THE TOROID CAVITY NMR DETECTOR

A cylindrical toroid cavity has been developed as an NMR detector for investigations at high temperature and high pressure in metal vessel p robes. With toroid cavity detectors, resonance frequencies up to 400 M Hz can easily be attained, which makes them particularly useful for hi gh-field H-1 and F-19 spectroscopy. Typically, static half-height line widths of 1.5 Hz are achieved, as measured on H-1 with standard soluti ons in cylindrical pressure vessels. Based on the radial dependency of the B-1 field inside a toroid detector, a mathematical equation was d erived that precisely predicts the signal intensity as a function of t he pulse width. Inversion-recovery measurements of the T-1 relaxation time of compressed gases (methane and hydrogen) were conducted by usin g composite inversion pulses. The results demonstrated the utility of toroid cavities for quantitative measurements in pressure probes. Pres sures up to 300 bar have been used successfully. Because of the streng th and regularity of the B-1 gradient, the toroid cavity detector is a lso suitable for one dimensional rotating-frame NMR microscopy. A spat ial resolution down to a few micrometers can be achieved. The spin con centration and spatial distribution of a chloroform solution were accu rately reconstructed from two-dimensional H-1 NMR data. Another simila rly accurate but even stronger B-1 gradient evolves as a result of the skin effect during high-frequency current transmission inside the cen tral conductor. This gradient makes it possible to perform rotating-fr ame microscopy inside the central conductor, as demonstrated with Cu-6 3 NMR spectroscopy. (C) 1994 Academic Press, Inc.