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Check the linewidth of the narrowest line in your spectrum. Quite often this is your solvent reference peak. If there are some broad lines and some narrow lines, the broad lines are possibly broad because they are undergoing chemical exchange, not because the resolution is poor.
Broad lines may also be caused by quadrupolar broadening such as if your compound contains a transition metal.
If you have not already done so, load the standard shims. You don't know what sort of state the previous user left the shims in! If you are using a Varian type bestshim in the input window. This macro is equivalent to typing rts('best') su. If you are using a Bruker type rsh best
If it still won't shim, remove the tube from the magnet and inspect your sample. Is the tube scratched? What type of tube are you using? The DCIF recommends Wilmad 528 or better for all of the facility NMR spectrometers. Is there anything floating in the sample? If so, filter it! Is the sample centered in the coil? If you are using a small amount of solvent to improve the concentration, you may need to add some more solvent to make it easier to shim.
Do you have paramagnetic ions in your sample?
Have other people been getting poor resolution? Check the sign-up sheet. If so, report it to a DCIF staff member.
If none of the above are true, change the NMR tube, filter your sample, and try again. If changing the tube solves the problem, throw the old tube away.
Have you placed your NMR tube in an oven to dry? We at the Facility do not recommend drying NMR tubes in an oven. If so, throw the tube away as it has become distorted. (Remember, glass is a liquid. It flows at high temperature). The correct way to dry a tube is via a stream of dry nitrogen through a glass wool filter.");
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