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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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