The field-frequency lock

Despite the impressive field stability provided by superconducting magnets, they still have a tendency to drift significantly over a period of hours, causing NMR resonances to drift in frequency leading to a loss of resolution. To overcome this problem, some measure of this drift is required so that corrections may be applied. On all modern spectrometers, the measurement is provided by monitoring the frequency of the deuterium resonance of the solvent. The deuterium signal is collected by a dedicated 2H observe spectrometer within the instrument that operates in parallel with the principle channels, referred to as the lock channel or simply the lock.
The lock system
The lock channel regulates the field by monitoring the dispersion mode deuterium resonance rather than the absorption mode signal that is usually considered in NMR and aims to maintain the center of

 

 

this resonance at a constant frequency (see above). A drift in the magnetic field alters the resonance frequency and therefore produces an error signal that has both magnitude and sign (unlike the absorption mode resonance that has only magnitude). This then controls a feedback system that adjusts the field setting. The dispersion signal also has the advantage of having a rather steep profile, providing the greatest sensitivity to change. Monitoring the deuterium resonance also provides a measure of the magnetic field homogeneity within the sample since only a homogeneous field produces a sharp, intense resonance.