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NMR generalities

NMR experiments are carried out in two steps

1 - signal acquisition. 2 - signal processing

Signal acquisition

The acquisition of data in NMR experiments from a sample placed in an intense B0 field requires two successive steps:

  • Excitation => application of a radio-frequency pulse (ν1) for a given time in the simplest case (“One-Pulse” experiment) => PW (usually lasting a few microseconds). Many multi-pulse experiments exist to specifically edit certain NMR data.
  • Detection => of the free induction decay (FID) which decreases over time until the system recovers its equilibrium state. This signal is recorded over a time period => AQ (usually lasting a few seconds).
Diagram of a FID signal. It is presented as a sinusoid whose amplitude decreases as the detection period AQ. The maximum amplitude at the beginning of the signal corresponds to the PW excitation.

Signal processing: the Fourier transform

The first step is to carry out a Fourier Transform (FT) on the FID, a mathematical operation that transforms a signal that changes with time (FID) into a frequency-dependent signal (spectrum). An NMR spectrum is the Fourier transform of the FID.

On the left an interferogram (FID) whose signal evolves in time. On the right, after Fourier transformation, an NMR spectrum, whose abscissa corresponds this time to the frequencies.

It is possible to improve certain characteristics of a spectrum by multiplying the FID by an apodization function before carrying out the FT. The most commonly used apodization functions can improve the signal-to-noise ratio or the digital resolution of the spectrum. After acquiring an NMR signal it is therefore possible to apply different treatments on the FID.

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