SETTING UP YOUR EXPERIMENT
   
Lock the field.
 

The homogeneity of the magnetic field immediately around the sample is one of the most important factors in the acquisition of high-quality NMR spectra. If the field differs even slightly from one part of the sample to another, the resonant frequencies of the corresponding nuclei will spread out, leading to broadening or other distortions of the observed lineshape. Homogeneity differences may arise from inconsistencies within the sample (such as concentration gradients or undissolved particles) or from changes in the laboratory setting (particularly if the magnet is unshielded). The effect of moving a metal-containing chair several feet away, or even of opening a steel bench drawer, can be seen in the NMR spectrum, if one looks closely enough. In the past, another significant contribution to inhomogeneity had been field drift, the slow decrease in magnetic field caused by the fact that the magnet coils are not perfect superconductors. For modern systems, this is not usually a significant problem, unless very long accumulations are being performed.
  In order to monitor the field homogeneity, the system frequency-locks onto the 2H resonance from the solvent. The exact resonant frequency of each commonly used solvent is known, and the lock system maintains the solvent resonance at that frequency, making small adjustments as necessary, so that the stability of the field is ensured. The lock frequency is also used to calculate chemical shifts in the final spectrum.
 

When the system locks onto the solvent, the 2H signal, an example of which is shown below (click here for a larger version) in its original state, is maximized (roll your mouse over the image to view this process):
 
  Note that, for other spectrometer systems, this information may be displayed differently. Once the lock has been established, the shimming routine will compensate for any field inhomogeneities by optimizing the level and lineshape of the lock signal.