SETTING UP YOUR EXPERIMENT
   
Shim the magnetic field.
 

While the frequency lock serves to maintain the stability of the NMR system and to provide a monitor for field homogeneity, it does not actually optimize the homogeneity. This correction is done by a process called shimming. During the shimming procedure, small wire coils are activated which add or subtract supplemental magnetic fields of particular shapes. These are designed so that combinations of shim settings can cmpensate for any unevenness in the field (more information on shimming is available here).
 

In the time domain, poor shimming is evidenced as an asymmetric FID and/or one an FID that decays too quickly. In the final spectrum, symptoms of poor shimming include symmetric line-broadening and shoulders to one or both sides of the peak, as shown below:
 

GOOD SHIM; NARROW, SYMMETRIC PEAK

POOR SHIM;
SYMMETRIC BROADENING
 

POOR SHIM;
ASYMMETRIC SHOULDER

POOR SHIM; SYMMETRIC SHOULDERS
 
While shimming can be performed manually, it is an iterative, often tedious, process which, for normal samples, can best be performed via an automated routine. If the inhomogeneity artifacts are not removed by the automated process, supplemental manual shimming should be performed. Consult your local spectroscopist for further instructions.