Field cycling relaxation

This chapter aims at showing the potential of the Field Cycling Relaxation approach in the food analysis. 

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 ... 

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).

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... 

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... 

Relaxation Measurements. Measurements of the magnetic fieldz dependence of the solvent water proton relaxation rate (T] l), i.e., nuclear magnetic relaxation dispersion (NMRD), were made by the field cycling method previously described (9,10). 

However, zero-field experiments suffer from low sensitivity and can only be applied to materials with reasonably long relaxation times T] to allow for the time it makes to shutde the sample in and out of the field. This field cycling technique measures the evolution of the magnetization at zero-field with the sensitivity of high-field NMR. Zero-field NMR is particularly suited to the investigation of nuclei with quadrupolar moments, since it avoids orientation-dependent broadening. 

If the relaxation is slow enough then magnetisation-applied field cycles can be performed to measure hysteresis loops (see later). In SMMs this slow relaxation is a result of the ZFS interactions in the ground spin state. Since magnetic anisotropy is being probed here, these measurements are ideally performed on single crystals. 

Whatever model is used, the problem with equation (1) is that it is intrinsically underdetermined since it contains the product of interaction and spectral density terms. The usual solution is to examine relaxation at different temperatures and assume a function (usually exponential) for the temperature dependence of the correlation times. This is problematic in foods, as the structure of food is typically very temperature dependent. The alternative is directly to determine the spectral density function of the material by determining 7, over a wide range of frequencies. This may be done by using fast field cycling NMR. 

Kimmich R (1979) Field cycling in NMR relaxation spectroscopy applications in biological, chemical and polymer physics. Bull Magn Reson 1 195-218... 

Fig. 5. Comparison of the frequency dependence of the total longitudinal proton relaxation time Ti and of the dipolar proton relaxation time Tto in the low-temperature nematic liquid crystal MBBA. r (v) was measured by the usual T] field-cycle with one B, r.f. pul%, shown by Rg. 1. Tto was measured by the usual Jeener-Broekaert sequence of three B r.f. pulses, in combination with a Bq field-cycle. which introduce an adjustable relaxation period between the second and third Bj pulse to give Tto(i )- The plots in the upper diagram show model fits according to equations (13a) 13d) with extensions described in the text. From the details at bottom about the experimental errors it can be clearly seen that the ratio T Tto significantly exceeds a value of 3 at medium frequencies, and in accordance with the model plot (frill line) approaches 1 in the low-frequency limit, where Bo is smaller than Bloc.

The preceding sections have shown that fast field-cycling NMR is not restricted to frequency dependent relaxation studies, as often believed. Although the four new kinds of applications illustrated are typical for the basic procedures, it should be pointed out that the method has already been applied in other directions, and so resulted in new growing fields of research in the recent NMR literature. Readers may encounter various headlines intimately related to FC techniques, which however is not always clearly seen or described, for instance zero-field NMR, , cross-relaxation... 

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