Experimental demonstration of wideband spin inversion

Abstract

Although spin inversion is widely used in many modem pulse sequences for highresolution NMR, a simple 180” pulse only provides efficient inversion if it is spatially homogeneous and relatively close to resonance. Recently, a family of phase-alternating composite 180” pulses has been proposed (1). These new sequences, devised by numerical nonlinear optimization, compensate for resonance offset effects over a surprisingly large bandwidth, as much as 6.4 rB,/2?r Hz. They are applicable to noninteracting spin-i nuclei in the liquid phase, neglecting the effects of relaxation during the sequence. In scalar-coupled spin systems the sequences are predicted to be effective provided the duration of the composite pulse is short compared with the inverse of a representative coupling constant. No compensation for miscalibration or spatial inhomogeneity of the B, field is offered, but for many situations of interest in highresolution spectroscopy these factors do not present a serious threat. At the time of the initial report (I) only the six-pulse sequence was checked experimentally. The more complex members of the family ran into the limitations imposed by the spectrometer pulse programmer. Since the theoretical calculations predict an unprecedented improvement in operating bandwidth for the longer sequences, it is important to establish that they can indeed be implemented in practice. Here we verify the predicted performance. A pulse programmer device recently developed for cyclic permutation of decoupling sequences (2) is easily adapted to the generation of almost arbitrarily long phasealternating composite pulses, providing ready access to the new spin-inversion sequences. A schematic of the circuit appears in Ref. (2). The pulse sequences are stored in two 2732 EPROMS in the form of 12-bit timer-words and 3 bits for the control of the transmitter gating, 90 and 180” phase-shifts. (Only 180” phase shifts are used in the present work.) A microcomputer (Sinclair QL) is used to generate the sequences which are then transferred directly to the EPROM programmer. The timerword is fed to three 74ALS 19 1 timers connected in a “ripple” configuration clocked at 10 MHz, giving a resolution of 0.1 I.LS. For decoupling experiments, the device performs a repetitive sequence of phase inversions independent of the spectrometer timing; for composite pulse experiments, it is necessary to synchronize the start of the sequence with the spectrometer pulse programmer. This was achieved by programming a repeated short delay (for example, 5 ps) prior to the start of the composite pulse; a trigger signal