Mass defect, definition

The measurements of mass defects carried out with very great accuracy by Aston (since 1920) supply information, therefore, on the heats of formation of nuclei, i.e. on the energy relations in nuclear construction. These are found to follow certain perfectly definite rules. It has proved to be convenient to divide up the various nuclei into four groups,... 

The relationship of energy and mass would indicate that in the formation of deuterium by the combination of a proton and neutron, the mass defect of 0.002 388 u would be observed as the liberation of an equivalent amount of energy, i.e. 931.5 X 0.002 388 = 2.224 MeV. Indeed, the emission of this amount of energy (in the form of y-rays) is observed when a proton captures a low ergy neutron to form jH. As a matter of fact, in this particular case, the energy liberated in the formation of deuterium has been used in the reverse calculation to obtain the mass of the neutron since it is not possible to determine directly the mass of the free neutron. With the definition (3.2) all stable nuclei are found to have negative AAf values thus the term "defect". 

Knowledge of nuclear forces is thus in one way very definite. The mass defects, which can be determined with considerable precision, yield quite reliable information about the relative energies of formation from possible eonstitutents. The quantitative application of the equation c Am — AE is well illustrated by the example of the reaction occurring when a fast proton causes the disintegration of... 

The intention of the Kendrick mass scale is to provide data reduction in a way that homologs can be recognzed by their identical Kendrick mass defect (KMD). Due to the steadily increasing resolution and mass accuracy of modem instrumentation this issue is again gaining importance for complex mixture analysis by MS. The Kendrick mass scale is based on the definition = 14.0000 u [54]. The conversion factor from the lUPAC mass scale, mnjpAc, to the Kendrick mass scale, Kendrick. IS therefore 14.000000/14.015650 = 0.9988834 ... 

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

A precise diagnosis. The urine of an infant gives a positive reaction with 2,4-dinitrophenylhydrazine. Mass spectrometry shows abnormally high blood levels of pyruvate, a-ketoglutarate, and the a-ketoacids of valine, isoleucine, and leucine. Identify a likely molecular defect and propose a definitive test of your diagnosis. 

Although alcaptonuria is a relatively harmless condition, such is not the ca.se with other errors in amino acid metabolism. In maple syrup urine dis-eim, the oxidative decarboxylation of cy-kctoacids derived from valine, isoleucinc, and leucine is blocked because the branched-chain dehydrogenase is missing or defective. Hence, the levels of these cx-ketoacids and the branched-chain amino acids that give rise to them are markedly elevated in both blood and urine. The urine of patients has the odor of maple syrup-hence the name of the disease (also called branched-chain ketoaciduria). Maple syrup urine disease usually leads to mental and physical retardation ss the patient is placed on a diet low in valine, isoleucinc, and leucine early in life. The disease can be readily detected in newborns by screening urine samples with 2,4-dinitrophenylhydrazine, which reacts with a-ketoacids to form 2,4-dinitrophenylhydrazone derivatives, A definitive sis can be made by mass spectrometry. 

In compound crystals, balanced-defect reactions must conserve mass, charge neutrality, and the ratio of the regular lattice sites. In pure compounds, the point defects that form can be classified as either stoichiometric or nonstoichiometric. By definition, stoichiometric defects do not result in a change in chemistry of the crystal. Examples are Schottky (simultaneous formation of vacancies on the cation and anion sublattices) and Frenkel (vacancy-interstitial pair). 

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