Figure 9.40(^13) C spectrum of 3-hydroxymethylbenzene (noise decoupled).
Pulsed Fourier Transform NMR (FT-NMR)
Conventional scanning (CW) NMR spectrometers take several minutes to scan a full-range spectrum
which is said to be in the frequency domain (absorption vs frequency). With a pulsed or Fourier
transform (FT) spectrometer the sample is subjected to a series of single (several microseconds
duration), high-power RF pulses of wide frequency range which excite all the nuclei of a particular
element (isotope). Between each pulse, the excited nuclei emit radiation in the form of a rapidly
decaying signal as they relax to re-establish the equilibrium population of spin states. This is known as
a free induction decay signal, or FID, and it contains spectral information from all of the nuclei excited
by the pulse, although it is not in the form of a CW spectrum. It is said to be in the time domain as the
signal is monitored as a function of time. Time and frequency domain spectra are mathematically
related by a set of Fourier transform equations. A dedicated computer can sample and digitize the FID
and transform it into the corresponding frequency domain spectrum in a second or less, compared to the
several minutes required to record a CW spectrum. This considerable saving in time can be used to
enhance the sensitivity of NMR by repeating the pulse sequence many times and co-adding the FIDs to
improve the signal-to-noise (S/N) ratio. For example, the accumulated data from one hundred pulses,
which can be collected and transformed in about two minutes, will increase the S/N ratio by a factor of
10, or in general by the square root of the number of pulses or FIDs (as below) accumulated.
Some examples of FID signals and their corresponding frequency domain spectra are shown in Figure
9.41. The FID signal related to a single resonance peak (Figure 9.41(a)) is seen to consist of a decaying
sinusoidal wave whose frequency corresponds to that of the resonance frequency. Two