Zero Quantum Nuclear Magnetic Resonance experiments utilizing a toroid cell and coil

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Virginia Tech


Over the past ten to fifteen years the area of Nuclear Magnetic Resonance (NMR) Spectroscopy has seen tremendous growth. For example, in conjunction with multiple quantum NMR, molecular structural mapping of a compound can be easily performed in a two dimensional (2D) experiment.

However, only two kinds of detector coils have been typically used in NMR studies. These are the solenoid coil and the Helmholtz coil. The solenoid coil was very popular with the permanent and electromagnet NMR instruments. With the advent of the superconducting magnets the popularity shifted to the Helmholtz coil, which remains the most common coil today for superconducting magnets. The Helmholtz coil, however, has been shown to have lower sensitivity than the solenoid coil. Hoult (1) has pointed out that potentially the Helmholtz coil represents a loss of signal-to-noise (S/N) by a factor of three in comparison to the solenoid coil. Since Hoult's original work, alternate methods for optimizing S/N have been explored. One of these has been the suggestion of toroid shaped resonators for NMR studies (2). A potential advantage of a toroid cell and coil is the confinement of the B₁ field to the torus region.It has been suggested that the toroid has a potential (S/N) advantage of 3.9 - 4.5 in comparison to the conventional Helmholtz cell (3). Since Zero Quantum (ZQ) experiments are independent of B₀ homogeneity, 2D ZQ experiments provide a convenient method of comparing the toroid and Helmholtz coils.

In these zero quantum studies, the toroid and Helmholtz probes will be characterized in terms of several factors 1) B1 homogeneity, 2) B₀ homogeneity, 3) flip angle dependence, and 4) sensitivity. Finally, the two probes will be contrasted using spectral analysis in the spin-spin mapping of a simple molecule (nâ butanol) and a complex molecular system (taxol).