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排序方式: 共有513条查询结果,搜索用时 343 毫秒
191.
Geraldine S.C. Turner Graham A. Mills Peter R. Teasdale Jonathan L. Burnett Sean Amos Gary R. Fones 《Analytica chimica acta》2012
Three adsorbents (Chelex-100, manganese dioxide [MnO2] and Metsorb), used as binding layers with the diffusive gradient in thin film (DGT) technique, were evaluated for the measurement of inorganic uranium species in synthetic and natural waters. Uranium (U) was found to be quantitatively accumulated in solution (10–100 μg L−1) by all three adsorbents (uptake efficiencies of 80–99%) with elution efficiencies of 80% (Chelex-100), 84% (MnO2) and 83% (Metsorb). Consistent uptake occurred over pH (5–9), with only MnO2 affected by pH < 5, and ionic strength (0.001–1 mol L−1 NaNO3) ranges typical of natural waters, including seawater. DGT validation experiments (5 days) gave linear mass uptake over time (R2 ≥ 0.97) for all three adsorbents in low ionic strength solution (0.01 M NaNO3). Validation experiments in artificial sea water gave linear mass uptake for Metsorb (R2 ≥ 0.9954) up to 12 h and MnO2 (R2 ≥ 0.9259) up to 24 h. Chelex-100 demonstrated no linear mass uptake in artificial sea water after 8 h. Possible interferences were investigated with SO42− (0.02–200 mg L−1) having little affect on any of the three DGT binding layers. PO43− additions (5 μg L−1–5 mg L−1) interfered by forming anionic uranyl phosphate complexes that Chelex-100 was unable to accumulate, or by directly competing with the uranyl species for binding sites, as with MnO2 and the Metsorb. HCO3− (0.1–500 mg L−1) additions formed anionic species which interfered with the performance of the Chelex-100 and the MnO2, and the Ca2+ (0.1–500 mg L−1) had the affect of forming labile calcium uranyl species which aided uptake of U by all three resins. DGT field deployments in sea water (Southampton Water, UK) gave a linear mass uptake of U over time with Metsorb and MnO2 (4 days). Field deployments in fresh water (River Lambourn, UK) gave linear uptake for up to 7 and 4 days for Metsorb and MnO2 respectively. Field deployment of the Metsorb-DGT samplers with various diffusive layer thicknesses (0.015–0.175 cm) allowed accurate measurements of the diffusive boundary layer (DBL) and allowed DBL corrected concentrations to be determined. This DBL-corrected U concentration was half that determined when the effect of the DBL was not considered. The ability of the DGT devices to measure U isotopic ratios with no isotopic fractionation was shown by all three resins, thereby proving the usefulness of the technique for environmental monitoring purposes. 相似文献
192.
Douglas P. Hardin Amos P. Kendall Edward B. Saff 《Discrete and Computational Geometry》2013,50(1):236-243
We prove a conjecture of Ambrus, Ball and Erdélyi that equally spaced points maximize the minimum of discrete potentials on the unit circle whenever the potential is of the form $$\begin{aligned} \sum _{k=1}^n f(d(z,z_k)), \end{aligned}$$ ∑ k = 1 n f ( d ( z , z k ) ) , where $f:[0,\pi ]\rightarrow [0,\infty ]$ f : [ 0 , π ] → [ 0 , ∞ ] is non-increasing and convex and $d(z,w)$ d ( z , w ) denotes the geodesic distance between z and w on the circle. 相似文献
193.
Amos W Webb S Liu Y Andrews JC LeDuc DL 《Analytical and bioanalytical chemistry》2012,404(5):1277-1285
Selenium hyperaccumulator Stanleya pinnata, Colorado ecotype, was supplied with water-soluble and biologically available selenate or selenite. Selenium distribution and tissue speciation were established using X-ray microscopy (micro-X-ray fluorescence and transmission X-ray microscopy) in two dimensions and three dimensions. The results indicate that S. pinnata tolerates, accumulates, and volatilizes significant concentrations of selenium when the inorganic form supplied is selenite and may possess novel metabolic capacity to differentiate, metabolize, and detoxify selenite concentrations surpassing field concentrations. The results also indicate that S. pinnata is a feasible candidate to detoxify selenium-polluted soil sites, especially locations with topsoil polluted with soluble and biologically available selenite. 相似文献
194.
Differential synthetic access to an advanced C26-C40 northern hemisphere fragment of spirastrellolide B and to a related [5,5,7]-bis-spiroketal analogue from a common intermediate has been achieved. Central to this venture is the regiocontrolled functionalization of a C(31-32) alkyne, exploiting different transition metal catalysts (cf. Pt(II) and Au(I)). 相似文献
195.
Amos N. Koeller 《Calculus of Variations and Partial Differential Equations》2012,43(1-2):265-309
We consider n-dimensional hypersurfaces flowing by the mean curvature flow with Neumann free boundary conditions supported on a smooth support surface. Under assumptions mirroring those for the case of the mean curvature flow without boundary we show that the n-dimensional Hausdorff measure of the singular set is zero. 相似文献
196.
197.
Tafadzwa Kaseke Umezuruike Linus Opara Olaniyi Amos Fawole 《Molecules (Basel, Switzerland)》2021,26(15)
Enzymatic pretreatment of seeds is a novel approach that enhances the health benefits of the extracted oil. The study investigated the influence of the enzymatic pretreatment of seeds on the quality of oil from different pomegranate cultivars. The quality of the ultrasound-assisted (and ethanol-extracted) oil was studied, with respect to the refractive index (RI), yellowness index (YI), conjugated dienes (K232), peroxide value (PV) ρ-anisidine value (AV), total oxidation value (TOTOX), total carotenoid content (TCC), total phenolic compounds (TPC), fatty acid composition, phytosterol composition, ferric reducing antioxidant power (FRAP), and 2.2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging capacity. The seeds of three different pomegranate cultivars (‘Wonderful’, ‘Herskawitz’, and ‘Acco’) were digested with an equal mixture of Pectinex Ultra SPL, Flavourzyme 100 L, and cellulase crude enzymes, at a concentration, pH, temperature, and time of 1.7%, 4.5, 40 °C, and 5 h, respectively. Enzymatic pretreatment of PS increased oil yield, PV, TPC, TCC, and DPPH radical scavenging capacity, but decreased the YI. The levels of K232, AV and TOTOX, fatty acids, phytosterols, RI, and FRAP, were not significantly affected by enzymatic pretreatment of PS. Principal component analysis (PCA) established that oil extracted from the ‘Acco’ seed after enzymatic pretreatment had higher yield, TPC, TCC, and DPPH radical scavenging capacity. Therefore, enzyme-pretreated ‘Acco’ pomegranate fruit seed is a source of quality seed oil with excellent antioxidant properties. 相似文献
198.
The metal-insulator transition of nanoscaled VO2 devices is drastically different from the smooth transport curves generally reported. The temperature driven transition occurs through a series of resistance jumps ranging over 2 decades in magnitude, indicating that the transition is caused by avalanches. We find a power law distribution of the jump sizes, demonstrating an inherent property of the VO2 films. We report a surprising relation between jump magnitude and device size. A percolation model captures the general transport behavior, but cannot account for the statistical behavior. 相似文献
199.
We recalculate the Ricci tensors of non-stationary axisymmetric space-times originally calculated by Chandrasekhar, and we
find that in the linear regime there are some common factors that did not appear in the original results. We also find some
discrepancies in the non-linear terms. However, these discrepancies do not affect the well-known results concerning linear
perturbations of a Schwarzschild black hole. 相似文献