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排序方式: 共有660条查询结果,搜索用时 31 毫秒
651.
A simple method for the determination of mercury in water samples after preconcentration using dispersive liquid-liquid microextraction is described. The procedure is based on the extraction of mercury in the form of a complex and its subsequent determination by spectrophotometry. The complex is formed between Hg(II) and 2-(2-benzothiazolylazo)-p-cresol. The detection at 650 nm is performed directly in the metal-rich phase, which is spread on a triacetylcellulose membrane. The method eliminates the need to use a cuvet or large quantities of samples and reagents. The parameters that influence the preconcentration were studied, and the analytical characteristics were determined. The enrichment factor and the consumptive index for this method were 64 and 0.16 mL, respectively. The LOD (3.3 microg/L) and LOQ (11.1 microg/L) were also determined. The accuracy of the method was tested by the determination of mercury in certified reference materials BCR 397 (Human Hair) and SRM 2781 (Domestic Sludge). The method was applied to the determination of mercury in samples of drinking water, sea water, and river water. 相似文献
652.
MR Seyedsayamdost S Cleto G Carr H Vlamakis M João Vieira R Kolter J Clardy 《Journal of the American Chemical Society》2012,134(33):13550-13553
Interrogation of the evolutionary history underlying the remarkable structures and biological activities of natural products has been complicated by not knowing the functions they have evolved to fulfill. Siderophores-soluble, low molecular weight compounds-have an easily understood and measured function: acquiring iron from the environment. Bacteria engage in a fierce competition to acquire iron, which rewards the production of siderophores that bind iron tightly and cannot be used or pirated by competitors. The structures and biosyntheses of "odd" siderophores can reveal the evolutionary strategy that led to their creation. We report a new Serratia strain that produces serratiochelin and an analog of serratiochelin. A genetic approach located the serratiochelin gene cluster, and targeted mutations in several genes implicated in serratiochelin biosynthesis were generated. Bioinformatic analyses and mutagenesis results demonstrate that genes from two well-known siderophore clusters, the Escherichia coli enterobactin cluster and the Vibrio cholera vibriobactin cluster, were shuffled to produce a new siderophore biosynthetic pathway. These results highlight how modular siderophore gene clusters can be mixed and matched during evolution to generate structural diversity in siderophores. 相似文献
653.
Indole alkaloids are the chemotaxonomic markers of the Aspidosperma genera. Those that have the simplest plumeran skeleton are classified as the precursors of biosynthetic routes and the intermediates for several synthetic reactions. This work aims to review the 1H and 13C-NMR data, up to 2011, describing the skeleton of 35 different plumeran indole alkaloids, from a group of 46 of them, and highlight the main spectral differences amongst them. 相似文献
654.
Douglas A. G. Vieira Ricardo H. C. Takahashi Rodney R. Saldanha 《Mathematical Programming》2012,131(1-2):131-161
This work presents an algorithm for multiobjective optimization that is structured as: (i) a descent direction is calculated, within the cone of descent and feasible directions, and (ii) a multiobjective line search is conducted over such direction, with a new multiobjective golden section segment partitioning scheme that directly finds line-constrained efficient points that dominate the current one. This multiobjective line search procedure exploits the structure of the line-constrained efficient set, presenting a faster compression rate of the search segment than single-objective golden section line search. The proposed multiobjective optimization algorithm converges to points that satisfy the Kuhn-Tucker first-order necessary conditions for efficiency (the Pareto-critical points). Numerical results on two antenna design problems support the conclusion that the proposed method can solve robustly difficult nonlinear multiobjective problems defined in terms of computationally expensive black-box objective functions. 相似文献
655.
Soraia Carvalho de Souza Mary Cristina Ferreira Alves A. Luiz M. de Oliveira E. Longo F. Ticiano Gomes Vieira Rodinei Medeiros Gomes L. E. B. Soledade A. G. de Souza Iêda M. Garcia dos Santos 《Journal of Thermal Analysis and Calorimetry》2009,97(1):185-190
In this work, the synthesis of Nd-doped SrSnO3 by the polymeric precursor method, with calcination between 250 and 700 °C is reported. The powder precursors were characterized
by TG/DTA and high temperature X-ray diffraction (HTXRD). After heat treatment, the material was characterized by XRD and
infrared spectroscopy. Ester and carbonate amounts were strictly related to Nd-doping. According to XRD patterns, the orthorhombic
perovskite was obtained at 700 °C for SrSnO3 and SrSn0.99Nd0.01O3. For Sr0.99Nd0.01SnO3, the kinetics displayed an important hole in the crystallization process, as no peak was observed in HTXRD up to 700 °C,
while a XRD patterns showed a crystalline material after calcination at 250 °C. 相似文献
656.
