Presaturation
One of the simplest and widely used ways to eliminate a strong water signal is to use presaturation. In this technique, the transmitter is set to the water resonance. a very long (seconds) low
power (mW) pulse is given. The
excitation profile of this pulse is very narrow due to its length and it saturates the water resonance at the transmitter frequency. A non-selective hard 90° pulse (with a wide
excitation profile) is then given to place all remaining spins in the transverse plane for detection. An example of this is shown in the figure below. The top trace is a standard 500 MHz 1H NMR spectrum of phenylalanine in 90% H2O / 10% D2O. The resonance due to the water is huge and off-scale in the figure. The bottom trace is the same sample run with presatutation.
Source:
University of Ottawa NMR Facility Blog