7.10:
NMR Spectrometers: Overview
NMR spectrometers consist of a strong magnet, a radiofrequency or rf transmitter, and a detector attached to a computer console.
First-generation NMR instruments called continuous-wave spectrometers record spectra using the frequency-sweep or the field-sweep methods.
While resonance frequencies are determined by fixing the magnetic field strength and varying the rf signal in the former, the field strength is swept in the latter.
In modern pulsed Fourier-Transform-NMR or FT-NMR instruments, the magnetic field is held constant while the sample is irradiated with a series of short high-power rf pulses.
Pulses have a large frequency spread that can simultaneously excite NMR-active nuclei with a range of resonance frequencies.
In the interval or delay between pulses, the nuclei relax and return to their original state, releasing rf energy in the form of a free induction decay or FID signal.
The FID signals recorded by the detector are converted by a Fourier transformation into a frequency-versus-amplitude signal, which is the NMR spectrum.
7.10:
NMR Spectrometers: Overview
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.