NMR relaxometry

Contrary to the findings for water, the relative change of the absolute values of Ti and of its frequency dependence is much more pronounced for n-heptane, which possesses a high affinity for the surface coke. One of the steps in the dispersion function can be interpreted as being related to the onset of full coverage of the surface. Because the maximum coke content corresponds to an average layer 

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HR. Kimmich, E. Anoardo 2004, (Fieldcycling NMR relaxometry), Prog. Nucl. Magn. Reson. Spectrose. 44, 257. 

NMR relaxometry Free induction decay (T2 ) or solid echo Spin echo decay (T2) Magnetization recovery curve (7)) Eads (1998)... 

NMR relaxometry to predict/assess meat quality traits 172... 

Finally, NMR relaxometry has also been used in the determination of composition of meat and meat products. Correlations between relaxation parameters and fat content in minced meat and meat emulsions,115 117 protein content in fresh meat115,118 and moisture content in sausages119 have been demonstrated results from fat determinations are summarised in Table 3. 

Many more such curves were measured in subsequent years, some of which were reported by Abragam (17). When Abragam s work was published it was already quite clear that the dispersion curves could become a valid tool for the study of molecular dynamics, thus laying down the foundation for variable field NMR relaxometry. In principle, the dispersion curves are potentially powerful tools to discriminate between various molecular dynamics models. 

Since, during an actual measurement, the shuttling process is repeated many times in a cyclic manner, the technique has been named field-cycling (FC) NMR relaxometry, a term which underlines the fact that it is the magnetic field variation that matters and not the manner in which it is achieved. 

The advent of the FFC instruments has opened a number of important application areas (molecular dynamics of liquid crystals, paramagnetic contrast MRI agents, proteins, polymers, etc.) and has thus provided a powerful impulse for further development of variable-field NMR relaxometry. Since 1996, Stelar entered the field and, building on the Noack-Schweikert technology (67), started producing the first commercial FFC NMR relaxometers. The availability of such instruments has further enhanced the drive towards new applications, apart from confirming the enormous potential of the technique as a primary tool for the study of molecular dynamics of even quite complex systems. 

In what follows, we wish to describe the most important technical aspects of FFC NMR relaxometry, including both the required special hardware (magnet, power supply, etc.) and the measurement methodology (data acquisition sequences and, to some extent, the subsequent data evaluation). Naturally, the description is based primarily on our own experience which has not yet been described in detail elsewhere. 

The second field-cycling method (65-67) uses electronic modulation of the current flowing through the coil of an electromagnet. This technique, commonly called Phst Field Cycling (FFC) NMR relaxometry, permits much faster variations of the field induction and thus extends the applicability of the field cycling approach to very short relaxation times Ti, at present, down to fractions of a millisecond. 

The instrumental aspects which will be discussed in the following sections concern primarily the fundamental characteristics and the functional behavior of those parts of an FFC NMR relaxometer which are characteristic of the FFC technique (Fig. 2). Subsequently, some of the methodological aspects of FFC NMR relaxometry, such as different data acquisition sequence and data accumulation and evaluation methods will be discussed. 

Using various detection sub-sequences, it is possible to select distinct nuclear magnetization components and thus discriminate between various sample components and/or distinct relaxation mechanisms. In combination with various possibilities of initial state preparation, this represents a powerful NMR relaxometry tool which, at present, is far from being completely exploited. 

The instruments currently produced by Stelar prove that Fast-field-cycling NMR relaxometry instrumentation is industrially viable in the sense that (i) its specifications are reasonably reproducible and (ii) once installed, the reliability of the instruments is comparable to that of any other NMR equipment. The overall performance, unthinkable-of only a few years ago, is best witnessed by the examples shown in Fig. 29. 

A drive towards high-resolution variable-field FFC NMR relaxometry (HR-FFC-NMRD), possibly combining the respective advantages of the FFC and the sample-shuttling methods. 

The increasing interest in FFC NMR relaxometry is also evidenced by the fact that there have been several international conferences in recent years specifically dedicated to the method. Even then, there are still many untapped application areas and additional ones will come to the front with further developments in technology. Opening such novel fields will undoubtedly provide a strong feedback, posing newer challenges to FFC NMR engineering. 

Vacuum magnetic permeability 245 Vacuum permittivity 31 Variable field NMR relaxometry 406 Volclay 306, 308-9... 

P.S. Hees, C.H. Sotak, Assessment of changes In murine tumor oxygenation in response to nicotinamide using F NMR relaxometry of a perfluorocarbon emulsion, Magn. Reson. Med. 29 (1993) 303-310and erratum 329 716 (1993). 

Davenel, A., Schuck, P., Mariette, F., and Brule, G. (2002). NMR relaxometry as non-invasive tool to characterize milk powders. Fait 82,465-473. 

Jaeger, F., Grohmann, E., and Schaumann, G. E. (2006). ll NMR relaxometry in natural humous soil samples Insights in microbial effects on relaxation time distributions. Plant Soil 280, 209-222. 

Abstract We use Nuclear Magnetic Resonance relaxometry (i.e. the frequency variation of the NMR relaxation rates) of quadrupolar nucleus ( Na) and H Pulsed Gradient Spin Echo NMR to determine the mobility of the counterions and the water molecules within aqueous dispersions of clays. The local ordering of isotropic dilute clay dispersions is investigated by NMR relaxometry. In contrast, the NMR spectra of the quadrupolar nucleus and the anisotropy of the water self-diffusion tensor clearly exhibit the occurrence of nematic ordering in dense aqueous dispersions. Multi-scale numerical models exploiting molecular orbital quantum calculations, Grand Canonical Monte Carlo simulations, Molecular and Brownian Dynamics are used to interpret the measured water mobility and the ionic quadrupolar relaxation measurements. 

Kehr M, Fatkullin N, Kimmich R (2007) Molecular diffusion on a time scale between nano-and milliseconds probed by field-cycling NMR relaxometry of intermolecular dipolar interactions Application to polymer melts. J Chem Phys 126 094903 Kirkwood JG, Riseman J (1948) The intrinsic viscosity and diffusion constant of flexible macromolecules in solution. J Chem Phys 16 565-573 Klein J (1986) Dynamics of entangled linear, branched, and cyclic polymers. Macromolecules 19(1) 105—118... 

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