USER-FRIENDLY SELECTIVE PULSES

The "spin-pinging" selective excitation scheme is described and analyzed with particular attention to its tolerance of phase errors, pulse-width misadjustment, and spatial inhomogeneity of the radiofrequency field. Through difference spectroscopy, this scheme generates pure-absorption-mode spectra at all offsets from resonance. The selectivity may be varied from very high (line-selective) to low (band-selective) simply by adjusting the duration of the soft pulse. When a rectangular soft pulse is employed, the excitation profile has only very weak side-lobe responses, and the profile may be considerably improved by shaping the pulse with an envelope that is symmetrical in time. An operator-guided evolutionary algorithm has been used to design a soft-pulse shape that gives uniform excitation over a broad central band of frequencies, flanked by narrow transition regions and negligible excitation elsewhere. In addition to its use as an excitation pulse, spin pinging can be employed for coherence transfer in a COSY experiment. This suggests an application for band-selective excitation and coherence transfer in multidimensional NMR with a view to limiting the size of the data table and the duration of the experiment. © 1992.