Field-cycling

it is necessary to obtain the spin-lattice relaxation rates at different frequencies in order to study the spectral density or to differentiate different relaxation mechanisms (Noack, 1971). With a tunable spectrometer, it is possible to cover a frequency range of one or two orders of magnitude 

We have discussed two ways in which spin-lattice relaxation rates can be measured at fields much smaller than the usual laboratory fields. In section IV.B.l. we introduced the concept of dipolar order in fields of neighboring nuclei and in IV.C.l. we talked about the spin-lattice relaxa- 

Another way to perform field cycling is to actually change the magnetic field without moving the sample. Most magnets are very sluggish because of the large inductance of the coil 

For any sizeable field, the field cycling experiment can be performed by pneumatically moving the sample between the two magnets. As mentioned above, variable temperature work is significantly more difficult. A typical transit time is of the order of 500 ms. See, for example, the papers by Jones, et al. (1969) and Edmonds (1977). 

As long as we are changing fields, there is no reason to use just two. We could, for example, have a very high field with only a minimal homogeneity in which the initial polarization takes place. Then that large polarization is inverted in a homogenous intermediate field and then allowed to relax 

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Koenig S H and Brown R D 1990 Field-cycling relaxometry of protein solutions and tissue— implications for MRI Prog. Nucl. Magn. Reson. Spectrosc. 22 487-567... 

E. Anoardo, G. Galli, G. Ferrante 2001, (Fast-field-cycling NMR Applications and instrumentation), Appl. Magn. Re-son. 20, 365. 

S. Stapf, X. Ren, E. Talnishnikh, B. Bliimich 2005, (Spatial distribution of coke residues in porous catalyst pellets analyzed by field-cycling relaxometry and parameter imaging), Magn. Reson. Imag. 23, 383. 

According to [18], 29% of emissions produced within the EU is related to the food production. However, these emissions arising within food production are related not only to the field cycle but also to the production of fertilizers and agrochemicals, processing or all process transport. 

Field cycling relaxometry Residual dipolar couplings... 

Figure 17 (A) A diagram describing the experimental setup for a magnetic-field cycling...

Belton, P.S. and Wang, Y. 2001. Fast field cycling NMR Applications to foods. In Magnetic Resonance in Food Science A View to the Future (G.A. Webb, P.S. Belton, A.M. Gil, and I. Delgadillo, eds), pp. 145-156. Royal Society of Chemistry, Cambridge, UK. 

Hills, B.P., Wang, Y.L., and Tang, H.R. 2001. Molecular dynamics in concentrated sugar solutions and glasses An NMR field cycling study. Mol. Phy. 99, 1679-1687. 

For spin-f nuclei, dipolar interactions may be modulated by intramolecular (DF, reorientation etc.) and/or intermolecular (TD) processes. In general, the intra- and inter-molecular processes can produce quite different Tj frequency dispersion curves. In practice, NMR field cycling experiments are often needed to extend the frequency domain from those employed in conventional spectrometers to a lower frequency range (i.e., the kHz regime) for unambiguous separation (and identification) of different relaxation mechanisms. The proton spin relaxation by anisotropic TD in various mesophases has been considered by Zumer and Vilfan.131 133,159 In the nematic phase, Zumer and Vilfan found the following expression for T ... 

Technical Aspects of fast Field Cycling Gianni Fermnte and Stanislav Sykora... 

Fig. 2. The different stages of a field-cycling experiment. Spins are pre-polarized in a relatively high static magnetic field (Bq). The relevant magnetization then decays to its equilibrium value in the field Bq according to the longitudinal relaxation time of interest (at a frequency equal to yBQl2n). For sensitivity reasons, magnetization is read again in the Bq field (a n/2 pulse yielding an fid Acq).

In the last ten years several novel theories for the analysis of NMRD profiles have been proposed for different paramagnetic systems. These have resulted in novel equations and computer programs for numerical calculations. In this review we tried to summarize the classical theory as well as some recent theoretical treatments, and to show their applicability to different metal ion systems by selecting some experimental profiles which exhibit the most characteristic features. In the last few years, dedicated instruments, called field-cycling relaxometers, have appeared on the market... 

There are a variety of models for the magnetic field dependence for polymers and variously ordered phases coming largely from the groups of Kimmich and Noack. We focus here on the protein system that provides unique structural and physical characteristics. Recent advances in the speed of current-switched field-cycling instruments have provided a direct measure of the MRD profile of the protein protons as shown in Fig. 18. The relaxation... 

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. 

A field-cycling experiment requires that the sample be subject at different times, and for variable durations, to different values of the magnetic field B. From the instrumental point of view, there are basically just two ways to achieve this goal. 

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. 

Any NMR field-cycling (FC) relaxometry experiment presumes that the sample is subject to a magnetic field of various intensities for time intervals of varying durations. More specifically, between the various intervals of a relaxation-time measurement, the external magnetic field induction... 

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