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.