Echo pulses for background suppression by optimal control

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.