7.11 : NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

A pulse is a short burst of radio waves distributed over a range of frequencies that simultaneously excites all the nuclei in the sample. Upon passing a radio frequency pulse along the x-axis, the nuclei absorb energy corresponding to their Larmor frequencies and achieve resonance. This shifts the net magnetization vector from the z-axis toward the transverse plane. This angle of rotation of the magnetization vector, or the flip angle, is proportional to the duration and intensity of the pulse. A 90° pulse shifts the net magnetization precisely onto the x-y plane, while a 180° pulse shifts it to the −z direction.

Pulses are applied in sequences to reveal detailed information about the sample under study. A simple pulse sequence begins with an excitation pulse from the transmitter, followed by an acquisition time during which the free induction decay signals are collected and digitized by the computer console. The nuclei relax and reestablish equilibrium before the next pulse is applied in the ensuing relaxation delay.