Variable rotation composite pulses for high resolution nuclear magnetic resonance using inhomogeneous magnetic and radiofrequency fields

An approach toward high-resolution NMR spectroscopy on samples located outside the physical confines of a magnet (ex situ NMR), has recently been described [C.A. Meriles, D. Sakellariou, H. Heise, A.J. Moulé, A. Pines, Science 293 (2001) 82]. Nutation echoes are generated by a train of z-rotation pulses in the presence of spatially matched static and rf field gradients. These pulses were based on a combination of `perfectʹ π/2 constant rotation composite pulses and `imperfectʹ ordinary pulses of variable length. Here we introduce a new class of `self-compensatedʹ z-rotation composite pulses based only on variable rotation inversion pulses that lead to an rf dependent phase shift. Experiments and simulations show that these new pulses perform well at high B0 gradients and require less rf power than previous schemes.