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Jörg Kärger - One of the best experts on this subject based on the ideXlab platform.
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Tracing Molecular Propagation in Dextran Solutions by Pulsed Field Gradient NMR.
Journal of Physical Chemistry Letters, 2012Co-Authors: Alexander Shakhov, Rustem Valiullin, Jörg KärgerAbstract:We have exploited the Pulsed Field Gradient (PFG) technique of NMR to measure molecular diffusion in aqueous solutions of a mixture of dextran molecules. From detailed studies by fluorescence correlation spectroscopy (FCS), the lighter component of such mixtures is known to undergo subdiffusion, up to diffusion path lengths on the order of 0.5 μm. Our studies provide clear evidence of a crossover to normal diffusion for diffusion path lengths from this range up to about 1 μm.
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Microdynamics of Guest Molecules in Zeolites Studied by Quasielastic Neutron Scattering and Nmr Pulsed Field Gradient Technique
Studies in Surface Science and Catalysis, 2008Co-Authors: Hervé Jobic, Jörg Kärger, Marc Bée, Jürgen Caro, Martin Bülow, Harry PfeiferAbstract:Summary Application of the quasi-elastic neutron scattering and NMR Pulsed Field Gradient technique to molecular self diffusion studies of hydrocarbons in zeolites ZSM-5 and NaX leads to coinciding results in both the absolute values, the concentration dependence and activation energy of the intracrystalline self-diffusion coefficients. Further agreement between the neutron and NMR diffusion data is obtained by comparing the mean molecular jump lengths which by both techniques are found to be of the order of 1.2…0.25 nm, slightly decreasing with increasing loading. Due to the different time scales of the two experimental methods applied, the mean diffusion paths followed are of different magnitude: in neutron scattering experiments molecular translation is measured up to 6 nm; in the NMR Pulsed Field Gradient technique the mean molecular displacements amount to some nm. However, in both methods, longrange self-diffusion is detected.
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Application of Pulsed Field Gradient NMR to characterize the transport properties of microporous membranes
Membrane Science and Technology, 2007Co-Authors: Wilfried Heink, Jörg Kärger, Sergey VasenkovAbstract:Abstract The potentials of Pulsed Field Gradient (PFG) NMR in elucidating molecular transport in microporous membranes are presented. Particular emphasis is given to structure-correlated diffusion anisotropy and single-file diffusion. First results of PFG NMR studies with A type zeolite membranes are presented.
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selective multi component diffusion measurement in zeolites by Pulsed Field Gradient nmr
Microporous and Mesoporous Materials, 2006Co-Authors: Andre Pampel, Frank Stallmach, Frank Engelke, Petrik Galvosas, C Krause, D Michel, Jörg KärgerAbstract:Abstract The potential of Pulsed Field Gradient (PFG) NMR for selective diffusion measurement in multi-component liquids is far from being fulfilled in multi-component diffusion studies with zeolites. We present two recent developments in PFG NMR instrumentation, which will significantly improve the measuring conditions for multi-component diffusion in zeolites and other nanoporous materials. They include options for an enhancement of the sensitivity with respect to smaller displacements by a novel principle of Field Gradient pulse matching and with respect to selectivity between different components by combining PFG NMR with magic angle spinning (MAS) NMR with a microimaging Gradient system. The potentials and limitations of the two options are demonstrated by the first results of selective PFG NMR self-diffusion measurements with zeolitic adsorbate–adsorbent systems containing as much as four different species of guest molecules.
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diffusion of n hexane in 5a zeolite studied by the neutron spin echo and Pulsed Field Gradient nmr techniques
Microporous and Mesoporous Materials, 2003Co-Authors: Hervé Jobic, Jörg Kärger, H Paoli, Alain Methivier, G Ehlers, C KrauseAbstract:The neutron spin-echo (NSE) and Pulsed-Field Gradient (PFG) NMR techniques are applied to investigate the microdynamics of n-hexane diffusion in a commercial sample of zeolite NaCaA. In the PFG NMR measurements molecular root mean square displacements in the range from 100 nm to 2 μm are covered, while the NSE experiments applied to deuterated guest molecules provide information about the rate of molecular fluxes over distances of a few nm. Taking into account these differences, the obtained diffusivity data are found to be in reasonable agreement.
