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61.
We determine the electronic structure of a semiinfinite crystal with a general coverage by adatoms, if there exists a partial order described by the long-range order parameter5. The tight-binding formalism is used, the presence of the surface is included via the surface Green function method, and the effect of disorder is described by a version of the coherent potential approximation. The local densities of states at/near the surface as well as the interaction energy of adatoms are calculated as functions of the coverage, and the long-range order parameterS. The numerical examples modelling the cases of a gas adsorption on the transition metal surfaces as well as the transition metal atoms on the transition metal surfaces, are investigated. 相似文献
62.
Kucera R Zizkovský V Sochor J Klimes J Dohnal J 《Journal of separation science》2005,28(12):1307-1314
Zirconia-based stationary phases represent an interesting alternative to silica-based materials. Two zirconia-based stationary phases were studied as an option for use in drug analysis. The different properties of zirconia material, distinct from RP silica-columns, were employed for the development of a novel and rapid stability monitoring HPLC method. This method enables simultaneous control of possible degradation processes of active substance (ibuprofen) as well as antimicrobial excipients (methyl-and propylparaben). The separation of ibuprofen, its two main degradation products 2-(4-isobutyrylphenyl)propionic acid and 4-isobutylacetophenone, parabens, and 4-hydroxybenzoic acid as their degradation product was successfully accomplished on a Zr-CarbonC18 column using a mobile phase consisting of acetonitrile-phosphate buffer (pH 4.8)-propan-2-ol (27:56:17, v/v/v). Detection was performed at 258 nm and the analysis was completed within 17 minutes. 相似文献
63.
Oriňák Andrej Vering Guido Arlinghaus Heinrich F. Andersson Jan T. Halas Ladislav Oriňáková Renáta Turčániová Ľudmila 《平面色谱法杂志一现代薄层色谱法》2005,18(1):44-50
JPC – Journal of Planar Chromatography – Modern TLC - A new hyphenated technique that enables coupling of thin-layer chromatography (TLC) with time-of-flight secondary-ion mass... 相似文献
64.
Wallace WT Wyrwas RB Whetten RL Mitrić R Bonacić-Koutecký V 《Journal of the American Chemical Society》2003,125(27):8408-8414
The discovery that supported gold clusters act as highly efficient catalysts for low-temperature oxidation reactions has led to a great deal of work aimed at understanding the origins of the catalytic activity. Several studies have shown that the presence of trace moisture is required for the catalysts to function. Using near-atmospheric pressure flow reactor techniques, we have studied humidity and temperature effects on the reactivity of gas-phase gold cluster anions with O2. Near room temperature, the humid source produces abundant gold-hydroxy cluster anions, Au(N)OH(-), and these have a reversed O2 adsorption activity: Nonreactive bare gold clusters become active when in the form Au(N)OH(-), while active bare clusters are inactive when -OH is bound. The binding energies for the stable structures obtained from density functional calculations confirm fully these findings. Moreover, the theory provides evidence that electron-transfer induced by the binding of a OH group enhances the reactivity toward molecular oxygen for odd anionic gold clusters and suppresses the reactivity for the even ones. The temperature dependence of O2 addition to Au(3)OH(-) and Au(4)(-) indicates deviations from equilibrium control at temperatures below room temperature. The effects of humidity on gold cluster adsorption activity support the conclusion drawn for the mechanism of O2 adsorption on "dry" gold cluster anions and provides insight into the possible role of water in the enhanced activity of supported gold cluster catalysts. 相似文献
65.
A potentiometric reductimetric method for the determination of platinum (Pt(IV)Pt(II)) with a standard Fe(II) solution in an alkaline medium of mannitol is described. The method, the error of which does not exceed 2%, can be used in the presence of palladium, iridium, and rhodium. 相似文献
66.
P. Vaňura V. Jedináková-Křížová M. Vobecký 《Journal of Radioanalytical and Nuclear Chemistry》2006,267(2):501-503
Summary An extraction technique for the separation of 90Sr from a high excess of90Y has been developed. This procedure can be used for the determination of trace amounts of90Sr in90Y prepared by a radiochemical90Y/90Sr generator by liquid scintillation. 相似文献
67.
