A yokeless flow cell for Laue crystallography

Laue crystallography has re-emerged as a powerful method for diffraction experiments when used in conjunction with the 'white' X-radiation available at a synchrotron facility (Moffat, Szebenyi & Bilderback, 1984). A shortcoming of the Laue method stems from the inability to determine the intensity of the X-ray spectrum at each wavelength along the continuum in order to produce accurate structure factors. However, relative structure factors may be determined accurately by recording two diffraction images of the same crystal, before and after the introduction of reactants to convert the crystal from the parent to the intermediate form. This is accomplished by the use of a flow cell (Wyckoff et al., 1967; Mitsui & Wyckoff, 1975; Hajdu, McLaughlin, Heiliwell, Sheldon & Thompson, 1985; Petsko, 1985). A typical flow cell is made from a piece of quartz capillary supported between the arms of a rigid U-shaped piece of metal referred to as a 'yoke'. This type of flow cell can be rather time consuming to set up and time is a serious constraint at a synchrotron laboratory. The flow cell described here is designed for Laue experiments, special attention having been given to rapid crystal mounting, simplicity of construction and flexibility of use. It does not use a yoke for structural integrity but instead relies on a novel support of the quartz capillary by a plastic cone and a wire harness. The effluent is allowed to drip out of the end of the capillary without being recollected. This is practical since flow rates are usually low, on the order of 5 pi min-1, and thus a 1 h experiment will produce only 300 pl of effluent. The absence of a yoke and a return tube allows 36ff" rotation of the crystal without production of a shadow. A diagram of the flow cell is shown in Fig. 1 and a photograph of it is shown in Fig. 2. An important feature is that a plastic pipette tip (2001al) is cut such that the capillary may be slid into it, making contacts at the narrow and wide ends of the capillary. This hard plastic covering around the capillary allows the assembly to be easily handled without breaking during crystal mounting. The capillary/pipette-tip assembly fits into a harness made from solid wire which is wound into coils and coated with wax. The wax helps to make a tight bond with the pipette tip, which remains secure even when wet. The attachment can be loosened by twisting the plastic with forceps. The ends of the wire come together and are soldered into the hole of a brass pin.