Continuous-Wave Nuclear Magnetic Resonance (NMR) Spectroscopy
Introduction
Continuous-wave NMR spectrometers have largely been
replaced with pulsed FT-NMR instruments. However due to the lower maintenance
and operating cost of cw instruments, they are still commonly used for
routine 1H NMR spectroscopy at 60 MHz. (Low-resolution cw instruments
require only water-cooled electromagnets instead of the liquid-He-cooled
superconducting magnets found in higher-field FT-NMR spectrometers.)
Instrumentation
A cw-NMR spectrometer consists of a control console, magnet, and two orthogonal
coils of wire that serve as antennas for radiofrequency (RF) radiation.
One coil is attached to an RF generator and serves as a transmitter. The
other coil is the RF pick-up coil and is attached to the detection electronics.
Since the two coils are orthogonal, the pick-up coil cannot directly
recieve any radiation from the generator coil. When a nucleus absorbs RF
radiation, it can become reoriented due to its normal movement in solution
and re-emit the RF radiation is a direction that can be recieved by the
pick-up coil. This orthogonal coil arrangement greatly increases the sensitivity
of NMR spectroscopy, similar to optical fluorescence.
Spectra are obtained by scanning the magnet and recording the pick-up
coil signal on paper at the control console.
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