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51.
Kayali-Sayadi MN Rubio-Barroso S Díaz-Díaz CA Polo-Díez LM 《Fresenius' Journal of Analytical Chemistry》2000,368(7):697-701
A rapid method for the determination of PAHs in soil samples based on their extraction with methylene chloride by sonication and subsequent separation by HPLC with fluorimetric detection is proposed. A Hypersil Green PAH column was used with a gradient of acetonitrile/water as the mobile phase, together with a program of nine excitation and emission wavelength pairs. Recoveries were in the range 70-98%, except for acenaphthene and naphthalene, at concentration levels 1.08-442 microg/kg with relative standard deviations in the range 2-15% (n = 4). Total PAHs found in soil samples were in the range 15-282 microg/kg. The results were compared with those obtained by applying the 3540 EPA method for two samples. 相似文献
52.
E. Tamez M. T. Olguín N. Segovia S. Bulbulian F. Abascal 《Journal of Radioanalytical and Nuclear Chemistry》1986,103(4):231-240
Experiments were designed to measure trace uranium concentration and the rate of radon exhalation from masonry structural materials, both bare and surface finished and coated. LR115 cellulose nitrate track detectors were used to record the alpha emission from structural material surface. Fission track, neutron activation and fluorometric analysis methods were used to determine the uranium content. Most types of paints studied will reduce alpha contribution and radon emanation from building materials. 相似文献
53.
Summary An X-ray diffraction method was used to study the structures of montmorillonites, the surfaces of which were covered to a systematically increasing extent with hexadecylpyridinium (HDP) cations. It was found that, depending on the degree of cover of the surface, the HDP cations form a mono- or bimolecular layer in the interlayer space. A detailed investigation was made of the selective sorption of binary mixtures on montmorillonite and on HDP-montmorillonites with different surface coverages. The adsorption excess isotherms were determined, as were the adsorption capacities from analysis of the isotherms, and also the equivalent specific surface areas.It was established that from benzene-n-heptane mixtures the benzene is adsorbed only on the external surface of montmorillonite, while in the case of organocomplexes it causes moderate disaggregation.From benzene, alcohols with different numbers of carbon atoms are also preferentially adsorbed on the internal surfaces of montmorillonite; with the rise in the number of carbon atoms, the extent of interlamellar adsorption decreases. In the case of HDP-montmorillonites in methanol-benzene mixtures, the adsorption capacity exhibits a monotonous increase with the degree of organopbility in the range under consideration. In the event of optimum organophility the disaggregation of the organocomplex is complete, and the equivalent specific surface area is 764-778 m2 · g–1. The swelling of the organocomplex, which can be characterized by X-ray diffraction measurement, is in close correlation with the interlamellar liquid adsorption. The results show modification of the surface of montmorillonite with HDP cations fundamentally determines the nature and extent of the selective liquid adsorption.
With 14 figures and 4 tables 相似文献
Zusammenfassung Die Struktur von mit Hexadecyl-pyridinium(HDP)-Kationen im systematisch gesteigerten Maße bedeckten Montmorilloniten wurde mit der Röntgendiffraktions-Methode untersucht. Es wurde festgestellt, daß die HDP-Kationen, von der Bedeckung der Oberfläche abhängend, im Schichtzwischenraum eine mono- bzw. bimolekulare Schicht bilden. Die selektive Sorption binärer Gemische wurde an Montmorilloniten und an HDP-Montmorilloniten verschiedener Bedeckung eingehend untersucht. Es wurden die Adsorptionsisothermen von Flüssigkeitsgemischen und aus deren Analyse die Adsorptionskapazitäten und die äquivalenten spezifischen Oberflächen bestimmt.Es wurde festgestellt, daß aus Benzol-n-Heptan-Gemischen das Benzol nur an der äußeren Montmorillonite-Oberfläche adsorbiert wird, während es bei Organokomplexen eine mäßige Desaggregation verursacht. Die Adsorption von Alkoholen verschiedener Kohlenstoffzahl aus Benzol erscheint auch an inneren Montmorillonit-Oberflächen bevorzugt; mit zunehmender Kohlenstoffzahl nimmt das Ausmaß der interlamellaren Adsorption ab. Bei HDP-Montmorilloniten zeigt die Adsorptionskapazität aus Methanol-Benzol-Gemischen mit zunehmender Organophilität im untersuchten Gebiet eine monotone Steigerung. Im Falle optimaler Organophilität ist die Desaggregation vollständig, die äquivalente spezifische Oberfläche beträgt 764-778 m2/g. Die durch Röntgenn-diffraktionsmessung charakterisierbare Quellung des Organokomplexes steht mit der interlamellaren Flüssig-keitsadsorption in engern Zusammenhang. Unsere Ergebnisse zeigen, daß Charakter und Ausmaß der selektiven Flüssigkeitsadsorption durch die Modifizierung der Montmorillonit-Oberfläche mit HDP-Kationen grundlegend verändert wird.
