Diffraction artefact

The dihedral angles about the glycosidic bonds in a-chitin were the same as those in p-chitin, " but in a-chitin there are two chitin chains in the unit cell. To account for an X-ray reflection which should not have been diffracted by crystals of the derived space group, it was proposed that the hydroxymethyl group was disordered. Subsequently, however, it was shown that this reflection was a double diffraction artefact. The detailed structure of a-chitin is therefore uncertain. 

For all these reasons diffraction gratings are used in most dispersive optical instruments. It is however essential to bear in mind the artefacts that can arise from harmonic transmissions. 

Catalyst surface areas were measured using the multi-point BET method on a Carlo-Erba Ins. Sorpty 1750. Before the measurements, the samples were heated under dynamic vacuum at 573 K for 1 h in order to remove adsorbed water and impurities. Measurements were made at liquid nitrogen temperature with nitrogen as the adsorbate gas. Powder X-ray diffraction measurements were performed on a Siemens Model D-500 diffractometer with Co Kc monochromatic radiation (X = 1.78901 A) and the high resolution electron microscopy was carried out on a Topcon EM-002B microscope. To prevent artefacts no solvents were used in the preparation and mounting of samples for HRTEM. 

Further work in the energy-dispersive CSCT area is needed to determine the limits to performance set by photon noise and reconstruction artefacts. Comparison of the plastic profiles of Fig. 20 with the diffraction profile of TNT (Fig. 5) derived from a direct tomographic energy-dispersive XDI device shown in Fig. 4 suggests that the latter has currently an advantage owing to its avoidance of reconstruction artefacts. 

An interesting structural problem also emerges from the study of the final sulfur-nitrogen cation to be considered, SsNs" ". First made in 1972, this was originally thought to contain a planar, heart shaped 10-membered heterocycle on the basis of X-ray diffraction studies on [S5N5]+[AlCl4] however, it now seems likely that this is an artefact of disorder within the crystals and that the structure of the cation is as in Fig. 15.41 which is the... 

This analysis breaks down if c(X, r) is not strictly constant. For example, the cell dimensions may change during the experiment due to thermal expansion, or the crystal may move very slightly, or the x-ray spectrum may vary due to fluctuations in the source or optics. The likelihood of these experimental artefacts and the magnitude of the errors they would introduce if present must be carefully evaluated in an individual experiment. Indeed, the entire experiment may fail if an order-disorder reaction occurs that leads to transient loss of the x-ray diffraction pattern [37],... 

Pavese A, Ferraris G, Pischedda V, Radaelli P (2000) Further powder neutron diffraction on phengite-Sr cation ordering and methodological thoughts. Mineral Mag 64 11-18 Pavese A, Ferraris G, Pischedda V, Fanth F (2001) Ml-site occupancy in 37 and 2A/j phengites by low temperature neutron powder diffraction Reahty or artefact Eur J Mineral (in press)... 

Sceletium Alkaloids.—Full details of an earlier briefly reported synthesis of ( )-mesembrine (3) have been published and a new synthesis has been described. The latter is outlined in Scheme 1 A new Sceletium alkaloid has been isolated from S. namaquense. It has been formulated as (4) on spectral evidence, the assignment of structure being confirmed by direct spectral comparison between the natural base and racemic (4), synthesized earlier. Channaine, another alkaloid of this family, from S. tortuosum, has structure (5) on the basis of an X-ray diffraction analysis of its hexahydrate. It may be an artefact arising from the condensation of a pair of iV-desmethylmesembrenone molecules (6) during isolation. ... 

Duran, A., Herrera, L.K., Jimenez de Haro, M.C., Justo, A., and Perez-Rodriguez, J.L. Nondestructive analysis of cultural heritage artefacts from Andalusia, Spain, by X-ray diffraction with Gobel mirrors. Talanta. 76 no. 1 (2008) 183-188. 

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