Precision occupancy determinations

The application of Mjy y absorption to valence measurements is still at its infancy at this time, although first studies were started already in 1973 by Ottewell et al. (1973) and recently continued by Kaindl et al. (1983b, 1984) and by van der Laan et al. (1986). Analytic procedures for the extraction of the 4f occupation numbers from two complicat l and superimposed Mjy y final-state multiplets require the existence of reliable theoretical calculations of their intensities and positions in each valence state, which are available only recently (Thole et al. 1985). In addition the special experimental problems of Mjy y absorption, e.g., saturation effects and surface shifts diminish drastically the potential of Mjy y absorption as a tool for high-precision valence determinations. 

To sum up then, straightforward structure refinement with X-rays can give occupancy determinations with a precision (a) of ... 

Most of the data located concerning the health effects of heptachlor and heptachlor epoxide in humans come from case reports and occupational epidemiology studies of workers engaged either in the manufacture or application of pesticides. There is some information on people who have consumed heptachlor-contaminated food or dairy products, but no adverse health effects have been related to these exposures. The occupational studies involve exposures that are predominantly inhalation with contributions from dermal exposure, whereas all the animal studies were conducted using oral or intraperitoneal exposures. The occupational and case reports provide no quantitation of dose or duration of exposure, which makes it impossible to determine with any precision the effect levels for humans. There are no data that indicate that heptachlor or heptachlor epoxide are carcinogenic to humans. However, human studies are limited by the long latency period of carcinogenesis and by ascertainment and follow-up biases. 

Using time-resolved crystallographic experiments, molecular structure is eventually linked to kinetics in an elegant fashion. The experiments are of the pump-probe type. Preferentially, the reaction is initiated by an intense laser flash impinging on the crystal and the structure is probed a time delay. At, later by the x-ray pulse. Time-dependent data sets need to be measured at increasing time delays to probe the entire reaction. A time series of structure factor amplitudes, IF, , is obtained, where the measured amplitudes correspond to a vectorial sum of structure factors of all intermediate states, with time-dependent fractional occupancies of these states as coefficients in the summation. Difference electron densities are typically obtained from the time series of structure factor amplitudes using the difference Fourier approximation (Henderson and Moffatt 1971). Difference maps are correct representations of the electron density distribution. The linear relation to concentration of states is restored in these maps. To calculate difference maps, a data set is also collected in the dark as a reference. Structure factor amplitudes from the dark data set, IFqI, are subtracted from those of the time-dependent data sets, IF,I, to get difference structure factor amplitudes, AF,. Using phases from the known, precise reference model (i.e., the structure in the absence of the photoreaction, which may be determined from... 

If an individual performs both radiographic procedures (i.e., procedures without use of a protective apron) and fluoroscopic procedures i.e., procedures with the use of a protective apron) during a given monitoring period, there may be no practical way to determine precisely the relative contribution each type of procedure made to the total /fp(lO) value recorded by a personal monitor. However, occupational exposure during radiographic procedures should be very low, since the worker is at a relatively large distance from the x-ray source and most often i.e., except for use of mobile x-ray systems) in a protective cubicle. 

Cation Site Distribution, Thin-film EDS analysis can also be used to quantitatively determine the site occupancy of atoms in a known crystal structure. Atom Location by Channeling Enhanced Microanalysis (ALCHEMI) is a technique which utilises electronchanneling enhanced X-ray emission for specific atoms in a crystal when appropriately oriented relative to the incident beam [43]. The method involves no adjustable parameters, can be used on relatively small areas of sample and provides fractional occupancies of atom positions [44] Unlike X-ray diffraction which has had limited success with adjacent elements in the periodic table [e.g. 45], ALCHEMI can provide site occupancies for adjacent elements and is relatively insensitive to sample thickness or the precise electron beam orientation [44] ... 

FAAS is a well established method for the determination of the major milk elements Ca, K, Mg, and Na after dissolution or mineralization of the samples [38-41], In fact, the practical advantages of FAAS, that is, speed, simplicity, precision, cost, and ease of operation, are ideal when large sample throughput is required. Thus, many practical milk measurements in clinical, occupational health, toxicology, and nutrition laboratories are still performed by FAAS. 

The oxygen atom was found to be disordered at the crystallographic inversion center and therefore refined with half occupation factors on each side of the B-B bond (model A, Space Group C2/c, No. 15). This disorder is connected with the centrosymmetry in the crystal lattice, which is also strictly valid for the C(SiMe3)3 substituents. In spite of the disorder of the oxygen atom in the crystal, the bond length of the B-B bond could be determined with satisfactory precision [1.601 (7)... 

Type A and type C centers can be readily distinguished. Type B centers are usually readily identified, but it may be difficult to determine the respective occupancies of the two shells precisely, because the contributions are typically not well resolved in the Fourier transform. Also, it is unfortunate that EXAFS cannot distinguish between car-boxylate oxygens and water as ligands of zinc. However, coordination by imidazole is usually clearly evident because of the multiple-scattering pathways involving all atoms of the five-membered ring (23, 26). 

Refinement of the crystal structure is, therefore, a powerful chemical analysis technique. Unlike conventional chemical analysis, which only yields the bulk composition of the sample, powder diffraction analysis facilitates accurate determination of the occupancies of different crystallographic sites by various chemical elements, or in other words, establishes precise chemical composition of the crystal at the atomic resolution. It should be noted that the results may be considered reliable only when the difference in the scattering ability of atoms in question is significant, in addition to a very high quality of experimental data. This is indeed the case here because scattering factors of Sn and Ni are related as-1.8 1. 

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