Improved background suppression in 1H MAS NMR using composite pulses

A well known feature of H-1 MAS NMR spectroscopy, particularly of solids where the concentration of H-1 nuclei is low, is the presence in the spectrum of a significant broad "background" signal arising from H-1 nuclei that are outside the MAS rotor and radiofrequency coil, probably located on the surfaces of the static components of the probehead. A popular method of suppressing this unwanted signal is the "depth pulse" method, consisting of a 90 degrees pulse followed by one or two 180 degrees pulses that are phase cycled according to the "Exorcycle" scheme, which removes signal associated with imperfect 180 degrees pulses. Consequently, only spins in the centre of the radiofrequency coil contribute to the H-1 MAS spectrum, while those experiencing a low B-1 field outside the coil are suppressed. Although very effective at removing background signal from the spectrum, one drawback with this approach is that significant loss of the desired signal from the sample also occurs. Here we investigate the H-1 background suppression problem and, in particular, the use of novel antisymmetric passband composite pulses to replace the simple pulses in a depth pulse experiment. We show that it is possible to improve the intensity of the H-1 signals of interest while still maintaining effective background suppression. We expect that these results will be relevant to H-1 MAS NMR studies of, for example, nominally perdeuterated biological samples or nominally anhydrous inorganic materials. (C) 2012 Elsevier Inc. All rights reserved.