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61.
S. Martínez-Gallegos S. Bulbulian 《Journal of Radioanalytical and Nuclear Chemistry》2005,266(2):285-287
Summary Electrophoresis followed by neutron activation analysis was utilized to determine chromium(III) and (VI) in mixed solutions.
These solutions proceeded from Cr(VI) adsorbed hydrotalcites heated at 800 °C to partially immobilize Cr in the Mg-Al oxide
solid solution. Immobilization was studied by Cr lixiviation with NaCl solutions through the heated hydrotalcites. The results
have shown that Cr lixiviated was in the form of CrO42- ions, mainly because some Cr(VI) was not completely reduced to Cr(III) during heating. Chromium lixiviated from HT-Cr sample,
heated at 800 °C and γ-irradiated at 1000 kGys, was found, as well, in the form of CrO42- ions. Although γ-irradiation increases Cr immobilization in the solid, it does not reduce completely all CrO42- ions present in the solid and, therefore, some Cr is lixiviated through the solid in the form of CrO42- ion. 相似文献
62.
Sitnikova NL Sprik R Wegdam G Eiser E 《Langmuir : the ACS journal of surfaces and colloids》2005,21(16):7083-7089
We studied the spontaneous emulsification and droplet growth mechanism in trans-anethol/water/ethanol solutions, also known as the beverage ouzo, using dynamic light scattering spectroscopy. This simple ternary mixture is a generic example of a system that forms microemulsions spontaneously when brought into the two-phase region. The volume fraction of the dispersed phase was found to profoundly affect the growth rates of the droplets, which is a new finding that has not been predicted by the Lifshitz-Slyozov-Wagner theory. Time-dependent measurements show that the droplet growth is governed by Ostwald ripening (OR), and no coalescence was observed. Furthermore, the OR rates increase with increasing oil concentration at low alcohol content. We attribute this behavior to enhanced droplet-droplet interactions. At high ethanol concentrations, we found that the measured rates decreased as the oil concentration increased. The OR growth mechanism completely correlates with changes in droplet size. The kinetics of droplet growth shows that the ripening has a saturation limit at a droplet radius of about 1.5 mum. Thus, formed emulsions remain stable for months. 相似文献
63.
To investigate the molecular details of the phosphoryl-transfer mechanism catalyzed by cAMP-dependent protein kinase, we performed quantum mechanical (QM) calculations on a cluster model of the active site and molecular dynamics (MD) simulations of a ternary complex of the protein with Mg(2)ATP and a 20-residue peptide substrate. Overall, our theoretical results confirm the participation of the conserved aspartic acid, Asp(166), as an acid/base catalyst in the reaction mechanism catalyzed by protein kinases. The MD simulation shows that the contact between the nucleophilic serine side chain and the carboxylate group of Asp(166) is short and dynamically stable, whereas the QM study indicates that an Asp(166)-assisted pathway is structurally and energetically feasible and is in agreement with previous experimental results. 相似文献
64.
Martín Y 《The Analyst》2000,125(12):2230-2235
A method for the determination of three anabolic hormones (diethylstilbestrol, dienestrol and trenbolone) in calf urine is described. After enzymatic hydrolysis, the samples were cleaned up by C18 solid-phase extraction. Drugs were extracted with hexane and analyzed by isocratic elution on a Discovery RP-Amide C16 5 microns column with photodiode-array detection at 240 and 347 nm. Both retention time and UV spectra were used for identification. Detection limits for the HPLC system were calculated to be 0.3 ng injected for all analytes in the standard mixture. However, for urine samples these limits increased because of the presence of unidentified matrix components. After extraction from urine, the limits of detection for the whole analytical procedure were 5 and 10 ng injected for trenbolone and stilbenes, respectively. The average recoveries of the hormones from spiked samples were in the range 53.1-56.7% with RSD between 11.3 and 14.5% for the whole procedure in the concentration range 25-2.5 ng ml-1. 相似文献
65.
J. L. Martínez Vidal A. Garrido Frenich T. López López I. Martínez Salvador L. Hajjaj el Hassani M. Hassan Benajiba 《Chromatographia》2005,61(3-4):127-131
An analytical method for determining residues of twenty pesticides by liquid chromatography (LC) coupled to electrospray ionisation (ESI) tandem mass spectrometry (MS-MS) in eight commodities, cucumber, tomato, pepper, green bean, eggplant, zucchini, melon and watermelon, has been developed and validated. On one hand, calibration curves prepared in solvent were compared with calibration curves prepared in a blank matrix extract of each target matrix. On the other hand, calibration curves and recoveries for each commodity were compared. Cucumber was selected as potential reference matrix for the target vegetables. 相似文献
66.
Isaac B. Bersuker Natalia N. Gorinchoi Victor Z. Polinger 《Theoretical chemistry accounts》1984,66(3-4):161-172
The vibronic origin of dynamic instability of molecular systems considered earlier, is here given a more complete and rigorous treatment. It is shown that the nonvibronic contribution to the curvature of the adiabatic potential arising due to nuclear displacements under fixed electronic density distribution, is always positive, and hence the only reason for dynamic instability is the pseudo Jahn-Teller effect. For some examples of special interest (planar equilateral NH3, planar square CH4 and linear H
3
+
) the molecular excited states, responsible for the instability of the ground state, are revealed by means of ab initio calculations. 相似文献
67.
Tatsuhiko Nakano A.C. Spinelli A. Martín A. Usubillaga Andrew T. McPhail Kay D. Onan 《Tetrahedron letters》1982,23(35):3627-3630
cleavage of the epoxide () of methyl (-)-kaur-9(11)-en-19-oate () with boron trifluoride-ether in benzene and in acetic anhydride yielded () and (), respectively. On epoxidation with -chloroperbenzoic acid in the presence of -nitrosomethyl urea, () suffered a backbone rearrangement to form (). 相似文献
68.
69.
Román LU Guerra-Ramírez D Morán G Martínez I Hernández JD Cerda-García-Rojas CM Torres-Valencia JM Joseph-Nathan P 《Organic letters》2004,6(2):173-176
[structure: see text] The triterpenes 8,14-seco-oleana-8(26),13-dien-3beta-ol (1) and its acetyl derivative 2 were isolated from Stevia viscida and Stevia eupatoria, respectively. Their structures were elucidated by 2D NMR, including carbon-carbon connectivity experiments, and confirmed by X-ray diffraction analysis of ketone 3. The absolute configuration was determined by NMR analysis of the Mosher esters of 1. The biogenetic implications of the new substances are discussed. 相似文献
70.
C. Martínez R. Cremer D. Neuschütz J. P. Servais D. Loison 《Surface and interface analysis : SIA》2002,34(1):524-526
The formation of oxides at the surface of Fe–1.5%Mn and Fe–0.6%Mn binary alloys was investigated as a function of the conditions of the heat treatments. Both the influence of temperature and the atmosphere under which the experiments were performed were studied. The range of annealing temperatures was adjusted to 800°C. The atmosphere consisted of a mixture of N2–5%H2 and traces of water vapour, with different fixed dew points ranging from −10°C to −30°C. The state of the annealed surfaces was determined using in situ analytical devices attached to the annealing reactor in order to avoid surface contamination or the formation of native oxides after the experiments due to contact with air. The structure and composition of the surfaces were determined by reflection high-energy electron diffraction (RHEED) and electron spectroscopy (XPS, AES). Copyright © 2002 John Wiley & Sons, Ltd. 相似文献