共查询到20条相似文献,搜索用时 0 毫秒
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G. H. Galbraith J. Guo R. C. McLean A. S. M. Y. Munsi C. H. Sanders 《Materials and Structures》2001,34(7):389-395
This study presents a numerical simulation of the performance of an NMR spectrometer in which the influence of equipment with
both low and high spatial resolutions is investigated. The results indicate that a low resolution NMR with a wide sensitivity
curve may be unable to provide an accurate estimation of the moisture profile along the whole length of a test sample where
the moisture gradient changes abruptly. It cannot provide accurate moisture information over a distance measured from each
end of the test sample equivalent to the effective width of the sensitivity curve. However, a low-resolution spectrometer
does not show any significant errors when calculating liquid diffusion coefficients, although the range of moisture contents
over which they can be measured is limited. This study suggests that to avoid errors and practical difficulties, NMR spectrometers
for moisture measurement should be selected carefully by matching the effective width of the sensitivity curve to the moisture
properties of the material to be measured.
Résumé Cette étude présente une simulation numérique de l'exécution d'un spectromètre RMN étudiant l'influence d'un matériel à basses et hautes résolutions spatiales. Les résultats indiquent qu'il se peut qu'une basse résolution du RMN avec une courbe large de sensibilité ne soit pas en mesure de fjurnir une évaluation précise du profil d'humidité sur la longueur entière d'un échantillon d'essai où le gradient d'humidité change soudainement. Il ne peut pas fournir d'informations précises sur l'humidité au-dessus d'une distance mesurée à partir de chaque extrémité de l'échantillon d'essai équivalant à la largeur effective de la courbe de sensibilité. Cependant, un spectromètre à basse résolution ne montre aucune erreur significative lors du calcul des coefficients de diffusion liquide, bien que l'intervalle du contenu d'humidité au-dessus duquel elles peuvent être mesurées soit limité. Cette étude suggère que pour éviter des erreurs et des difficulté pratiques, les spectromètres RMN mesurant l'humidité soient choisis soigneusement en associat la largeur effective de la courbe de sensibilité aux propriétés d'humidité du matériel à mesurer.相似文献
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Pigments of titanium dioxide rutile, coated with SiO2>-Al2O3, have been characterized by high-resolution solid state NMR. The model samples were prepared following four synthesis routes differing by the aluminate and silicate salts addition sequence. From 27Al single pulse MAS and 3Q-MAS NMR experiments, we characterize short range order around Al and distinguish several types of environment: AlIV, AlV, AlVI and AlVI linked to Ti, depending upon synthesis route and thermal treatment. The major difference between the different samples is observed after heat treatment at 750 °C. Rotational echo double resonance 27Al-1H and cross-polarization H-Si experiments provide longer range distance information through dipolar coupling to proton. Two types of surface treatment can be distinguished from 27Al-1H Redor by the presence of proton free alumina domain in the surface treatment. 相似文献
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Multivariate curve resolution is proposed for the study of complex chemical reactions monitored by two-dimensional (2D) NMR spectroscopy. In particular, in this work, multivariate curve resolution is applied to the study of the reaction between (15)N-labeled cisplatin and the amino acid-nucleotide hybrid (Phac-Met-linker-p(5')dG). At several stages of the reaction, 2D [(1)H,(15)N] HSQC NMR spectra were acquired and stored in data matrices. In a first step, multivariate curve resolution was applied to analyze individually each one of these 2D spectra, allowing the resolution of the corresponding (1)H and (15)N one-dimensional correlation spectra. In a second step, the whole set of 2D spectra recorded along the reaction were simultaneously analyzed by multivariate curve resolution, allowing the resolution of the kinetic concentration profiles and of the pure 2D NMR spectra of each of the species detected along the reaction. Results finally obtained confirmed previously postulated reaction mechanisms involving the existence of two monofunctional adducts and of two bifunctional adducts, with the structure of one of them not completely resolved. 相似文献
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Vacuum line components such as demountable seals, mechanical feed-throughs and valves suitable for ultra-high vacuum application are reviewed, with emphasis on the most recent developments. 相似文献
