This section has been included to give the user a basic understanding of the principles of the lock system. Practical aspects such as how to actually lock the sample will be dealt with in the section Locking the Sample.

The aim of the  lock system is to ensure that the strength of the magnetic field surrounding the sample does not change during an experiment or that the field is not modulated by external disturbances. NMR analysis involves measuring the precise frequency of signals that are emitted by the sample. The frequencies of these signals are directly proportional to the magnetic field strength, i.e. if the field strength varies, so does the emitted frequency. Therefore, the user must be confident that the magnetic field strength is always maintained at precisely the same strength, which is referred to as " locking" the sample. The lock system is essentially a separate spectrometer designed to observe deuterium. It should be mentioned that the signals emitted by deuterium are normally far removed from frequencies of interest. However, if the deuterium frequency is unsuitable, then a  Fluorine (19F) lock may be used. Since it is by far the most popular, only the deuterium lock will be dealt with here, but the reader should note that the principle of the deuterium and the fluorine lock are identical.

In AVANCE systems the BSMS provides the hardware required to implement the lock and a separate deuterium module in the HPPR  transmits and receives the lock signals. Some deuterium must of course be introduced into samples that are under analysis. This can most easily be done by dissolving the sample in a deuterated solvent. A  deuterated solvent is one in which a large percentage of the hydrogen atoms have been replaced by deuterium. Commonly used deuterated solvents are acetone-d6, benzene-d6, chloroform-d and DMSO-d6, although many other solvents are available. The sample which will be used to illustrate some basic NMR techniques in this manual is Menthyl Anthranilate in DMSO-d6.

The frequency of the signals emitted by deuterium for a particular size of magnet is precisely known. Therefore, if the magnetic field strength is correct, any deuterium nucleus in the sample should emit this exact frequency. If the magnet strength varies so will the deuterium frequency. The  lock system uses a receiver (housed in the BSMS rack) to monitor this deuterium frequency and makes adjustments to the magnetic field strength accordingly.

The receiver in the lock system is designed so that when the field strength is correct (i.e. the correct deuterium frequency is detected), no adjustments are made to the field. However, should the field strength vary (drift ), a current is changed in a special coil (the H0 coil) located within the shim system of the magnet, which has the effect of returning the field strength to the correct value. The deuterium frequency is measured several thousand times per second. Hence, as long as the system is locked, the user can be confident that the field is maintained at a constant strength during acquisition.