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.
[U. of Ottawa NMR Facility Blog] Watergate vs Presaturation
Watergate vs Presaturation
Biochemists and protein chemists are often interested in observing the NH protons in their samples. Since the NH protons usually undergo slow chemical exchange with water, it is desirable to run the samples in H2O rather than D2O so the NH protons will not exchange with the deuterium in the solvent which would make them invisible in the 1H NMR spectrum. In practice, a mixture of 10% D2O and 90% H2O is used as a solvent so that a deuterium lock can be established and used while running the spectrum. The very high concentration of water compared to the very low...
Presaturation
Linear Mode
