Solvent suppression comes in many forms, the most common is when a single peak (usually from the protonated fraction of your solvent) is removed from the spectrum. Presaturation is the method where a low power long pulse is applied at the frequency to be obliterated. Gradient based techniques where the magnetization due to the offending solvent is “scattered” while in the transverse or XY plane, this results in a net zero magnetization. What is common to both of these methods is that they both on resonance. This means that the centre field is set to frequency of interest, this is not an absolute requirement and offsets can be used to retain a reasonable sweep width. In the case of suppressing a residual peak from D2O the centre would be set to approximately 4.7 ppm which is conveniently in the middle of the proton window. However, if the residual peak was from dimethyl sulfoxide (DMSO), located at 2.5 ppm then the an offset would be required or an extremely large sweep width could be used to compensate.
Another problem that arises in solvent like DMSO is the second solvent peak, here it is actually H2O in the DMSO. Try as you might your DMSO will always be a little wet and sometimes a lot wet depending on how it has been stored. It would be nice to do something like presaturation to get rid of both solvent peaks. Luckily a number of methods are available to do multiple solvent peak suppression. Here I will describe how to use the systems set up for liquid chromatography NMR (LC-NMR). Bruker have developed a suite of experiments to allow for the acquisition of NMR data after LC separation using non or minimally deuterated solvents. The experiments work perfectly well with fully deuterated solvents and have the advantage of being preconfigured for 1H, COSY, TOCSY and HSQC. Generally the experiments also come set up to do carbon decoupling which is a necessity for the non deuterated solvent systems as the carbon satellites would be very noticeable under these conditions. Luckily this is not likely to be a problem when using normal deuterated NMR solvents.
The double presat Bruker parameter set is either LC1D12 or LC1D12GP. After loading the parameter set go through the acquisition parameters and set them according to you sample just as you would if acquiring a regular proton experiment. When ready to acquire type xaua au_lc1d this is the magic of this method. The preparation program quickly obtains a 1H spectrum and determines the chemical shift of the two largest peaks in the spectrum. This is actually the Achilles heal of the method, if the peaks that are to be suppressed are not in fact the two largest in the spectrum then the experiment will fail. Once the preparation program obtains the chemical shifts to be suppressed it creates a custom shape pulse that will specifically saturate the two frequencies in question.
The LC-NMR manual has a list of all the different experiments that Bruker have prepared for double supression. The 2D experiments can be prepared for use in exactly the same way with the minor variation that the preparation program is xaua au_lc2d.
These experiments can also be implemented using automation without any further modifications.
At UNSW the four experiment types named about have been implemented on all instruments and have the suffix “lcpr” to differentiate them from other solvent suppression routines.
