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71.
Total energies of Laves phases Cr2X, CrX2, CrCr2 and XX2 (X=Nb,Ta) in all three structural forms C14, C15 and C36 have been calculated ab initio by pseudopotential VASP code with a complete relaxation of structural parameters. The calculated values were used in a two-sublattice model for re-modeling of Gibbs energies of Laves phases and subsequently for calculation of phase diagrams of Cr–Nb and Cr–Ta systems by CALPHAD method. It turns out that application of ab initio calculated values of total energy of hypothetical “end-members” in a two-sublattice model substantially simplifies the modeling and lowers the number of necessary parameters. Comparison of phase diagrams obtained by a model using first-principles results with previous empirical approach as well as relative stability of Cr2X polytypes is presented. 相似文献
72.
73.
ABSTRACT ABSTRACT Cut-off points between protons in a-methyl and or-methylene groups were proposed for the evaluation of 'H NMR spectra of coal tars. The position of the cut-off points depended on the proton aromaticity and related structural features of the samples. Equations for the calculation of the cut-off points as a function of proton and carbon aromaticity, and atomic H/C ratio were proposed for the range of proton aromaticity between 0.2 and 0.6. 相似文献
74.
Pavlásková K Strnadová M Strohalm M Havlícek V Sulc M Volný M 《Analytical chemistry》2011,83(14):5661-5665
This work reports on a new and extremely simple approach for determination of a double bond position by a laser desorption ionization mass spectrometry. It is solely based on the catalytic properties of nanostructured surfaces used in nanoassisted laser desorption ionization experiments. These surfaces can induce oxidation of analytes, which results in a mass shift that can be detected by mass spectrometry. If a site of unsaturation is oxidized and cleaved, the m/z difference is diagnostic of the position of a double bond. By demonstrating that the oxidation depends on the analyte surface dwell time, it was proven that it is caused by the surface activity and not by the laser desorption ionization process itself. Control matrix-assisted laser desorption/ionization (MALDI) experiment showed only a limited partial oxidation and no time dependency of the process. The ability to determine a position of a double bond was demonstrated on polyunsaturated phospholipids and cyclosporine A. In some other cases, however, the unexpected oxidation could cause confusion, as demonstrated for a glycosphingolipid from a porcine brain extract. 相似文献
75.
Josef Kameník Alena Saláková Zdeněk Pavlík Gabriela Bořilová Radka Hulanková Iva Steinhauserová 《European Food Research and Technology》2014,239(3):395-402
Physicochemical, instrumental and microbiological examinations of steaks of beef and pork (m. longissimus lumborum) in vacuum skin packaging (VSP), conventional vacuum packing (CVP) and modified atmosphere packaging (MAP) were performed. Samples were stored at 2 ± 0.5 °C for 3 (pork) or 5 (beef) weeks. No statistically significant changes in pH values were recorded. Statistically significant (p < 0.001) changes in the thiobarbituric reactive substances content in modified atmosphere packed beef samples, and differences between samples in MAP and VSP or CVP were found from week 2 of the experiment onwards. The biggest changes in colour parameters were found in beef samples in MAP. The lowest and highest purge loss was recorded in samples in VSP and CVP, respectively. Vacuum packing enhanced the growth of lactic acid bacteria (LAB). At the end of the experiment, their numbers ranged from 4.31 log10 cfu g?1 (pork in CVP) to 5.14 log10 cfu g?1 (beef in VSP). LAB populations reached 2 log10 cfu g?1 in MAP beef and pork samples. On the other hand, MAP enabled the development of bacteria of the genus Pseudomonas and Brochothrix thermosphacta. The highest increase in coliform bacteria counts was recorded in vacuum-packed pork. 相似文献