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Tried an excitation sculpted watergate on a sample in H2O with 150 mM salt and I found that it is very hard to suppress the water signal (we have a room temperature H{C,N} gradient probe, and 800 MHz instrument). Why presense of salt makes it so much more difficult to suppress water? I understand that 90 degree pulse will be longer due to the RF losses. High power 90 on water increased by ~25% in this particular case, doesn't seem to be a huge increase... Thanks. |
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High dielectric samples (e.g. salty samples) dissipate the RF energy very efficiently. This is the reason for the longer pulse widths observed by you. Water suppression in salty samples by presaturation causes extensive heating of the sample, due to dissipation of the RF energy within the sample. The resultant thermal gradients are an additional source of poor water suppression as the chemical shift of water is sensitive to temperature. However, you are not using presaturation. Based on your observations, I would guess that the problem lies in the non-linearity of the NMR response due to power dissipation within the sample, IF the excitation sculpting scheme involved proton RF pulses of different powers (soft pulses) in the watergate sequence. If only hard pulses were used for excitation sculpting then this is not the source of your problem. The second problem is the loss of magnetic homogeneity due to introduction of a sample with high dielectric medium. Though the line-width at half height may not change significantly, the peak shape may be distorted. Even minor distortions in the peak shape can lead to very poor water suppression, even with a watergate based solvent suppression scheme. Therefore, the shimset used for this sample has to be optimized extensively and will be different from that of other samples in routine use. Thanks. Yes there are two soft 90 degree pulses flanking the hard 180 refocusing pulse. - Evgeny Fadeev (Jul 10 '10 at 09:05) |
