NMR Spectrometers: Radiofrequency Pulses and Pulse Sequences

A radiofrequency pulse is a short burst of high-power radio waves distributed over a range of frequencies that simultaneously excites all nuclei in a sample. The pulse includes the Larmor frequencies of the observed nuclei, causing them to absorb energy and achieve resonance. This energy absorption shifts the net magnetization vector from the z-axis toward the transverse plane.

The flip angle, or angle of rotation of the magnetization vector, is proportional to both 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 negative z direction. Longer, more intense pulses produce larger flip angles.

After a radiofrequency pulse excites the nuclei, the magnetization returns to equilibrium along the z-axis. During this relaxation process, free induction decay signals are collected during the acquisition time. The nuclei then undergo a relaxation delay, allowing them to completely reestablish equilibrium before the next pulse is applied.

A pulse sequence begins with an excitation pulse from the transmitter, followed by an acquisition time during which free induction decay signals are collected and digitized. After signal collection, a relaxation delay allows nuclei to relax completely and reestablish equilibrium before the next pulse cycle begins.

Multi-channel pulse sequences are used when observing different NMR-active nuclei in a sample. Each channel operates at a frequency matched to one specific type of nucleus, allowing simultaneous or sequential excitation of different nuclei. This enables researchers to study multiple nuclear species within the same sample.

The relaxation delay is critical because it allows nuclei to completely relax and reestablish equilibrium along the z-axis before the next pulse is applied. Without sufficient relaxation delay, nuclei would not fully recover their magnetization, reducing signal intensity and affecting the accuracy of subsequent measurements.

The free induction decay signal is collected during the acquisition time after a radiofrequency pulse excites the nuclei. This signal contains detailed information about the sample under study, including chemical shifts and coupling patterns. The computer console digitizes these signals for analysis and interpretation.