NMR spectrometers consist of a strong magnet, a radiofrequency transmitter, and a detector attached to a computer console for recording spectra of samples containing NMR-active nuclei. In first-generation NMR instruments called continuous-wave spectrometers, the resonance frequencies of the nuclei are determined by frequency-sweep or field-sweep methods. The magnetic field strength is fixed and the rf signal is swept in the former, while the radiofrequency signal is fixed and the magnetic field strength is gradually increased in the latter.
In modern pulsed-Fourier-Transform-NMR instruments (FT-NMR), the magnetic field is held constant while a sample is irradiated with a series of short pulses. A pulse is a short burst of energy distributed over a range of radio frequencies that excites the nuclei into higher energy states. As each nucleus relaxes in the interval or delay between pulses and returns to its original spin state, the energy is released as an electrical impulse called free induction decay or FID. The FID is then converted into a frequency-versus-amplitude signal via a mathematical technique called Fourier transform and recorded as an NMR spectrum.