Sergey Vasenkov - One of the best experts on this subject based on the ideXlab platform.
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self diffusion of heptane inside aggregates of porous alumina particles by Pulsed Field Gradient nmr
Microporous and Mesoporous Materials, 2016Co-Authors: Evan M Forman, Matthias A Trujillo, Kirk J Ziegler, Steven A Bradley, Haiyan Wang, Sesh Prabhakar, Sergey VasenkovAbstract:Abstract 1 H and 13 C Pulsed Field Gradient (PFG) NMR was used to study the self-diffusion of heptane in porous alumina exhibiting a hierarchy of pore sizes that cover the range from micropore to macropore sizes. Diffusion measurements were performed for the lengths scales of displacements comparable with and smaller than the sizes of aggregates of porous alumina particles. The measured heptane diffusivities inside the particle aggregates were found to be several times smaller than the diffusivity in the bulk liquid heptane. The diffusivities inside the aggregates were correlated with the aggregate structural characteristics, which include pore volume, pore size and packing density of the individual particles in the aggregates.
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Application of Pulsed Field Gradient NMR to characterize the transport properties of microporous membranes
Membrane Science and Technology, 2007Co-Authors: Wilfried Heink, Jörg Kärger, Sergey VasenkovAbstract:Abstract The potentials of Pulsed Field Gradient (PFG) NMR in elucidating molecular transport in microporous membranes are presented. Particular emphasis is given to structure-correlated diffusion anisotropy and single-file diffusion. First results of PFG NMR studies with A type zeolite membranes are presented.
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determination of genuine diffusivities in heterogeneous media using stimulated echo Pulsed Field Gradient nmr
Journal of Magnetic Resonance, 2001Co-Authors: Sergey Vasenkov, Frank Stallmach, Petrik Galvosas, Oliver Geier, Nikolaus Nestle, Jörg KärgerAbstract:Pulsed Field Gradient (PFG) NMR diffusion measurements in heterogeneous media may lead to erroneous results due to the disturbing influence of internal magnetic Field Gradients. Here, we present a simple theoretical model which allows one to interpret data obtained by stimulated spin echo PFG NMR in the presence of spatially varying internal Field Gradients. Using the results of this theory, the genuine self-diffusion coefficients in heterogeneous media may be extrapolated from the dependence of the apparent diffusivities on the dephasing time of the simulated echo PFG NMR sequence. Experimental evidence that such extrapolation yields satisfactory results for self-diffusion of hexadecane in natural sediments (sand) and of n-octanol in doped MgO pastes is provided.
Partha P. Mitra - One of the best experts on this subject based on the ideXlab platform.
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Simulations of Pulsed Field Gradient spin-echo measurements in porous media.
Magnetic Resonance Imaging, 2004Co-Authors: Lawrence M. Schwartz, Pabitra N. Sen, Partha P. MitraAbstract:Under ideal conditions, in an isotropic homogeneous system, Pulsed Field Gradient spin-echo (PFGSE) experiments measure the probability P(R,t), that a diffusing water molecule with travel a root mean square distance, R, in a time interval of length t. We will show that random walk simulations on model porous media provide valuable insight to the sensitivity of this measurement to (1) complex pore geometries and (2) enhanced relaxation at the pore-grain interface.
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multiple wave vector extensions of the nmr Pulsed Field Gradient spin echo diffusion measurement
Physical Review B, 1995Co-Authors: Partha P. MitraAbstract:Multiple wave-vector extensions of NMR Pulsed-Field-Gradient diffusion measurements are discussed in the context of diffractionlike effects of restricted diffusion. In the case of two independent wave vectors, it is shown that the dependence of the amplitude on the relative angle between the wave vectors carries information that is absent in the usual single-wave-vector amplitude. It is shown that a two-wave-vector measurement is sensitive to restricted diffusion even at small wave vectors, in contrast with the single-wave-vector case. It is proposed that the angular dependence noted above may be used to distinguish effects of restricted diffusion from those arising from a distribution of diffusion constants. © 1995 The American Physical Society.
Peter J. Basser - One of the best experts on this subject based on the ideXlab platform.
