Presaturation is one of the most common methods of solvent suppression. A long selective low power pulse is applied at the solvent frequency followed by a hard non-selective read pulse (or composite pulse). Aside from a well-shimmed homogeneous magnet, there are two important parameters required for effective presaturation: saturation power and saturation time. The selection of saturation power was addressed in a previous post. With a properly selected saturation power, the appropriate choice for the saturation time depends on the relaxation properties of the solvent and the B1 field inhomogeneity of the probe. To determine an appropriate saturation time experimentally, one can run a pseudo 2D saturation pulse sequence like the one in the figure below.
power of 38.4 Hz (54 dB) and a recycle time, D1, of 5 seconds is shown in the figure below.
Nyquist sampling condition is not met with sampling intervals of 50 msec, one observes an aliased nutation curve with a much lower frequency. The overall decay is due to relaxation and B1 inhomogeneity. After ~1.3 seconds, the water signal is saturated and the data are invariant for longer presatutation times. These data suggest that the minimum saturation time should be set >1.3 sec.