Field inhomogeneity

For condensed species, additional broadening mechanisms from local field inhomogeneities come into play. Short-range intermolecular interactions, including solute-solvent effects in solutions, and matrix, lattice, and phonon effects in soHds, can broaden molecular transitions significantly. 

The spin-echo experiment therefore leads to the refocusing not only of the individual nuclear resonances but also of the field inhomogeneity components lying in front or behind those resonances, a maximum negative amplitude being observed at time 2t after the initial 90° pulse. The frequency of rotation of each signal in the rotating frame will depend on its chemical shift and after the vector has been flipped by the 180° pulse, it... 

Figure 2.2 Effect of 180 pulse on phase imperfections resulting from magnetic field inhomogeneities. Spin-echo generated by 180 refocusing pulse removes the effects of magnetic field inhomogeneities.

The spin-echo is used to suppress the production of spurious signals due to field inhomogeneities or to eliminate errors in the setting of pulse widths. It is also possible to use the spin-echo to follow the decay of transverse magnetization and to determine the transverse relaxation time (7 2). How might we do this in practice ... 

Moreover, precession under selective irradiation occurs in the longitudinal plane of the rotating frame, instead of rotation in the transverse plane, which occurs during the evolution of the FID. The magnitude of the vector undergoing precession about the axis of irradiation decreases due to relaxation and field inhomogeneity effects. 

Figure 5.18 (A) Pulse sequence for homonuclear 2D y-resolved spectroscopy. (B) Effect of 90° H and 180° H pulses on an H doublet. (C) In the absence of coupling, the vectors are refocused by the 180° H pulse after t. This serves to remove any field inhomogeneities or chemical shift differences.

Composite pulse A composite sandwich of pulses that replaces a single pulse employed to compensate for B] field inhomogeneities, phase errors, or offset effects. 

Spin-echo The refocusing of vectors in the xy-plane caused by a (t-180°-t) pulse sequence produces a spin-echo signal. It is used to remove field inhomogeneity effects or chemical shift precession effects. 

Since these terms are proportional to tr, they increase with decreasing temperature.1 There are several line-width contributions, included in oc0, which do not depend on m,-. These include magnetic field inhomogeneity and the spin rotation interaction, the latter increasing with 1/tr and thus with increasing temperature. These and other line-width effects have been studied in some detail and are discussed elsewhere.13... 

The widths of resonances arising from the lie residue (Table III) are fairly uniform, and do not show much solvent dependence. Those numbers probably reflect a significant contribution from the field inhomogeneity in this heterogeneous sample since the normally sharp (less than 1 Hz) solvent line is 20 Hz wide. 

One can further increase the amount of transferred polarization if one carries out the cross polarization in an adiabatic fashion. In this experiment, the amplitude of one of the spin-lock fields is usually varied in a tangential shape [33-35]. In addition to the compensation of instabilities in the amplitude and rf field inhomogeneities, one can also obtain a gain in signal by a up to a factor of two. The concept of adiabatic polarization transfer will be discussed in more detail in Sect. 11.3.1. 

The SIN defined by Equation 7.6 for a given NMR resonance is proportional to the square of the nuclear precession frequency (mo, rad/s), the magnitude of the transverse magnetic field (Bi) induced in the RE coil per unit current (/), the number of spins per unit volume (Ns), the sample volume (Vs), and a scaling constant that accounts for magnetic field inhomogeneities. The SIN is inversely proportional to the noise generated in the RE receiver and by the sample (Vnoise) as defined by the Nyquist theorem,... 

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