Fourier-Transform Nuclear Magnetic Resonance (FT-NMR) Spectroscopy
Introduction
Fourier-transform NMR spectrometers use a pulse of
radiofrequency (RF) radiation to cause nuclei in a magnetic field to flip
into the higher-energy alignment. Due to the Heisenberg uncertainty principle,
the frequency width of the RF pulse (typically 1-10 µs) is wide enough
to simultaneously excite nuclei in all local environments. All of the nuclei
will re-emit RF radiation at their respective resonance frequencies, creating
an interference pattern in the resulting RF emission versus time, known
as a free-induction decay (FID). The frequencies are extracted from the
FID by a Fourier transform of the time-based data.
Instrumentation
An FT-NMR spectrometer consists of a control console, magnet, and a coil
of wire that serves as the antenna for transmitting and receiving the RF
radiation. (Only one coil is necessary because signal reception does not
begin until after the end of the excitation pulse.) Because the FID results
from the emission due to nuclei in all environments, each pulse contains
an interference pattern from which the complete spectrum can be obtained.
Because of this multiplex (or Fellgett) advantage, repetitive signals can
be summed and averaged to greatly improve the signal-to-noise ratio of
the resulting FID.
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