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排序方式: 共有337条查询结果,搜索用时 15 毫秒
41.
Decroos K Vincken JP Heng L Bakker R Gruppen H Verstraete W 《Journal of chromatography. A》2005,1072(2):185-193
A novel method utilizing high-performance liquid chromatography (HPLC) with evaporative light scattering detection (ELSD) and electrospray ionisation mass spectrometry (ESI-MS) was developed for the analysis of soyasaponins, a divers group of triterpenic compounds with one or two sugar side chains, occurring in soy. Group A soyasaponins in different degrees of acetylation, as well as group B soyasaponins in both their 2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP)-conjugated and non-conjugated forms could be separated and quantified using authentic soyasaponin standards, in one single run. The method was tested by the determination of the soyasaponin content and composition of eight soygerm samples of different origin. Differences in the composition and the degree of acetylation of the group A soyasaponins were observed among these samples. The group B soyasaponins showed much less variability and they were mainly present in their DDMP-conjugated form. 相似文献
42.
43.
Bernard L. G. Bakker 《Few-Body Systems》2011,49(1-4):177-181
Calculations in light-front quantization are sometimes found to lead to singularities that are not present in the corresponding manifestly covariant treatment. We give some examples that were found in the framework of perturbation theory, but must also occur in nonperturbative calculations. In the case of anomalies, regularization-scheme dependences were found that not only occur between the light-front approach and manifestly covariant calculations, but also among the latter ones. 相似文献
44.
Domingos SR Panman MR Bakker BH Hartl F Buma WJ Woutersen S 《Chemical communications (Cambridge, England)》2012,48(3):353-355
Vibrational circular dichroism is a powerful technique to study the stereochemistry of chiral molecules, but often suffers from small signal intensities. Electrochemical modulation of the energies of the electronically excited state manifold is now demonstrated to lead to an order of magnitude enhancement of the differential absorption. Quantum-chemical calculations show that increased mixing between ground and excited states is at the origin of this amplification. 相似文献
45.
We investigate the reflection near the reststrahlen band of the optical phonon in bulk GaAs in the time domain, using time-resolved terahertz spectroscopy. We find that the dynamics of the reflection measured for GaAs differs strongly from the reflection dynamics that would be expected for a TO phonon with a frequency-independent dephasing time. 相似文献
46.
Karin Y. Chumbimuni-Torres Eric Bakker Joseph Wang 《Electrochemistry communications》2009,11(10):1964-1967
This Communication demonstrates the ability of potentiometric ion-selective electrodes (ISE) to probe the growth dynamics of metal nanoparticles in real-time. The new monitoring capability is illustrated using a solid-contact silver ISE for monitoring the hydroquinone-induced precipitation of silver on gold nanoparticle seeds. Potential-time recordings obtained under different conditions are used to monitor the depletion of the silver ion during the nanoparticle formation and shed useful insights into the growth dynamics of the nanoparticles. Such potentiometric profiles correlate well with the analogous optical measurements. The new real-time electrochemical probing of the particle growth process reflects the direct, rapid and sensitive response of modern ISE to changes in the level of the precipitated metal ion from the bulk solution and holds considerable promise for probing the preparation of different nanoscale materials. 相似文献
47.
Coulometry belongs to one of the few known calibration-free techniques and is therefore highly attractive for chemical analysis. Titrations performed by the coulometric generation of reactants is a well-known approach in electrochemistry, but suffers from limited selectivity and is therefore not generally suited for samples of varying or unknown composition. Here, the selective coulometric release of ionic reagents from ion-selective polymeric membrane materials ordinarily used for the fabrication of ion-selective electrodes is described. The selectivity of such membranes can be tuned to a significant extent by the type and concentration of ionophore and lipophilic ion-exchanger and is today well understood. An anodic current of fixed magnitude and duration may be imposed across such a membrane to release a defined quantity of ions with high selectivity and precision. Since the applied current relates to a defined ion flux, a variety of non-redox active ions may be accurately released with this technique. In this work, the released titrant's activity was measured with a second ionophore-based ion-selective electrode and corresponded well with expected dosage levels on the basis of Faraday's law of electrolysis. Initial examples of coulometric titrations explored here include the release of calcium ions for complexometric titrations, including back titrations, and the release of barium ions to determine sulfate. 相似文献
48.
L. H. Bowen E. De Grave P. M. A. De Bakker R. E. Vandenberghe 《Hyperfine Interactions》1990,54(1-4):467-472
Six goethite samples α(Fe, Al)OOH prepared by a similar synthetic method but varying in Al substitution from 0 to 31 mole percent, have been examined by Mössbauer spectroscopy at ≌ 4 K in an external field of 6 T. The resulting spectra have been fitted by two methods: first a simplified model consisting of three distributions of magnetic fields (parallel, perpendicular and antiparallel orientations of internal and external field) and second, a model-independent bi-dimensional distribution of both the internal hyperfine field and the orientation angle ? with the external field. Both fitting methods show that the angular distribution of spins is not random in the case of high Al substitution, but canted towards an antiparallel orientation (?>90°). 相似文献
49.
P. M. A. de Bakker E. De Grave R. E. Vandenberghe L. H. Bowen 《Hyperfine Interactions》1990,54(1-4):493-498
Two small-particle maghemite (λ-Fe2O3) samples have been investigated with the Mössbauer effect. From the results of the model-independent hyperfine-field distribution fits some characteristic temperature-dependent parameters have been obtained. The spectra at the lowest temperatures could be fitted with two strongly overlapping hyperfine-field distributions with different isomer shifts. Spectra in applied magnetic fields ranging from 40 to 60 kOe, and at 4.2 K showed non vanishing Δm 1=0 absorption lines. The hyperfine-field distribution and canting-angle distribution method for fitting these spectra did not yield reasonable results. Therefore, a bi-dimensional hyperfine-field-canting-angle distribution has been applied, and was found to reproduce the experimental line shapes with remarkable adequacy. The resulting distribution profiles revealed a linear correlation between Hhf and the angle between the magnetic moments and the external field. 相似文献
50.
Planar Raman imaging through a spectrograph is demonstrated as a diagnostic tool for quantitative flow visualisation of internal supersonic wedge flow. A dedicated Bayesian deconvolution filter is used to remove the spectral structure that is introduced by the spectrograph. The 2D density field is determined with ca. 10% precision using average images over 6,000 laser pulses, down to 0.5 mm from the surface of the wedge. Direct interpretations of Raman intensities provide more precise density data than indirect interpretations based on shock geometry in 2D inviscid flow.
相似文献
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