Data collection diffraction

Crystal Data collection - Diffraction images Measure X-ray diffraction intensities from a crystal... 

Quantitative Phase Analysis. Once the identity of the components in a sample are known, it is possible to determine the quantitative composition of the sample. There are several different methods for doing a quantitative analysis, but the most rehable method is to use mixtures of known composition as standards. The computer can determine quantitatively the relative amounts of each component in the unknown sample. For accurate calculations of relative amounts in the unknown sample, it is necessary that the sample and standards have uniform distributions of crystaUites. Often the sample and standards are rotated during data collection to provide a more even distribution of crystaUites which diffract. 

Area Detectors. A two-dimensional or area detector attached to a powder diffractometer can gready decrease data collection time. Many diffraction appHcations require so much time with a conventional detector that they are only feasible if an area detector is attached to the iastmment. The Siemens General Area Detector Diffraction System (GADDS) uses a multiwire area detector (Fig. 17). This detector measures an x- and ajy-position for each x-ray photon detected. The appHcations foUow. 

Laue Method for Macromolecule X-Ray Diffraction. As indicated above it is possible to determine the stmctures of macromolecules from x-ray diffraction however, it normally takes a relatively long period of data collection time (even at synchrotrons) to collect all of the data. A new technique, the Laue method, can be used to collect all of the data in a fraction of a second. Instead of using monochromated x-rays, a wide spectmm of incident x-rays is used. In this case, all of the reflections that ate diffracted on to an area detector are recorded at just one setting of the detector and the crystal. By collecting many complete data sets over a short period of time, the Laue method can be used to foUow the reaction of an enzyme with its substrate. This technique caimot be used with conventional x-ray sources. 

Figure 4 Plot of time-resolved decomposition of titanium-enriched slags as extracted from neutron diffraction data collected at 1000° C.

Final structure Fit eiectron density Solve structure Collect diffraction data... 

When a diffracted X-ray beam hits a data collection device, only the intensity of the reflection is recorded. The other vital piece of information is the phase of the reflected X-ray beam. It is the combination of the intensity and the phase of the reflections that is needed to unravel the contributions made to the diffraction by the electrons in different parts of the molecule in the crystal. This so-called phase problem has been a challenge for theoretical crystallographers for many decades. For practical crystallography, there are four main methods for phasing the data generated from a particular crystal. 

Otwinowski Z, Minor W. Processing of X-ray diffraction data collected in oscillation mode. Methods Enzymol 1997 276 307-26. 

There are many variants of this system which can be envisaged as means by which the current possibilities for automation in data collection can be applied for specific purposes. There are considerable dangers in this approach in that it may be all too easy to build in restrictions which predetermine the results. These dangers, however, are not likely to be worse than those normally encountered in electron microscopy or single crystal diffraction where the one particularly "good-looking picture is taken as being "typical" of a sample. 

Figure 4 Diagrammatic representation of single-crystal x-ray diffraction and data collection.

When low-temperature studies are performed, the maximum resolution is imposed by data collection geometry and fall-off of the scattered intensities below the noise level, rather than by negligible high-resolution structure factor amplitudes. Use of Ag Ka radiation would for example allow measurement of diffracted intensities up to 0.35 A for amino-acid crystals below 30 K [40]. Similarly, model calculations show that noise-free structure factors computed from atomic core electrons would be still non-zero up to O.lA. 

The advent of CCD detectors for X-ray diffraction experiments has raised the possibility of obtaining charge density data sets in a much reduced time compared to that required with traditional point detectors. This opens the door to many more studies and, in particular, comparative studies. In addition, the length of data collection no longer scales with the size of the problem, thus the size of tractable studies has certainly increased but the limit remains unknown. Before embracing this new technology, it is necessary to evaluate the quality of the data obtained and the possible new sources of error. The details of the work summarized below has either been published or submitted for publication elsewhere [1-3]. 

The data structures and the control flow through the data handling activities are depicted in Figure 3. We use a batch approach to data collection so that the instrument can perform a series of data acquisition tasks without requiring operator intervention. A batch of tasks may take several days when diffraction is weak or when there are interferences. The structure of the data handling activities is governed by the batch nature of the data collection process. 

The information stored in the specimen database is sufficient to identify the particular specimen and the material from which it is made. Other parameters provide information on the orientation of the specimen and on its unit cell parameters. These latter parameters are used by the data collection tasks and the crystal geometry calculation function to determine diffraction angles, the angles between crystal planes, etc. The user can store information on several specimens in the specimen database, thus permitting him to easily remount and rerun a specimen after looking at the collected data. 

Each data collection task creates a file of output data (the diffraction data file in Figure 3). The file header contains information from the specimen database and parameters set by the data collection task using the data in the run database. The diffraction data files are, therefore, completely self-contained. 

The molecular structure of 1,2,9,10-tetragerma[2.2]paracyclophane 17 was determined by the X-ray diffraction study. The single crystals of 17 for X-ray crystallography were obtained from a toluene solution. Similar to 12, the crystal belongs to the space group P2Jn, and the data collection was carried out at 13°C. The ORTEP drawing of 17 is shown in Fig. 7. 

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