The lock system

Recording an NMR spectrum requires working through a series of steps, one of which is "locking", but what exactly is locking? Locking is monitoring one of the NMR signals in a sample so that any fluctuations in the strength of the magnetic field can be compensated for.

NMR spectra measure frequency differences of much less than one part per million (ppm). Since the frequency depends on the magnetic field, variations as small as one in a billion in the field strength will result in broadening or even shifting of the peaks. To prevent this the lock system continuously monitors a signal, typically ²H, so that the magnetic field can be precisely controlled. If you use deuterated solvents then there is no need to add additional ²H, but for samples dissolved in H₂O or buffers 5-10% D₂O is added to provide a lock signal.

On Bruker systems the lock signal is shown as a frequency domain signal, like that produced by continuous wave NMR spectrometers that sweep through a frequency range. As the system sweeps past the lock frequency the signal spikes, then as the sweep moves on the signal rings down. The range over which the system sweeps looking for the lock signal is known as the sweep amplitude. If the lock signal is not visible the first thing to try is to increase the sweep amplitude until you can see the signal. After the lock signal is found it needs to be adjusted so that it is centered and symmetrical. The lock field is used to move the position of the signal and the lock phase adjusts the twist of the signal. With the lock signal symmetrical and centered the lock can be turned on.

Once "locked" the lock system stops sweeping back and forth and should maintain a constant position. This position is proportional to the height of the lock signal. Improving the shimming will narrow the lock signal and increase it's height, which is why the height of the lock signal is often used to monitor the quality of the shimming. If the lock signal does not maintain a constant position, but instead slowly oscillates up and down, then this indicates that the lock power is too high. The lock power should be reduced until the oscillations stop. The lock gain can then be used to scale up the signal.

When locked, the lock system measures the frequency of the lock signal thousands of times per second. If the signal starts to drift, then the current in the Z_o shim coil is adjusted to alter the strength of the magnetic field and bring the lock signal back to its starting point. This maintains a constant field strength in the magnet and ensures the peaks are as sharp as possible.