Field cycling technique

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. 

Only a few relaxation dispersion measurements have thus far been carried out on smectic phases. However, the influence of smectic order on director fluctuations has been detected. Blinc et al. [6.43] were first to study proton Ti frequency dispersion in the smectic phases of terephthal-bis-p-butylaniline (TBBA) using a field cycling technique. Figure 6.6 shows the... 

Tir) contributions to Ti have rather complicated and quite different dispersive behavior (see Chap. 6), the separation of two effects from a Ti frequency dispersion curve is a formidable task. It is therefore preferable to identify and separate the intermolecular and intramolecular contributions to relaxation before applying theories. Isotopic dilution experiments [7.73, 7.74] have been done to isolate Tir in liquid crystals. Unfortunately, this technique has not yet been used in conjunction with the field-cycling technique to obtain Tir over a wider frequency range. 

A similar study has been performed in MBBA by Dong et al. [93]. The dispersion of the proton in the isotropic phase of 5CB, PAA and MBBA has been precisely studied by the field cycling technique by Noack and coworkers [94-96]. 

The proton spin-lattice relaxation dispersion data (Fig. 9) in MBBA at 18°C [ 135] obtained by the field cycling technique clear-... 

Transverse and longitudinal NMR relaxometry of and nuclei was shown to be governed by three components of chain dynamics. With respect to tests of theoretical model concepts, the most important one is component B which reflects the influence of chain modes and can be probed with the field-cycling technique. 

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 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. 

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

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