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Using double Pulsed-Field Gradient MRI to study tissue microstructure in traumatic brain injury (TBI).
Microporous and Mesoporous Materials, 2017Co-Authors: Michal E. Komlosh, Dan Benjamini, Elizabeth B. Hutchinson, Sarah King, Margalit Haber, Alexandru V. Avram, Lynne A. Holtzclaw, Abhishek Desai, Carlo Pierpaoli, Peter J. BasserAbstract:Double Pulsed-Field Gradient (dPFG) MRI is proposed as a new sensitive tool to detect and characterize tissue microstructure following diffuse axonal injury. In this study dPFG MRI was used to estimate apparent mean axon diameter in a diffuse axonal injury animal model and in healthy fixed mouse brain. Histological analysis was used to verify the presence of the injury detected by MRI.
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microscopic anisotropy revealed by nmr double Pulsed Field Gradient experiments with arbitrary timing parameters
Journal of Chemical Physics, 2008Co-Authors: Evren Ozarslan, Peter J. BasserAbstract:We consider a general double Pulsed Field Gradient experiment with arbitrary experimental parameters and calculate an exact expression for the NMR signal attenuation from restricted geometries, which is valid at long wavelengths, i.e., when the product of the gyromagnetic ratio of the spins, the Pulsed Gradients’ duration, and their magnitude is small compared to the reciprocal of the pore size. It is possible to observe microscopic anisotropy within the pore space induced by the boundaries of the pore, which can be used to differentiate restricted from free or multicompartmental diffusion and to estimate a characteristic pore dimension in the former case. Explicit solutions for diffusion taking place between parallel plates as well as in cylindrical and spherical pores are provided. In coherently packed cylindrical pores, it is possible to measure simultaneously the cylinders’ orientation and diameter using small Gradient strengths. The presence of orientational heterogeneity of cylinders is addressed, and a scheme for differentiating microscopic from ensemble anisotropy is proposed.
H Van As - One of the best experts on this subject based on the ideXlab platform.
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microscopic displacement imaging with Pulsed Field Gradient turbo spin echo nmr
Journal of Magnetic Resonance, 2000Co-Authors: Tom W J Scheenen, D Van Dusschoten, P A De Jager, H Van AsAbstract:Abstract We present a pulse sequence that enables the accurate and spatially resolved measurements of the displacements of spins in a variety of (biological) systems. The pulse sequence combines Pulsed Field Gradient (PFG) NMR with turbo spin–echo (TSE) imaging. It is shown here that by ensuring that the phase of the echoes within a normal spin–echo train is constant, displacement propagators can be generated on a pixel-by-pixel basis. These propagators accurately describe the distribution of displacements, while imaging time is decreased by using separate phase encoding for every echo in a TSE train. Measurements at 0.47 T on two phantoms and the stem of an intact tomato plant demonstrate the capability of the sequence to measure complete and accurate propagators, encoded with 16 PFG steps, for each pixel in a 128 × 128 image (resolution 117 × 117 × 3000 μm) within 17 min. Dynamic displacement studies on a physiologically relevant time resolution for plants are now within reach.
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study of transport phenomena in chromatographic columns by Pulsed Field Gradient nmr
Journal of Physical Chemistry B, 1998Co-Authors: Ulrich Tallarek, D Van Dusschoten, H Van As, Ernst Bayer, Georges GuiochonAbstract:Pulsed Field Gradient NMR has been applied to study mass transfer, flow, and dispersion in packed chromatographic columns. A single measurement allows the determination of the full displacement probability distribution of all fluid particles located in the measurement volume. Depending on the orientation of the Pulsed magnetic Field Gradient with respect to the net flow direction, the so-called averaged propagator is obtained independently and quantitatively for either the axial or the transverse fluid particle displacements, over an experimentally adjustable observation time. Thus, this technique can act on a dynamic time scale ranging from a few to several hundred milliseconds. This enabled us to detect the stagnant mobile phase in packed chromatographic columns and to follow the mass transfer between the stagnant solvent and the stream of mobile phase percolating through the column bed. With Field Gradients in the direction of net flow velocity, mean fluid particle displacements ranging between 0.07 an...
