Echo pulses for background suppression by optimal control

Abstract

Materials used to construct magic‐angle‐spinning NMR probes can contain NMR active nuclei that produce a significant amount of background signal. Because these materials are located outside the sample coil, the use of spatially selective pulses to remove the background is a popular approach for background suppression. However, previously suggested spatially selective pulses suffer from limited excitation bandwidths, which may make them unsuitable for the acquisition of nuclei with a large chemical shift range. Here, a pulse (OC‐BACK) is presented, which has been developed by optimal control, which has a flat profile of ~120 kHz with respect to off‐resonance effects and extended pass and suppression bands with respect to the nominal nutation frequency. The presented solution is large enough to be effective for background suppression schemes in 19F magic‐angle‐spinning NMR at medium and low magnetic fields.