Double Presaturation
Presaturation is a common method of reducing the water signal in the 1H NMR spectra of aqueous samples. Sometimes, a sample may contain more than one undesirable resonance which a user may want to presaturate. In such a case, one must presaturate at multiple frequencies simultaneously. On a two-channel Bruker spectrometer, two signals can be presaturated. This is accomplished by using both Signal Generation Units (SGUs). One of the undesirable signals is put on-resonance and is presaturated with the signal from SGU1 after which the hard pulse is given (also through SGU1). The second undesirable resonance is presaturated using SGU2. The configuration is as follows:
If one has a three-channel system, one can presaturate three resonances using three SGUs. The figure below shows an example of double presaturation on a two-channel system.
The sample consisted of phenylalanine dissolved in D2O contaminated with methanol. The one-pulse spectrum in the bottom left panel shows the intense HDO and methanol signals. The double presaturation spectrum in the top left panel is on the same vertical scale as the one on the bottom left. One can see that both solvent signals have been almost completely eliminated. The spectra on the right-hand side are the same data as on the left except the vertical scale has been increased by a factor of 100. You can try this on your Bruker spectrometer using the pulse program "lc1prf2".
Source:
University of Ottawa NMR Facility Blog