Mechanical artifacts

The absence of an electron from a covalent bond leaves a hole and the neighboring valence electron can vacate its covalent bond to fill the hole, thereby creating a hole in a new location. The new hole can, in turn, be filled by a valence electron from another covalent bond, and so on. Hence, a mechanism is estabUshed for electrical conduction that involves the motion of valence electrons but not free electrons. Although a hole is a conceptual artifact, it can be described as a concrete physical entity to keep track of the motion of the valence electrons. Because holes and electrons move in opposite directions under the influence of an electric field, a hole has the same magnitude of charge as an electron but is opposite in sign. 

AH of these technologies require careful attention to the film transport mechanism to avoid imaging defects. The sizes of the features produced on the film, typicaHy resolution of 1000 spots/cm or greater, and the accuracy of spot placement both require high precision equipment. Even minute misplacements of spots can produce objectionable visible artifacts. 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

An example of mechanism (1) is given in Section 1.1.2 Essentially, numerical artifacts are due to computational operations that result in a number, the last digits of which were corrupted by numerical overflow or truncation. The following general rules can be set up for simple operations ... 

A diazodiphenoquinone is also produced as a chemical artifact (Knackmuss and Beckmann 1973). There are, however, a nnmber of unresolved features of the degradation, including the mechanisms for production of 2,6-dihydroxypyridine and oxidative fission of the pyridine ring. 

SEC analysis shows that some samples have a blmodal MWD. At this time it is not possible to tell whether the bimodallty is an artifact of the polymerization mechanism or, perhaps, a consequence of partial hydrolysis of the polymer i.e., the high MW shoulder in Figure 5 may be due to the formation of aggregates through Intermolecular dipolar interactions of P-OH side groups or to polymer molecules crosslinked by P-O-P bonds. 

The lattice gas model of Bell et al. [33] neither gave any detailed mechanism of the orientational ordering nor separated the contributions of the headgroup and the acyl chain. Lavis et al. [34] discussed Ref. 33 critically and concluded that the sharp kink point in the isotherm at transition was an artifact of the mean field approximation used. An improved correspondence to experimental data was claimed by the use of the real-space renormalization group method [35]. The same authors returned to the problem [35] and concluded that in addition to the orientation of the molecules, chain melting had to be included in a model which could interpret the phase transitions. 

Domanski, M., J. A. Webb, and J. Boland (1994), Mechanical properties of stone artifact materials and the effect on heat treatment, Archaeometry 36, 177-208. 

Interestingly, although many transition state analogs bind noncovalently to the target enzyme s active site via a one-step kinetic mechanism (Scheme la) and would therefore be expected to exhibit no time-dependent properties of inhibition, inhibitors with Kj values of < 10 10 M (like coformy-cin) usually have a slow onset of inhibition kobserved < 10 2 s 1 (i.e., an approach to equilibrium inhibition of > 1 min).161 This is merely an assay artifact due to... 

Finally, a feedback mechanism has often been used to explain observed (negative and positive) deviations from the Scatchard type plots or nonunity slopes of the nonsaturated portion of the logarithmic Michaelis-Menten plots (e.g. [209]). When no artifacts are present (cf. [197,198]), deviations can indeed be interpreted to indicate that the intrinsic stability or dissociation rate constants vary with the number of occupied transport sites. Nonetheless, several other physical explanations, including multiple carriers, non 1 1 binding, carrier aggregation, etc. must also be considered. 

X-ray diffraction uses X-rays of known wavelengths to determine the lattice spacing in crystalline structures and therefore directly identify chemical compounds. This is in contrast to the other X-ray methods discussed in this chapter (XRF, electron microprobe analysis, PIXE) which determine concentrations of constituent elements in artifacts. Powder XRD, the simplest of the range of XRD methods, is the most widely applied method for structural identification of inorganic materials, and, in some cases, can also provide information about mechanical and thermal treatments during artifact manufacture. Cullity (1978) provides a detailed account of the method. 

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