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M. Laridjani E. Lafontaine J. P. Bayle P. Judeinstein 《Journal of Materials Science》1999,34(24):5945-5953
Hybrid organic-inorganic materials, silica-poly(ethylene glycol) (PEG) blends, were prepared by the sol-gel process from mixtures of tetraethoxysilane and PEG of low molecular mass. The synthesis scheme (acidic [HCl] or nucleophilic [NH4F] catalysis) influences the structure of these materials and consequently their properties. Two different methods were used to investigate the structure of these blends: a) X-ray diffraction techniques; b) 29Si NMR spectroscopy. A new x-ray diffractometry technique identifies precise interference functions and radial distribution functions of these blends. The comparison of predicted radial distribution functions of the Bell and Dean's physical model refined by Gaskell with the radial distribution function obtained from this technique is implemented to identify the structure of these blends. Analysis by amorphography has identified the existence of SiO2 silica grains and provides only about the positional disorder of these grains in continuum random network. The NMR spectroscopy discriminates the different silicon sites and demonstrates the changes of the morphology and structure when the nature of the catalyst is modified. These results indicate that the structure of non-crystalline SiO2 aggregates inside nanocomposites differs from fused glass by their compositional disorders. These nanocomposites could be described as an agglomerate of SiO2 objects with the pores filled by disordered polymer chains. When these materials are obtained under acidic conditions, the polymer chains are linked to the SiO2 grains forming an ideal composite. 相似文献
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The NMR chemical shift of spin 1/2 nuclei in a polyprotic molecule represents a sensitive probe of microscopic protonation equilibria. However, these equilibria are commonly parametrized in terms of microscopic equilibrium constants, whose number increases very rapidly with the number of ionizable groups. For that reason their determination was considered to be basically impossible except for the cases of the simplest molecules. On the basis of a cluster expansion of the free energy of a microstate, we propose a novel parametrization of this problem that drastically reduces the number of necessary parameters needed to specify the microscopic equilibria. Such cluster parameters can be extracted from NMR titration data in a straightforward way. Once these parameters are known, all microscopic equilibrium constants can be obtained. 相似文献
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Wong A Jiménez B Li X Holmes E Nicholson JK Lindon JC Sakellariou D 《Analytical chemistry》2012,84(8):3843-3848
High-resolution magic-angle sample spinning (HR-MAS) (1)H NMR spectroscopy of tissue biopsies combined with chemometric techniques has emerged as a valuable methodology for disease diagnosis and environmental assessments. However, the tissue mass required for such experiments is of the order of 10 mg, and this can compromise the metabolic evaluation because of tissue heterogeneity. Tissue availability is often a limitation for clinical studies due to histopathological requirements, which are currently the gold standard for diagnosis, for example, in the case of tumors. Here, we introduce the use of a rotating micro-NMR detector that optimizes the coil filling factor such that mass-limited samples can be measured. We show the results for measuring nanoliter volume tissue biopsies using a commercial HR-MAS probe for the first time. The method has been tested with bovine muscle and human gastric mucosal tumor tissue samples. The gain in mass sensitivity is approximate up to 17-fold, and the adequate spectral resolution (3 Hz) allows the measurement of the metabolite profiles in nanoliter volume samples, thereby limiting the ambiguity resulting from heterogeneous tissues; thus, the approach presents diagnostic potential for studies by metabonomics of mass-limited biopsies. 相似文献
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Ryohei Tanaka 《Materials at High Temperatures》2013,30(4):237-238
An encapsulation technique for high-temperature controlled-environment testing is introduced. This encapsulation technique is combined with the DC potential drop technique to monitor the crack growth during the experiment. This technique eliminates the complicated retort systems of conventional mechanical testing and possesses the advantages of compactness, transportability, ease of manufacture and low cost. Test results in vacuum using this technique are compared with results of similar experiments in air and in a retort system. 相似文献
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