Hard-pulse sequences for solvent suppression which only need a zero-order phase correction

Abstract

Among the various methods which have been proposed to achieve solvent suppression, the binomial hard-pulse sequences (1-8) are rather attractive. Their popularity in conventional NMR spectroscopy is undoubtedly due to several advantages. They produce little radiofrequency power dissipation in the sample, they lead to a water suppression factor which is insensitive to Bi inhomogeneities, and they can be easily implemented on most FT NMR spectrometers. Furthermore, the short total duration of binomial sequences allows the observation of slowly exchanging protons or of resonances with short T2. However, their main drawback is that, except for the jump and return sequence (i.e., 90°-T-900), they generally require a linearly frequencydependent (first-order) phase correction. In the case of a large residual solvent peak such a correction results in a rolling baseline (2, 3). Recently pulse sequences which only require a small frequency-dependent phase correction have been reported (9, HO). In this contribution we propose a new class of hard-pulse sequences which achieve good suppression of the solvent signal and which only require a frequency-independent (zero-order) phase correction. The basic concept is to exploit some interesting refocusing properties of phasecycled pulse sequences symmetrical or antisymmetrical in time (II). Let us consider the BX--7-8X binomial pulse sequence (2,5). Neglecting off-resonance effects during the pnlses and effects of spin-spin coupling, the corresponding propagator can be written as P = exp(+iU,)exp( iwTl,)exp( iU,). Cl