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 probes. With toroid cavity detectors, resonance frequencies up to 400 MHz can easily be attained, which makes them particularly useful for high-field 1H and 19F spectroscopy. Typically, static half-height linewidths of 1.5 Hz are achieved, as measured on 1H with standard solutions in cylindrical pressure vessels. Based on the radial dependency of the B1 field inside a toroid detector, a mathematical equation was derived that precisely predicts the signal intensity as a function of the pulse width. Inversion-recovery measurements of the T1 relaxation time of compressed gases (methane and hydrogen) were conducted by using composite inversion pulses. The results demonstrated the utility of toroid cavities for quantitative measurements in pressure probes. Pressures up to 300 bar have been used successfully. Because of the strength and regularity of the B1 gradient, the toroid cavity detector is also suitable for one dimensional rotating-frame NMR microscopy. A spatial resolution down to a few micrometers can be achieved. The spin concentration and spatial distribution of a chloroform solution were accurately reconstructed from two-dimensional 1H NMR data. Another similarly accurate but even stronger B1 gradient evolves as a result of the skin effect during high-frequency current transmission inside the central conductor. This gradient makes it possible to perform rotating-frame microscopy inside the central conductor, as demonstrated with 63Cu NMR spectroscopy