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NMR Imaging (nmr + imaging)

Distribution by Scientific Domains
Life Sciences50%
Polymers and Materials Science33%

Selected Abstracts

NMR Imaging in Chemical Engineering

MACROMOLECULAR MATERIALS & ENGINEERING, Issue 2 2008
Peter Blümler
[source]

Xylem Flow and its Driving Forces in a Tropical Liana: Concomitant Flow-Sensitive NMR Imaging and Pressure Probe Measurements

PLANT BIOLOGY, Issue 6 2000
N. Wistuba
Abstract: Flow-sensitive NMR imaging and pressure probe techniques were used for measuring xylem water flow and its driving forces (i.e., xylem pressure as well as cell turgor and osmotic pressure gradients) in a tropical liana, Epipremnum aureum. Selection of tall specimens allowed continuous and simultaneous measurements of all parameters at various distances from the root under diurnally changing environmental conditions. Well hydrated plants exhibited exactly linearly correlated dynamic changes in xylem tension and flow velocity. Concomitant multiple-probe insertions along the plant shoot revealed xylem and turgor pressure gradients with changing magnitudes due to environmental changes and plant orientation (upright, apex-down, or horizontal). The data suggest that in upright and - to a lesser extent - in horizontal plants the transpirational water loss by the cells towards the apex during the day is not fully compensated by water uptake through the night. Thus, longitudinal cellular osmotic pressure gradients exist. Due to the tight hydraulic coupling of the xylem and the tissue cells these gradients represent (besides the transpiration-induced tension in the xylem) an additional tension component for anti-gravitational water movement from the roots through the vessels to the apex. [source]

,-Irradiation of ultrahigh-molecular-weight polyethylene: Electron paramagnetic resonance and nuclear magnetic resonance spectroscopy and imaging studies of the mechanism of subsurface oxidation

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2004
Todd M. Alam
Abstract The shelf aging of irradiated ultrahigh-molecular-weight polyethylene (UHMWPE) causes subsurface oxidation, which leads to failure in UHMWPE orthopedic components, yet the mechanisms causing subsurface oxidation remain unclear. The shelf aging of ,-irradiated UHMWPE bars has been studied with electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) imaging and with microtoming and Fourier transform infrared microscopy. The bars initially contained only allyl radicals, and upon air exposure, a surface layer of peroxyl radicals formed through the reaction of allyl radicals with oxygen. Importantly, a band of low radical intensity just beneath the peroxyl layer became apparent. NMR imaging showed a zone of altered proton relaxation in this zone. With increasing time, surface peroxyl radicals persisted in comparison with the interior allyl radicals, although oxygen did not appear to penetrate any more deeply into the bar. The area of maximal oxidation and mechanical disruption, measured after 3 years, was at the interface between the zone of exterior peroxyl radicals and the zone of low radical intensity. We present a mechanism involving the intermediacy of sterically strained reactive dialkyl peroxides at this interface to explain subsurface oxidation. We also demonstrate that EPR and NMR imaging provides information that could potentially be used to identify subsurface oxidized UHMWPE components before failure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5929,5941, 2004 [source]

Xylem Flow and its Driving Forces in a Tropical Liana: Concomitant Flow-Sensitive NMR Imaging and Pressure Probe Measurements

PLANT BIOLOGY, Issue 6 2000
N. Wistuba
Abstract: Flow-sensitive NMR imaging and pressure probe techniques were used for measuring xylem water flow and its driving forces (i.e., xylem pressure as well as cell turgor and osmotic pressure gradients) in a tropical liana, Epipremnum aureum. Selection of tall specimens allowed continuous and simultaneous measurements of all parameters at various distances from the root under diurnally changing environmental conditions. Well hydrated plants exhibited exactly linearly correlated dynamic changes in xylem tension and flow velocity. Concomitant multiple-probe insertions along the plant shoot revealed xylem and turgor pressure gradients with changing magnitudes due to environmental changes and plant orientation (upright, apex-down, or horizontal). The data suggest that in upright and - to a lesser extent - in horizontal plants the transpirational water loss by the cells towards the apex during the day is not fully compensated by water uptake through the night. Thus, longitudinal cellular osmotic pressure gradients exist. Due to the tight hydraulic coupling of the xylem and the tissue cells these gradients represent (besides the transpiration-induced tension in the xylem) an additional tension component for anti-gravitational water movement from the roots through the vessels to the apex. [source]

Effects of cold-girdling on flows in the transport phloem in Ricinus communis: is mass flow inhibited?

PLANT CELL & ENVIRONMENT, Issue 1 2006
ANDREAS D. PEUKE
ABSTRACT The effects of cold girdling of the transport phloem at the hypocotyl of Ricinus communis on solute and water transport were investigated. Effects on the chemical composition of saps of phloem and xylem as well as of stem tissue were studied by conventional techniques and the water flow in the phloem was investigated by NMR imaging. Cold girdling reduced the concentration of sucrose but not that of inorganic solutes or amino acids in phloem saps. The possibility that cold treatment inhibited the retrieval of sucrose into the phloem, following leaching from the sieve tubes along a chemical gradient is discussed. Leaching of other solutes did not occur, as a result of missing promoting gradients in stem tissue. Following 3 d of cold girdling, sugar concentration increased and starch was synthesized and accumulated in stem tissue above the cold girdling region and along the cold-treated phloem pathway due to leaching of sugars from the phloem. Only in the very first period of cold girdling (< 15,30 min) was mass flow inhibited, but recovered in the rest of cold treatment period to values similar to the control period before and the recovery period after the cold treatment. It is concluded that cold treatment affected phloem transport through two independent and reversible processes: (1) a permanent leaching of sucrose from the phloem stem without normal retrieval during cold treatment, and (2) a short-term inhibition of mass flow at the beginning of cold treatment, possibly involving P proteins. Possible further mechanisms for reversible inhibition of water flow are discussed. [source]

Axial Dispersion and Wall Effects in Narrow Fixed Bed Reactors: A Comparative Study Based on RTD and NMR Measurements

CHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 8 2004
D. Tang
Abstract Axial dispersion and wall effects in narrow fixed beds with aspect ratios < 10 were investigated, both by classical methods and by NMR imaging. The residence time distribution (RTD) in the center and at the wall was measured, system water/NaCl-solution as tracer, and subsequently compared with radial velocity profiles based on NMR imaging. The influence of the aspect ratio and Rep on dispersion and on the degree of non-uniformity of the velocity profile was studied. The NMR results are consistent with the RTD and also with literature data of numerical simulations. For low aspect ratios, dispersion/wall effects have a strong influence on the reactor behavior, above all, in cases where a low effluent concentration is essential, as proven by breakthrough experiments with the reaction of H2S with ZnO. [source]