NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

A pulse is a short burst of high-power rf radiation that includes the Larmor frequencies of the observed nuclei in a sample.

As the nuclei absorb energy, the net magnetization vector shifts toward the transverse plane.

This angle of rotation, or the flip angle, of the magnetization vector, is proportional to the pulse duration and intensity.

A 90-degree pulse shifts the net magnetization precisely onto the x–y plane, while a 180-degree pulse shifts it to the −z direction.

As the magnetization returns to equilibrium along the z-axis, the emitted FID signals are collected during the acquisition time.

A pulse sequence begins with an excitation pulse from the transmitter, followed by the acquisition time.

Then relaxation delay allows the nuclei to relax completely and reestablish equilibrium before the next pulse is applied.

Experiments involving different NMR-active nuclei use multi-channel pulse sequences, where the frequency of each channel is matched to one type of nucleus.