal, mounted on a goniometer head, is placed in the appropriate portion of the instrument that will be used to measure intensities. Then the crystal is viewed through the eyepiece of a telescope mounted 

FIGURE 7.8. A goniometer head for orienting and centering a crystal in an X-ray camera or diffractometer. The arcs and lateral adjustments provide the means for the crystallographer to orient the crystal as needed. Note the directions of (j and Z. These define the goniometer head orientation. 

FIGURE 7.10. Diagram of the relative arrangement of the X-ray source, the detector, and the crystal. 

Preliminary photographs of a crystal may be taken in order to check for a cracked or twinned crystal, or for thermal diffuse scattering or superlattice formation. Some of the instruments for doing this are those that were historically used for data collection. It is now debated whether it is necessary to take such preliminary photographs, because the more sophisticated data-collecting devices, together with a high-speed computer, can provide much of the same information. The reader, however, may encounter these other methods, which are briefly described here. More details can be obtained from the listed references. 

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