Cameras and Diffractometers

The x-ray analyses were obtained using powder camera and diffractometer techniques. In the former, powdered shale was loaded into a glass capillary which then was exposed to 20 hr of Cu(Ka) radiation. The x-ray pattern was taken with a standard Norelco 114.6-mm diameter powder camera equipped with a nickel filter. 

Hajdu, J., McLaughlin, P. J., Helliwell, J. R., Sheldon, J., and Thompson,. 4 W. Universal crystal cooling device for precession cameras, rotation camera and diffractometer. J. Appl. Cryst. 18, 528-532 (1985). 

G.36 A. J. C. Wilson. Elements of X-Ray Crystallography (Reading, Mass. Addison-Wesley, 1970). Powder cameras and diffractometers single-crystal cameras and diffractometers structure determination diffraction by imperfect crystals. 

G.39 Harold P. Klug and Leroy E. Alexander. X-Ray Diffraction Procedures, 2nd ed. (New York Wiley, 1974). Contains a great deal of useful detail on the theory and operation of powder cameras and diffractometers. Covers the following topics in depth chemical analysis by diffraction, parameter measurement, line-broadening analysis, texture determination, stress measurement, and studies of amorphous materials. Single-crystal methods are not included. 

The crystal is placed in an X-ray diffraction apparatus (camera or diffractometer) where the X-ray pattern is recorded photographically or by measuring the intensity of the X-rays electronically. The resulting intensity values are used to obtain the observed structure factors which constitute the fundamental experimental data from which the crystal and molecular structures are derived. The structure derived is used for calculating structure factors that are compared with the experimental structure factors during the period when the derived structure is being modified to fit the experimental data. 

In the following sections, I will discuss briefly a few of the major instruments employed in data collection. These include the X-ray sources, which produce an intense, narrow beam of radiation detectors, which allow quantitative measurement of reflection intensities and cameras or diffractometers, which control the orientation of the crystal in the X-ray beam and thus direct reflections having known indices to detectors. 

The X-ray patterns were recorded (with Cu Kct radiation) at different temperatures on a General Electric diffractometer fitted with a high-temperature camera and a temperature programmer. The a0 values of the Pniim and Fm3m phases of the CsCl+KCl solid solutions were determined at different temperatures, and that a0 values of the CsCl+ CsBr solid solutions were determined at 25 and 560°C. All the calculations employing the Born treatment were carried out on the IBM 1620 and IBM 7044 computers in this Institute. 

X-ray patterns can be obtained using either a powder diffractometer or a camera. Currently, diffractometers find widespread use in the analysis of pharmaceutical solids. The technique is usually nondestructive in nature. The theory and operation of powder diffractometers is outside the scope of this discussion, but these topics have received excellent coverage elsewhere. Instead, the discussion will be restricted to the applications of X-ray powder diffractometry (XRD) in the analysis of pharmaceutical solids. The United States Pharmacopeia provides a brief but comprehensive introduction to X-ray dif-fractometry. The use of XRD in the physical characterization of pharmaceutical solids and in the characterization of controlled release delivery systems have been discussed earlier. ... 

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. 

There is a fundamental difference between the operation of a powder camera and a diffractometer. In a camera, all diffraction lines are recorded simultaneously, and variations in the intensity of the incident x-ray beam during the exposure can have no effect on relative line intensities. On the other hand, with a diffractometer, diffraction lines are recorded one after the other, and it is therefore imperative to keep the incident-beam intensity constant when relative line intensities must be measured accurately. Since the usual variations in line voltage are quite appreciable, the x-ray tube circuit of a diffractometer must include a voltage stabilizer and a tube-current stabilizer. 

The crystal-setting procedure illustrated in Fig. 8-21 can be carried out whether or not the indices of the various poles are known. If the Laue pattern of a crystal is difficult to solve, any spot on it can be indexed by using a Laue camera and a diffractometer in sequence [8.4]. In addition, a goniometer is required that fits both instruments. The procedure is as follows ... 

For reasons to be discussed in Chap. 11, the observed values of sin 6 always contain small systematic errors. These errors are not large enough to cause any difficulty in indexing patterns of cubic crystals, but they can seriously interfere with the determination of some noncubic structures. The best method of removing such errors from the data is to calibrate the camera or diffractometer with a substance of known lattice parameter, mixed with the unknown. The difference between the observed and calculated values of sin 6 for the standard substance gives the error in sin 9, and this error can be plotted as a function of the observed values of sin 6. Figure 10-1 shows a correction curve of this kind, obtained with a particular specimen and a particular Debye-Scherrer camera. The errors represented by the ordinates of such a curve can then be applied to each of the observed values of sin 0 for the diffraction lines of the unknown substance. For the particular determination represented by Fig. 10-1, the errors shown are to be subtracted from the observed values. 

The diffractometer is a more complex apparatus than a powder camera and therefore more subject to misalignment of its component parts. A further difficulty with most commercial diffractometers is the impossibility of observing the same back-reflected cone of radiation on both sides of the incident beam. Thus, the experimenter has no automatic check on the accuracy of the angular scale of the instrument or the precision of its alignment. 

To find the relation between diffracted intensity and concentration, we must go back to the basic equation for the intensity diffracted by a powder specimen. The form of this equation depends on the kind of apparatus used, namely, camera or diffractometer we shall consider only the diffractometer here. The exact expression for the intensity diffracted by a single-phase powder specimen in a diffractometer is... 

The Biology Department beam line includes a station for protein crystallography (with Supper oscillation camera and FAST TV diffractometer) and a station for small angle diffraction (with a three-circle goniostat and MWPC electronic area detector). The latter station may be available for optimised anomalous dispersion crystallographic studies. The optical design for each consists of a bent pre-mirror, double crystal monochromator and bent post-mirror the mirrors have rhodium coatings (Wise and Schoenborn 1982). 

X-ray powder diffraction studies are perfonned both with films and with counter diffractometers. The powder photograph was developed by P Debye and P Scherrer and, independently, by A W Hull. The Debye-Scherrer camera has a cylindrical specimen surrounded by a cylindrical film. In another commonly used powder... 

---