Distortions due to Lock Saturation

The amplitude of the ²H lock signal provides information for an electronic feedback circuit which continuously corrects the magnetic field strength (by way of a B₀ shim) to compensate for environmental instability.   A poor ²H lock signal will provide unreliable input for the feedback circuit and B₀ compensation will be erratic.  This leads to undesirable effects in NMR spectra.  For example, noisy lock signals will lead to undesirable noise at the base of the observed NMR peaks.  If one uses too much lock power, the ²H lock signal gets saturated and the lock amplitude is unstable.  A saturated ²H lock will lead to problems in the NMR spectrum since the input to the B₀ compensation feedback circuit is unstable.  This is demonstrated in the figure below.

When one scan is collected, there are spectral distortions at the base of the NMR resonances.  When 16 scans are collected these artifacts average to produce a general broadening at the base of the NMR resonances.  Be careful not to saturate the ²H lock.