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161.
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Heidi L. Shimp 《Tetrahedron》2009,65(31):5908-5111
A convergent formal synthesis of the antimitotic macrolide dictyostatin has been achieved. The C11-C26 fragment of dictyostatin was prepared via convergent assembly of the central deoxypropionate motif utilizing a site- and stereoselective titanium-mediated reductive cross-coupling and an asymmetric hydrogenation of the resulting stereodefined 1,3-diene. 相似文献
164.
The accurate and precise determination of methanol in crude oils at concentrations less than 10 ppm is of economic value to the petroleum industry. This report presents the optimization, results and long term performance of a flow switching device MDGC hardware, the Swafer?, for the rapid and precise analysis of methanol from approximately 0.4 ppm (w/w) to 1000 ppm. The use of low temperature injection and backflush technique decreases maintenance and increases sample throughput. The short term quantitative percent relative standard deviations at 1, 30 and 1000 ppm (w/w) methanol in crude oils are 5, 3 and 0.3, respectively. The MDGC procedure follows closely that described in ASTM D7059 for determination of crude oils; however, D7059 previously has not been evaluated at concentrations of less than 10 ppm. This work further extends the application of D7059 to concentrations of <10 ppm. 相似文献
165.
Garcia-Sartal C Taebunpakul S Stokes E Barciela-Alonso Mdel C Bermejo-Barrera P Goenaga-Infante H 《Analytical and bioanalytical chemistry》2012,402(10):3359-3369
Edible seaweed consumption is a route of exposure to arsenic. However, little attention has been paid to estimate the bioaccessibility
and/or bioavailability of arsenosugars in edible seaweed and their possible degradation products during gastrointestinal digestion.
This work presents first use of combined inductively coupled plasma mass spectroscopy (ICP-MS) with electrospray ionization
tandem mass spectrometry (ESI-MS/MS) with two-dimensional HPLC (size exclusion followed by anion exchange) to compare the
qualitative and quantitative arsenosugars speciation of different edible seaweed with that of their bioavailable fraction
as obtained using an in vitro gastrointestinal digestion procedure. Optimal extraction conditions for As species from four
seaweed namely kombu, wakame, nori and sea lettuce were selected as a compromise between As extraction efficiency and preservation
of compound identity. For most investigated samples, the use of ammonium acetate buffer as extractant and 1 h sonication in
a water bath followed by HPLC-ICP-MS resulted in 40–61% of the total As to be found in the buffered aqueous extract, of which
86–110% was present as arsenosugars (glycerol sugar, phosphate sugar and sulfonate sugar for wakame and kombu and glycerol
sugar and phosphate sugar for nori). The exception was sea lettuce, for which the arsenosugar fraction (glycerol sugar, phosphate
sugar) only comprised 44% of the total extracted As. Interestingly, the ratio of arsenobetaine and dimethylarsinic acid to
arsenosugars in sea lettuce extracts seemed higher than that for the rest of investigated samples. After in vitro gastrointestinal
digestion, approximately 11–16% of the total As in the solid sample was found in the dialyzates with arsenosugars comprising
93–120% and 41% of the dialyzable As fraction for kombu, wakame, nori and sea lettuce, respectively. Moreover, the relative
As species distribution in seaweed-buffered extracts and dialyzates was found to be very similar. Collection of specific fractions
from the size-exclusion column to be analysed using anion-exchange HPLC-ESI-MS/MS enabled improved chromatographic selectivity,
particularly for the less retained arsenosugar (glycerol sugar), facilitating confirmation of the presence of arsenosugars
in seaweed extracts and dialyzates. Using this approach, the presence of arsenobetaine in sea lettuce samples was also confirmed. 相似文献
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Phillips H Zheng S Hyla A Laine R Goodson T Geva E Dunietz BD 《The journal of physical chemistry. A》2012,116(4):1137-1145
Recent advances in the ability to functionalize octahedral silsesquioxanes with different photoactive ligands, and thereby tune their optical properties, suggest that these molecules may serve as potential building blocks of light-harvesting, photovoltaic, and photonic devices. In this paper we report extensive ab initio calculations of the excitation energies underlying the absorption spectra of these systems. The calculations are based on density functional theory for the ground electronic state and time-dependent density functional theory for the excited electronic states. The ability of the commonly used B3LYP functional to reproduce the experimentally observed absorption excitation energies is compared to that of recently developed range-separated hybrid functionals. The importance of pairing the range-separated hybrid functionals with basis sets that include diffuse and polarization basis functions is demonstrated in the case of vinyl-functionalized silsesquioxanes. Absorptive excitation energies are then calculated and compared with experiment for octahedral silsesquioxanes functionalized with larger ligands. The tunability of optical properties is demonstrated by considering the effect on the excitation energies of functionalizing the ligands with electron-donating or -withdrawing groups. 相似文献
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Julien Heroult Volker Nischwitz Dorota Bartczak Heidi Goenaga-Infante 《Analytical and bioanalytical chemistry》2014,406(16):3919-3927
This work represents a first systematic approach to the size-based elemental quantification and size estimation of metal(loid) oxide nanoparticles such as silica (SiO2) in a real food matrix using asymmetric flow field-flow fractionation coupled online with inductively coupled plasma mass spectrometry (ICP-MS) and multi-angle light scattering (MALS) and offline with transmission electron microscopy (TEM) with energy-dispersive X-ray analysis (EDAX). Coffee creamer was selected as the model sample since it is known to contain silica as well as metal oxides such as titania at the milligramme per kilogramme levels. Optimisation of sample preparation conditions such as matrix-to-solvent ratio, defatting with organic solvents and sonication time that may affect nanoparticle size and size distribution in suspensions was investigated. Special attention was paid to the selection of conditions that minimise particle transformation during sample preparation and analysis. The coffee creamer matrix components were found to stabilise food grade SiO2 particles in comparison with water suspensions whilst no significant effect of defatting using hexane was found. The use of sample preparation procedures that mimic food cooking in real life was also investigated regarding their effect on particle size and particle size distribution of silica nanoparticles in the investigated food matrix; no significant effect of the water temperature ranging from ambient temperature to 60 °C was observed. Field-flow fractionation coupled to inductively coupled plasma-mass spectrometry (FFF-ICP-MS) analysis of extracts of both unspiked coffee creamer and coffee creamer spiked with food grade silicon dioxide, using different approaches for size estimation, enabled determination of SiO2 size-based speciation. Element-specific detection by ICP-MS and post-FFF calibration with elemental calibration standards was used to determine the elemental composition of size fractions separated online by FFF. Quantitative data on mass balance is provided for the size-based speciation of the investigated inorganic nano-objects in the complex matrix. The combination of FFF with offline fractionation by filtration and with detection by ICP-MS and TEM/EDAX has been proven essential to provide reliable information of nanoparticle size in the complex food matrix. Figure
Characterisation of silica nanoparticles in a coffee creamer matrix using FFF-based methodology 相似文献