Inhomogeneity in the static magnetic

Line broadening due to inhomogeneity in the static magnetic field. Ho, as well as in the rf pulse Hj, can contribute to the observed resonance. However, studies of standard sairples, of known natural linewidths, enable the contributions from this source to be determined. In the present case these causes contribute only a few percent, i.e., a few Hz, to the total linewidth and are thus inconsequential to the present problem. Before discussing the different motional contributions to the linewidth. 

The use of the 180°, pulse instead of a 180° pulse in the above sequences was employed to provide a more convenient picture of events, yet it is important to realise that the refocusing effects on chemical shift and couplings described above would also have occurred with a 180° pulse except that the refocused vectors would now lie along the —y-axis instead of the -fy-axis. One further feature of the spin-echo sequence is that it will also refocus the deleterious effects that arise from inhomogeneities in the static magnetic field, as these may be viewed as just another contribution to chemical shift differences throughout the sample. The importanee of the spin-echo in modem NMR teehniques can... 

The principal source of external relaxation or coherence in the transverse plane for liquids and solutions is inhomogeneity in the static magnetic field. Using a spin-echo sequence (Fig. 6.6) can eliminate this source of loss. 

The deuterium lock prevents changes in the static field (Bq) and radiofrequency (B,) by maintaining a constant ratio between the two. It therefore ensures long-term stability of the magnetic field. If the lock is not applied, a drastic deterioration in the shape of the NMR lines is expected, due to magnetic and radioffequency inhomogeneities, (a) With deuterium lock... 

An inhomogeneous line is one in which separate parts of the line can be identified as due to separate contributions. The most common example is a powder pattern due to anisotropies such as quadrupolar or chemical shifts in which crystallites with different orientations with the static magnetic field contribute to different parts of the line. 

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