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排序方式: 共有840条查询结果,搜索用时 15 毫秒
831.
Michael Polet Wim Van Gansbeke Koen Deventer Peter Van Eenoo 《Biomedical chromatography : BMC》2013,27(2):259-266
The administration of anabolic steroids is one of the most important issues in doping control and is detectable through a change in the carbon isotopic composition of testosterone and/or its metabolites. Gas chromatography‐combustion‐isotope ratio mass spectrometry (GC‐C‐IRMS), however, remains a very laborious and expensive technique and substantial amounts of urine are needed to meet the sensitivity requirements of the IRMS. This can be problematic because only a limited amount of urine is available for anti‐doping analysis on a broad spectrum of substances. In this work we introduce a new type of injection that increases the sensitivity of GC‐C‐IRMS by a factor of 13 and reduces the limit of detection, simply by using solvent vent injections instead of splitless injection. This drastically reduces the amount of urine required. On top of that, by only changing the injection technique, the detection parameters of the IRMS are not affected and there is no loss in linearity. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
832.
Anne Vuorema Mika Sillanpää Marianna Vehviläinen Taina Kamppuri Pertti Nousiainen Wim Thielemans Frank Marken 《Electroanalysis》2013,25(7):1773-1779
Reconstituted cellulose spontaneously self‐assembles at surfaces from an alkaline cellulose solution (ca. 1 wt%, pH 14, prepared with an enzymatic method from wood pulp) into porous films with approximately 300 nm thickness per layer, for example onto immersed tin‐doped indium oxide (ITO) electrodes. Sequential multi‐layer deposition allows control over the thickness of the assembled films. The hydrophilic properties of the cellulose film electrodes are utilised here (i) as dip‐probe with capillary force picking up sample solution and (ii) as flow‐through generator‐collector probe, for example for future application in in situ chromatographic separation in end‐column detection with nano‐molar sensitivity. 相似文献
833.
Atomic force microscopy (AFM) was used to measure single interaction forces between corrole (host) and phenol derivatives (guests) in aqueous media. A gold tip was modified with thiol derivatives of corrole via the Au–S covalent bond. Such a tip was used to measure adhesion forces with a planar gold substrate modified with thiol derivatives of phenol and ortho-nitrophenol in aqueous solutions. The mean force between the corrole and ortho-nitrophenol was higher than that between corrole and phenol, probably reflecting stronger hydrogen bond interaction in the former complex. In the presence of a supporting electrolyte (0.1 M K2SO4), the mean force increased, suggesting that electrostatic and π–π interactions play an essential role in the adhesion force. In addition, the adhesion force measured at pH 6.0 was larger than that at pH 10, reflecting the electrostatic repulsion at the higher pH. These behaviours are consistent with the potentiometric responses of a liquid membrane based on corrole to phenolic compounds. Also, the values of forces for the interaction between corrole and phenol derivatives showed the same tendency as energy calculated for these complexes. The Poisson method was used for the calculation of the single force of the chemical bond between the corrole host and the phenolic guests. 相似文献
834.
835.
α-Allyloxyaldoximes 8, formed by the reduction of β-nitrostyrenes 5 with SnCl2-2H2O in the presence of an unsaturated alcohol, undergo either thermally induced intramolecular oxime olefin cycloaddition (IOOC) to bicyclic isoxazolidines 7, with stereospecific introduction of three stereocenters, or nitrile oxide olefin cycloadditions (INOC) to bicyclic isoxazolines 6. This provides an entry into functionalized tetrahydrofurans and tetrahydropyrans. 相似文献
836.
Pieter A.M. van der Klein Wim Filemon Helgo J.G. Broxterman Gijs A. van der Marel Jacques H. van Boom 《合成通讯》2013,43(12):1763-1771
Polyhydroxylated pyrrolidines are readily accessible by ring opening of a 1,4-cyclic sulfate function in pentitol derivatives by nitrogen nucleophiles and further processing of the in situ generated charged sulfate group. 相似文献
837.
The Responsive Particle Dynamics model is a very efficient method to account for the transient forces present in complex fluids, such as solutions of entangled polymers. This coarse-grained model considers a solution of particles that are made of a core and a corona. The cores typically interact through conservative interactions, while the coronae transiently penetrate each other to form short-lived temporary interactions, typically of entropic origin. In this study, we reformulate the resulting rheological model within the general framework of nonequilibrium thermodynamics called General Equation for the Nonequilibrium Reversible–Irreversible Coupling. This allows us to determine the consistency of the model, from a mechanistic and thermodynamic point of view, and to isolate the reversible and irreversible contributions to the dynamics of the model system. 相似文献
838.
Michael Wübbenhorst Ernout M. Van Koten John C. Jansen Wim Mijs Jan van Turnhout 《Macromolecular rapid communications》1997,18(2):139-147
The molecular dynamics of amorphous and liquid-crystalline (LC) side-chain polycarbonates was studied by dielectric spectroscopy at frequencies from 10−2 to 106 Hz and at temperatures from −160 to 180°C. ‘Model’ compounds containing no mesogenic side-groups showed two relaxations, which originate from the carbonate group (α, βm-relaxation). By contrast, in LC-polycarbonates bearing a mesogenic nitrostilbene side group around and above the glass transition temperature Tg up to three relaxation modes were distinguished (α-, λ1-, λ2-process); below Tg four secondary relaxations (γ-, βm-, βs-, βsc-relaxation) were observed. The γ-relaxation was found only in compounds possessing an aliphatic spacer linked to the backbone by an ether bond. Apart from βm-, two additional β-processes were identified as relaxations associated with the mesogenic unit in the glassy (βs) or in the crystalline state (βsc). 相似文献
839.
Peter A. Cuypers George M. Willems H. Coenraad Hemker Wim Th. Hermens 《Macromolecular Symposia》1988,17(1):155-159
A model for protein adsorption kinetics is presented. This model includes diffusion limited adsorption, adsorption and desorption rate constants which are dependent on the surface concentration and an interaction term for the mutual influence of the adsorbed protein molecules. It is shown that, in first approximation, the values of the adsorption and desorption rate constants are exponential functions of the surface concentration. Assuming an adequate interaction term it is possible to show with this model for the adsorption kinetics of a mixture of proteins that the ratio of the adsorbed proteins is strongly dependent on the overall surface concentration even if the ratio of the bulk concentrations of these proteins is kept constant. Differences in interaction terms for the different proteins offer a possible explanation for the peculiar behaviour of plasma protein adsorption on a surface at different dilutions of the plasma, the so called “Vroman effect”. 相似文献
840.