When a new superconducting magnet is brought up to field one of the specifications for assessing it is magnetic field drift. A cryomagnet's magnetic field is not constant but changes with time, and if this change is too rapid then the magnet becomes difficult to use. Measuring the drift rate of a new magnet is one criterion for judging its quality.
NMR spectra measure frequency differences in the parts per million range. To obtain such precision the strength of the magnetic field needs to be precisely known. The lock system is used to compensate for small fluctuations in the magnetic field while recording spectra as well as long term drift in the magnetic field and this enables recording of multidimensional spectra that may take several days. However, it is also good to monitor the typical drift rate of a magnet as any changes can indicate developing problems.
To measure magnetic field drift a spectrum is recorded at periodic intervals with the lock system turned off. With the lock off, the drift will not be compensated for and the peaks in the spectrum will shift with time. Measuring the change in the frequency and plotting it against time gives the drift rate.
The figure below shows a series of spectra recorded on the npa600 NMR using a sample of ethylbenzene in chloroform-d. A single scan spectrum was recorded every hour with the first spectrum at the bottom. The peaks move to the right, or upfield. This is because the magnet is getting weaker so the signals appear to be at a higher frequency with each successive spectrum.
The rate of change of the frequency of the peaks gives the drift rate. The figure below shows the drift of the npa600 magnet taken from the spectra above, as well as the drift of the new neo600 magnet in NSB. The central line in the methyl triplet at 1.3 ppm was used to measure the frequency. The drift rate of npa600 is -6.1±0.4 Hz/hour, while neo600 drifts at +1.5±0.4 Hz/hour. Interestingly, the neo600 magnet drifts in the opposite direction to the npa600 magnet, i.e. it is getting stronger. It remains to be seen if this will continue.
An acceptable drift rate for a new 600 MHz magnet is >10 Hz/hour. The npa600 magnet has been drifting at 6 Hz/hour since installation and periodically the basic magnetic field has been reduced, but despite this the lock system has been able to compensate for the drift and the instrument has generated useful data for nearly 20 years.
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