With 14 figures and 4 tables 相似文献
54.
Herein, we present results from MD simulations of the Michaelis complex formed between the B. cereus zinc-beta-lactamase enzyme and benzylpenicillin. The structural and dynamical effects induced by substrate-binding, the specific role of the conserved residues, and the near attack conformers of the Michaelis complex are discussed. Quantum chemical methods (HF/6-31G* and B3LYP/6-31G*) are also applied to study the hydrolysis reaction of N-methylazetidinone catalyzed by a monozinc system consisting of the side chains of the histidine residues (His86, His88, and His149) complexed with Zn-OH and the side chains of Asp90 and His210. From this model system, we built molecular-mechanics representations of the prereactive complex and transition state configurations docked into the active site. Linear-scaling semiempirical calculations coupled with a continuum solvent model were then performed on these static models. We propose that the experimental rate data for the B. cereus enzyme is compatible with a one-step mechanism for the hydrolysis of beta-lactam substrates in which His210 acts as a proton donor. 相似文献
55.
The ability of enzymes to work in non-aqueous media offers new and almost unexploited possibilities for the development of new optical biosensors. The advantages of performing biocatalytic reactions in non-aqueous media are discussed in relation to their possible application in optical biosensor design. Attention is focused on the factors that influence enzymatic catalysis in organic solvents, including the role of enzyme-associated water, criteria for solvent selection and the alteration of enzyme specificity. Recent examples of relevant applications and future prospects of organic-phase optical biosensing are discussed. 相似文献
56.
When excesses of ammonium thiocyanate and tetrabutylammonium chloride are added to a cobalt(II) solution, a water-insoluble ion-pair is formed; this compound is soluble in chloroform, isoamyl alcohol, and other solvents. The variables which affect the extractions by these two solvents are studied in order to obtain the optimal conditions and two alternative extraction procedures are proposed. Thus, the separation of cobalt as a previous step for its spectrophotometric determination is studied. 相似文献
57.
Lszl Hazai Gyula Dek Jzsef Tams Gbor Doleschall Gabor Tth 《Journal of heterocyclic chemistry》1985,22(6):1491-1495
4-Benzylidene-1-phenyl-1,4-dihydro-3(2H)-isoquinolinone, the intermediary product of the carbonyl condensation reaction between 1-phenyl-1,4-dihydro-3(2H)-isoquinolinone and benzaldehyde, rearranges in the presence of an equivalent quantity of sodium hydride into 4-benzyl-1-phenyl-3(2H)-isoquinolinone. As the possibility of the migration of the hydrogen at C-1 in the form of a proton or a hydrogen atom (radical reaction) was excluded, the mechanism of the rearrangement could be depicted as an intermolecular hydride anion migration. In case of the 1-(4-pyridyl)- and 1-(3-pyridyl)-1,4-dihydro-3(2H)-isoquinolinones, however, the rearrangement can be carried out also in polyphosphoric acid and in this case a proton loss-proton gain mechanism was proved. 相似文献
58.
Graciela Díaz de Delgado Ramírez V. Belkis Miguel Delgado José Rosquete P. Carmelo 《Journal of chemical crystallography》1995,25(7):371-374
The title compound, formula C15H20O2, is orthorhombic, P212121 witha=8.747(2),b=12.025(3),c=12.554(3)Å,Z=4, andD
m
=1.32(2)g/ml. The structural analysis shows that the compound corresponds to eudesma-4(15),7(11)-dien-8,12-olide, a sesquiterpene lactone previously isolated fromAster umbellatus but whose crystal structure was unknown. 相似文献
59.
60.
González-Díaz PF 《Physical review D: Particles and fields》1995,51(10):5698-5706