Ladislav Sojk 《Journal of chromatography. A》1980,190(2)
The optimal separation properties of Carbowax 20M as a stationary phase in 300-m glass capillary columns at temeperatures below its lower temperature limit (60°) in terms of the selectivity, efficiency and speed of the gas chromatographic analysis of isomeric C10---13n-alkenes were investigated.In the separation of isomeric n-alkenes two possible states of the Carbowax stationary phase below 60° were used (solid state and intermediate liquid state), and the dependence of the retention of the isomers on temperature was studied. The region of the hysteresis portion of the ICW = f(T)diagram permits the continuous setting of column selectivity. The efficiency of the column at temperatures below the lower temperature limit of Carbowax 20M depends on the spatial cis-trans configuration of n-alkenes and, in the solid state, for cis-isomers is comparable to the high efficiency of this column at higher temperatures (115°). Utilization of Carbowax 20M in the solid state made it possible to achieve faster and more complete separations of the eleven possible positional and spatial isomers of n-tridecenes in comparison with previous methods. 相似文献
68.
Zusammenfassung Die bei 1850, 2000 bzw. 2300°C durch die Reduktion von Scandiumoxid mittels Kohlenstoff in dem der Scandiumdicarbidbildung entsprechenden Molverhältnis hergestellten Produkte wurden mit Wasser zersetzt und mit Hilfe der gleichzeitigen gaschromatographischen und massenspektrometrischen Analyse untersucht. Aus den Ergebnissen geht hervor, daß bei der Scandiumdicarbidbildung gleichzeitig ein weiteres Scandiumcarbid, höchstwahrscheinlich ein Sesquicarbid, entsteht.
Herrn Prof. Dr.H. Nowotny gewidmet.
7. Mitt.: Coll. Czech. Chem. Commun., im Druck. 相似文献
Scandium(III) dicarbide problem. Further experimental results
Products obtained by the reduction of Sc2O3 with carbon at 1850, 2000 and 2300°C, resp., in the molar ratio corresponding to the scandium dicarbide were hydrolysed with water. Gaseous reaction products were analysed using gas chromatography combined with mass spectrometry. Results show, that during dicarbide formation another scandium carbide, probably sesquicarbide, is also formed.
Herrn Prof. Dr.H. Nowotny gewidmet.
7. Mitt.: Coll. Czech. Chem. Commun., im Druck. 相似文献
69.
Kyselová Z Kysel'ová Z Krizanová L Soltés L Stefek M 《Journal of chromatography. A》2005,1084(1-2):95-100
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) profiles of eye lens proteins showed that both progression of diabetic cataract in rats in vivo and precipitation of soluble eye lens proteins stressed by free radicals in vitro were accompanied by significant protein cross-linking. There was a noticeable contribution of disulfide bridges to protein cross-linking in diabetic eye lens in vivo. In contrast, under conditions in vitro, when eye lens proteins were exposed to hydroxyl or peroxyl radicals, we showed that the participation of reducible disulfide linkages in the formation of high molecular mass products was markedly lower. These in vivo--in vitro differences indicate that the generally accepted role of reactive oxygen species in diabetic cataractogenesis may be overestimated in connection with the processes of protein cross-linking. 相似文献
70.
Thorsteinn Loftsson Dagný Hreinsdóttir Már Másson 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):545-552
Studies have shown that cyclodextrins form both inclusion and non-inclusion complexes and that several different types of
complexes can coexist in aqueous solutions. In addition, both cyclodextrins and cyclodextrin complexes are known to form aggregates
and it is thought that these aggregates are able to solubilize drugs through micellar-type mechanism. Thus, stability constants
determined from phase-solubility profiles are rarely true stability constants for of some specific drug/cyclodextrin complexes.
A more precise method for evaluation of the solubilizing effects of cyclodextrins is to determine their complexation efficiency
(CE). CE can be determined by measuring the solubility of a given drug at 2–3 cyclodextrin concentrations in pure water or
a medium constituting the pharmaceutical formulation such as parenteral solution or aqueous eye drop formulation. Based on
the CE value the drug:cyclodextrin ratio in the complexation medium can be determined as well as the increase in the formulation
bulk in a solid dosage form. Determination of CE is a simple method for quick evaluating the solubilizing effects of different
cyclodextrins and/or the effects of excipients on the solubilization. Here we report the CE of 43 different drugs with mainly
2-hydroxypropyl-β-cyclodextrin but also with randomly methylated β-cyclodextrin as well as few other cyclodextrins. Calculation of CE, drug:cyclodextrin molar ratio and the increase in the
formulation bulk is discussed, as well as the influence of the intrinsic solubility and drug lipophilicity on the CE. 相似文献