Nuclear magnetic resonance transmitter

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

Nuclear magnetic resonance (NMR) spectroscopy can be used as a powerful noninvasive tool for clinical and other investigations of tissue biochemistry in vivo. There are many techniques for obtaining spectra from only a localized region of tissue, the simplest being to use a surface coil probe as both radio-frequency (RF) transmitter and receiver. This article reviews briefly surface coil spectroscopy, covering design, spectroscopic techniques, and clinical applications. 

Rapid non-destructive methods for measuring oil percentage based on nuclear magnetic resonance have made selection for oil percentage much simpler. Infrared reflectance and transmittance techniques can be used for both oil and protein percentages (Hammond, 1991). 

In c.w.-n.m.r. spectroscopy, a relatively weak, but rapidly oscillating, magnetic field is produced on the x axis by the application of a continuous, low-powered radiofrequency (r.f.) to the transmitter coil(s). As this radiofrequency approaches the resonance frequency, the magnetization vector is very slightly tipped out of the z axis, and precesses about this axis. When this frequency of precession is matched by the r.f. applied (the resonance condition), some of the individual, nuclear moments undergo transitions to the less-stable energy-level represented by precession about the — z direction, accompanied by absorption of energy from the transmitter coil. 

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