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1.
p palan BS 《Physical review letters》1987,58(16):1597-1599
2.
The size distribution, elemental composition, shape and surface texture of harmattan particulates of distinctly located points have been examined. The characterized particle size ranges are 0.7–16.4 μm and 0.6–19.7 μm for Kano and Ife, respectively. A total of 12 elements (Al, Si, P, S, Cl, K, Ca, Mn, Ti, Fe, Cu, and Zn) were detected with varying degrees of concentration. The plotted element concentration curves favored micron and submicron particle diameters. The particle shapes exhibit high/low sphericity. The plotted size distribution curves exhibited a marked downward shift in particle diameter from Kano to Ife. 相似文献
3.
Akpa BS Holland DJ Sederman AJ Johns ML Gladden LF 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2007,186(1):160-165
PFG NMR methods are frequently used as a means of probing both coherent and incoherent molecular motions of fluids contained within heterogeneous porous media. The time scale over which molecular displacements can be probed in a conventional PFG NMR experiment is limited by the relaxation characteristics of (1)H - the nucleus that is typically observed. In multiphase systems, due to its sensitivity to susceptibility gradients and interactions with surfaces,(1)H signal is frequently characterized by rapid T(1) and T(2) relaxation. In this work, a heteronuclear approach to PFG NMR is demonstrated which allows the study of molecular displacement over extended time scales (and, consequently, length scales) by exploiting the longer relaxation time of (13)C. The method presented employs the DEPT technique of polarization transfer in order to enhance both the sensitivity and efficiency of (13)C detection. This hybrid coherence transfer PFG technique has been used to acquire displacement propagators for flow through a bead pack with an observation time of up to 35 s. 相似文献
4.
Magin RL Akpa BS Neuberger T Webb AG 《Communications in Nonlinear Science & Numerical Simulation》2011,16(12):4581-4587
We report the appearance of anomalous water diffusion in hydrophilic Sephadex gels observed using pulse field gradient (PFG) nuclear magnetic resonance (NMR). The NMR diffusion data was collected using a Varian 14.1 Tesla imaging system with a home-built RF saddle coil. A fractional order analysis of the data was used to characterize heterogeneity in the gels for the dynamics of water diffusion in this restricted environment. Several recent studies of anomalous diffusion have used the stretched exponential function to model the decay of the NMR signal, i.e., exp[-(bD)(α)], where D is the apparent diffusion constant, b is determined the experimental conditions (gradient pulse separation, durations and strength), and α is a measure of structural complexity. In this work, we consider a different case where the spatial Laplacian in the Bloch-Torrey equation is generalized to a fractional order model of diffusivity via a complexity parameter, β, a space constant, μ, and a diffusion coefficient, D. This treatment reverts to the classical result for the integer order case. The fractional order decay model was fit to the diffusion-weighted signal attenuation for a range of b-values (0 < b < 4,000 s-mm(-2)). Throughout this range of b values, the parameters β, μ and D, were found to correlate with the porosity and tortuosity of the gel structure. 相似文献
5.
Molar extinction coefficients of some carbohydrates viz. l-arabinose (C5H10O5), d-glucose (C6H12O6), d-mannose (C6H12O6), d-galactose (C6H12O6), d(-) fructose (C6H12O6) and maltose (C12H24O12) in aqueous solutions have been determined at 81, 356, 511, 662, 1173 and 1332 keV by gamma ray transmission method in a
narrow beam good geometry set-up. These coefficients have been found to depend upon the photon energy following a 4-parameter
polynomial. These extinction coefficients for different sugars having the same molecular formula have same values varying
within experimental uncertainty. Within concentration ranges studied, Beer-Lambert law is obeyed very well. 相似文献
6.
van Eis MJ van Der Linde BS de Kanter FJ de Wolf WH Bickelhaupt F 《The Journal of organic chemistry》2000,65(14):4348-4354
In contrast to the terminal phosphinidene complex PhPW(CO)(5) (2), which adds to [5]metacyclophane (1) in a 1,4-fashion, dichlorocarbene preferentially adds in a 1,2-fashion to the formal "anti-Bredt" type double bond of the aromatic ring of 1 to afford the norcaradiene 11b, which immediately rearranges to the bridged cycloheptatriene 12b and further by a [1,5] sigmatropic chlorine migration to the isomeric 13b as the first observable product. More slowly, the latter isomerizes via a dissociative mechanism to give 15b. A computational study supports the notion that the [1,5] chlorine migration in the rearrangement 12b --> 13b, for which an activation barrier of 70.2 kJ mol(-)(1) was calculated, is essentially concerted with minor charge separation. In contrast, the analogous [1,5] chlorine migration in the flat model compound 7,7-dichlorocycloheptatriene (12a) displays features of a dissociative pathway. 相似文献
7.
Akpa Eric Jacques Philippe Wathelet Bernard Paquot Michel Fuchs Regine Budzikiewicz Herbert Thonart Philippe 《Applied biochemistry and biotechnology》2001,91(1-9):551-561
Bacillus subtilis produces various families of lipopeptides with different homologous compounds. To produce “new molecules” with improved activities
and to select strains that produced a reduced number of homologs or isomers, we studied the effects of different media on
the nature of the synthesis of fatty acid chains for each lipopeptide family. This study focused on two B. subtilis strains cultivated in flasks. Optimized medium for lipopeptide production and Landymedium modified by replacing glutamic
acid with other α-amino acids were used. We found that the intensity of production of homologous compounds depends on the
strain and the culture medium. Analysis of these lipopeptides by high-performance liquid chromatography showed that the strain
B. subtilis NT02 yielded various homologous compounds when cultivated in Landy medium (L-Glu), but primarily one homologous product in
high relative amounts when cultivated in the optimized medium. Mass spectrometric analysis and determination of the amino
acid composition of this molecule enabled us to identify it as Bacillomycine L c15. 相似文献
8.
I. G. E. Ibeanu L. A. Dim S. P. Mallam T. C. Akpa J. Munyithya 《Journal of Radioanalytical and Nuclear Chemistry》1997,221(1-2):207-209
Nigerian and Kenyan clays were analysed using energy dispersive X-ray fluorescence (EDXRF) technique and thirteen elements (Ca, Ti, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr, Zr, Nb and Pb) were detected. The health implications, since the substance is mostly consumed by pregnant women in both countries, are also reviewed and discussed. 相似文献
9.
10.
The size distribution, elemental composition, shape and surface texture of harmattan particulates of distinctly located points have been examined. The characterized particle size ranges are 0.7–16.4 m and 0.6–19.7 m for Kano and Ife, respectively. A total of 12 elements (Al, Si, P, S, Cl, K, Ca, Mn, Ti, Fe, Cu, and Zn) were detected with varying degrees of concentration. The plotted element concentration curves favored micron and submicron particle diameters. The particle shapes exhibit high/low sphericity. The plotted size distribution curves exhibited a marked downward shift in particle diameter from Kano to Ife. 相似文献