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I am working with carbohydrates and in 1D NMR we have anomeric protons very close to the HOD peak, sometimes even right under there. In the lab where I used to work we had a bruker machine with an adapted WEFT pulse (described by Hard et al, 1995), which basically does a 180 degree pulse on the HOD, followed by a HS pulse of 50 usec, d1 times to have 0 magnetization on HOD, then a detection pulse. Since the HOD relaxes much slower than the glycan H-C protons you get about 80% signal on nearby glycan protons and your HOD is usually nearly gone. Glycan protons farther away from the HOD are at full signal strength (which is not so in the normal WEFT pulse described in 1972) Where I work now, we have Varian machines with pre-installed PRESAT and WET1D pulses, however, these pulses do not only suppress the HOD signal, but also remove any carbohydrate signal that is near, or under, the HOD signal. Does anyone know of a way to adjust settings in the WET1D, which uses a shaped pulse for the HOD suppression much like the WEFT, to avoid the glycan signal suppression? Or do I have to try and get our old bruker pulse from my previous employer and try to translate it into Varian language? |
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Hi, There are few important things in setting up the experiment in varian spectrometers. The most important thing one must optimize it the transmitter offset (tof variable) and the saturation frequency (satfrq, and its corresponding power satpwr and delay satdly). As the tof is set up depending upon the solvent you chose, make sure you have set the solvent variable right (depending upon the version of VNMRJ you should be able to do it from a drop down menu in the "Lock" tab). Optimization of satfrq is simple, you have to array the satfrq variable, check the response of water suppression from the experiment and chose the value at which the best water suppression is achieved (remember that you suppression is dependent on the goodness of the shims). The cases where water is the solvent, the tof and the satfrq are set to the same value, however the varian pulse program also gives you the option to suppress signals other than the solvent (by having a satfrq different from tof). WET1D is similar, if I remember right it is exclusively written for water as solvent, therefore the tof decides the resonance that is saturated. As I understand from your observation of solute signals missing and water not being suppressed, I think it is because the tof and/or satfrq is mis-set. Make sure you optimize it for the first time, the remaining times the variable shouldn't move too much (unless and otherwise there is significant change in sample conditions like pH, ionic strength, etc.). WEFT is slightly different from WET, but I have not seen the pulse sequence in varian before, so cannot comment. But I would assume presat or wet (and may be even watergate) should work for your case. Good luck, Waj |
