Measuring 1H NMR spectra of samples in water (or mixtures of H2O and D2O) usually requires some form of solvent suppression of which selective presaturation is the most common method. In this technique, a long low power selective pulse is applied before the high power excitation pulse (or composite pulse). If too little power is used for presaturation, the water signal will not be sufficiently suppressed. If too much power is used, one loses intensity of signals close to the water resonance and foregoes the quantitative nature of the NMR data. The question then arises as to how much power is required for presaturation. The figure below shows the 300 MHz 1H NMR spectra of a plant extract in H2O/D2O measured as a function of presaturation power. The data were acquired on a Bruker AVANCE II console with the Bruker zgcppr pulse sequence using a two second presaturation pulse. The power levels are reported in dB and Hz.
One can see that at higher presaturation powers, one loses intensity for peaks near the water signal. In this case, where the nearest resonance of interest is 52 Hz from the water signal, one can obtain unattenuated signals with a presaturation field strength of 27 Hz. This corresponds to 58 dB on this instrument.