SAMPLE-INDUCED RF PERTURBATIONS IN HIGH-FIELD, HIGH-RESOLUTION NMR-SPECTROSCOPY

Conducting dielectric samples are often used in high-resolution experi ments at high held. It is shown that significant amplitude and phase d istortions of the RF magnetic field may result from perturbations caus ed by such samples. Theoretical analyses demonstrate the spatial varia tion of the RF field amplitude and phase across the sample, and compar isons of the effect are made for a variety of sample properties and op erating field strengths. Although the effect is highly nonlinear, it t ends to increase with increasing field strength, permittivity, conduct ivity, and sample size. There are cases, however, in which increasing the conductivity of the sample improves the homogeneity of the amplitu de of the RF field across the sample at the expense of distorted RF ph ase. It is important that the perturbation effects be calculated for t he experimental conditions used, as they have the potential to reduce the signal-to-noise ratio of NMR experiments and may increase the gene ration of spurious coherences. The effect of RF-coil geometry on the c oherences is also modeled, with the use of homogeneous resonators such as the birdcage design being preferred, Recommendations are made conc erning methods of reducing sample-induced perturbations. Experimental high-field imaging and high-resolution studies demonstrate the effect. (C) 1997 Academic Press.