Jose V. Silveira Luciana L. Vieira Josue Mendes Filho Antonio J. C. Sampaio Oswaldo L. Alves Antonio G. Souza Filho 《Journal of Raman spectroscopy : JRS》2012,43(10):1407-1412
This paper reports a study of vibrational, structural and morphological properties of molybdenum oxide nanoribbons. Temperature‐dependent Raman spectroscopy measurements in MoO3 nanoribbons revealed morphological changes in the 150–350 °C temperature range. No structural phase transitions were observed, thus showing that the orthorhombic phase is stable from room temperature (nanoribbons) up to 650 °C (bulk‐like phase) where large plates have been formed by the coalescence of the nanoribbons. The interpretation of temperature‐dependent Raman data (wavenumber and linewidths) is supported by scanning electron microscopy that is used to directly probe the morphological changes in MoO3 samples. The observed phenomena in the Raman data for MoO3 nanoribbons can be applied to other nanomaterials. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
657.
J.A. Galvis M. Crespo I. Guillamón H. Suderow S. Vieira M. García Hernández S. Bud'ko P.C. Canfield 《Solid State Communications》2012,152(12):1076-1079
We present measurements of the superconducting upper critical field Hc2(T) and the magnetic phase diagram of the superconductor ErNi2B2C made with a scanning tunneling microscope (STM). The magnetic field was applied in the basal plane of the tetragonal crystal structure. We have found large gapless regions in the superconducting phase diagram of ErNi2B2C, extending between different magnetic transitions. A close correlation between magnetic transitions and Hc2(T) is found, showing that superconductivity is strongly linked to magnetism. 相似文献
658.
Koehler PE Reifarth R Ullmann JL Bredeweg TA O'Donnell JM Rundberg RS Vieira DJ Wouters JM 《Physical review letters》2012,108(14):142502
We obtained the total radiation widths of s-wave resonances through an R-matrix analysis of (147)Sm(n,γ) cross sections. Distributions of these widths differ markedly for resonances below and above E(n)=300 eV, which is in stark contrast to long-established theory. We show that this change, as well as a similar change in the neutron-width distribution reported previously, is reflected in abrupt increases in both the average (147)Sm(n,γ) cross section and fluctuations about the average near 300 eV. Such effects could have important consequences for applications such as nuclear astrophysics and nuclear criticality safety. 相似文献
659.
Vieira Manuel V. C. Carvalho Margarida 《4OR: A Quarterly Journal of Operations Research》2023,21(3):491-512
4OR - Motivated by a real-world application, we present a multi-container loading problem with 3-open dimensions. We formulate it as a biobjective mixed-integer nonlinear program with lexicographic... 相似文献
660.
Priscila Fernanda Pereira Barbosa Valmor Roberto Mastelaro Eduardo Guimarães Vieira Devaney Ribeiro do Carmo 《Electroanalysis》2023,35(1):e202100628
A synthesis of β-cyclodextrin (β-Cd) functionalized with polyamide dendrimer PAMAM G.0 complexed with silver and potassium hexacyanoferrate (III) forming a mixed-valence complex (β-Cd-PAMAM-Ag/Fe) was proposed. This material was characterized by scanning electron microscopy (SEM), infrared spectroscopy (FT-IR), X-Ray diffraction (XRD), Energy Dispersion Spectroscopy (EDS) and Electron Paramagnetic Resonance (EPR). The hybrid complex was electrochemically investigated using cyclic voltammetry and some parameters (electrolyte, concentration of electrolyte, pH and scan rate) were evaluate in order to obtain optimum analytical responses. The β-Cd-PAMAM-Ag/Fe material was successfully applied in the electrocatalytic oxidation of dopamine (DOP) using Cyclic Voltammetry (CV) and Square Wave Voltammetry (SWV). 相似文献