Reduction of the water hump using a composite refocusing pulse

Abstract

.B, field inhomogeneity gives rise to a broad hump in a NMR spectrum. This hump is ;a particular nuisance in recording the proton NMR spectrum of dilute samples dissolved in Hz0 because the intense hump from H20, referred to as a “water hump,” cannot be saturated with a monochromatic RF field so that it remains as an intense an’d broad signal in the spectrum, which makes it difficult to observe weak solute signals. Dykstra has demonstrated that the modification of a NMR probe by shielding the leard wire of RF coil can significantly reduce the water hump ( 1) . Many recent NMR probes have been modified according to the procedure of Dykstra, and we also use thi.s kind of a modified NMR probe on our spectrometer in the present study. The use of the modified probe efficiently reduces the water hump by 90-95% (1). However, for dilute solutions, especially those with high ionic strength, the remaining water hnmp is still a severe obstacle. Under the condition of high ionic strength, RF power in the sample solution is reduced due to dielectric loss and then the resulting inhomogeneous B, field becomes significant, which gives a larger water hump. Several pulse sequences have been devised for obtaining the spatial selectivity of signals in in vivo spectroscopy. Bendall and Gordon (2) have proposed a pulse sequence which can selectively observe signals from the region of homogeneous B, field. Their pulse sequence is based on a (90”-180” -Acq.) scheme, where the phases of 180” pulse and receiver are cycled in a Exorcycle manner as described by Bodenhausen (3). To improve spatial selectivity further, both Shaka and Freeman (4) and Tycko and Pines (5 ) have developed composite narrow-band inversion pulses that are very sensitive to B1 field strength. The replacement of a regular 180” pulse in the (90°18;O”-Acq.) scheme with these narrow-band composite pulses makes for more efficient spatially selective observation. In a different type of pulse sequence, Bax has shown a spatially selective composite 90” pulse (6). Recently, Zagorski has demonstrated that the wideband inversion pulse GROPE-16, by Shaka and Freeman ( 7), is applicable to solving the problem of B, inhomogeneity (8). The requirement of the pulse scheme for the water hump suppression in highresolution NMR spectroscopy is to eliminate only the water hump efficiently without attenuating signals over a relatively wide frequency range. However, these requirements