The Effects of Bad Shimming

When the magnet shims are not optimized, the lines in your NMR spectra can take on distinct shapes. The shape of the peak will give you a hint as to which shims need adjustment. Note that lineshapes can also be much more complicated than those below and can even show splittings.

If you have used gradient shimming or another type of auto-shimming and you have peculiar line shapes, then you should check the following:
1. Was the depth set correctly on the tube?
2. Does the tube have a column of solution of approximately 5 cm? Tubes that are underfilled or overfilled will cause problems.
3. Is the sample well mixed? (see BLOG entry for September 4, 2007).
4. Is the solution cloudy?
5. Are there floaters or sinkers?
6. Is the sample paramagnetic?
7. Is there any slow chemical or dynamic exchange?