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The lock is a separate NMR spectrometer within the spectrometer which is specific to deuterium. In order to produce a high resolution NMR spectrum of a sample, it is key to keep a constant magnetic field strength over the duration of the experiment. This is achieved by "locking" the spectrometer to the frequency of deuterium, ie by centering the deuterium signal to a predefined frequency. This signal is constantly monitored and compensations for the external magnetic field (B~0~B0) drift are made in order to keep the resonance frequency constant.
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