Slide 5 of 16
Notes:
A review of the literature of peak supression would reveal a bewildering array of methods, for this has been a very popular field for experimentalists. For the longest time, however, basically only one method fulfilled the most important criteria and was basically exclusively used: presaturation.
It was technically easy to implement, easy to setup, stable, and reliably provided sufficient supression of the water signal, usually by at least 2000 fold in a single scan. The principle is the usage of a very low power thus selective pulse directly on the water resonance. Thus water would be continuously excited, until an equilibrium between excitation and relaxation is reached = near/basically equal population of the energy levels, which means there is no observable magnetization. Requirement= long irradiation time, (few times T2)
sufficient power, so that the saturation factor (gB2T2 ) large enough (approx 1000).
For water (although solute dependent) :
T1= 5s; T2 = 1s; ==> gB2 = 20 Hz and 2-3 s sufficient.
Remember the filed strength of a pulse is given by : gB1/2 = 1/pw90,
thus if your standard power 58 db gives you a 9usec pulse, then the minimum powerlevel for presat (20 Hz) would be : -5 db !
