Improved Solvent Suppression with Composite Pulses
A hard rf pulse delivers an rf field to the NMR sample inside of the coil.
The rf field is not perfectly homogeneous nor does it end abruptly at the edge of the coil. When a long, selective, low power
presaturation pulse is given to suppress a water signal, the water in the coil may be fully saturated whereas water outside of the coil will not be. Furthermore, unsaturated water outside of the coil may be outside of the carefully shimmed region of the magnet and give rise to a broadened residual signal when
presaturation is used prior to a hard 90° excitation pulse. One long known* way to avoid this residual broad signal is to use a
composite 90° pulse after
presaturation. One of the simplest such pulses is a (90°x - 90°y - 90°-x - 90°-y) composite pulse which is designed to excite sample inside of the coil but not outside of the coil. As a result, when it is used after a water
presaturation pulse, it will not excite the broad signal from the unsaturated water outside of the coil and it will provide a spectrum with better
presaturation performance. The figure below shows a small portion of the 1H spectrum of a plant extract in H2O/D2O.
The water signal is highlighted in pink. The left panel shows a conventional spectrum acquired with a 90° pulse. The center panel is a spectrum of the same sample where a two second low power
presaturation pulse preceded the 90° hard pulse (Bruker pulse program = zgpr). One can see that most of the water is suppressed from the
presaturation pulse however, a broad water signal remains from the water outside of the coil. The spectrum in the right-hand panel is the same as that in the center except that the 90° pulse was replaced with a composite 90° pulse (Bruker pulse program = zgcppr). Clearly, the broad signal from the unsaturated water outside of the coil is essentially gone providing a spectrum with much better water suppression.
*A. Bax.
J. Magn. Res. 65, 142 (1985).
Source:
University of Ottawa NMR Facility Blog