How does the lock work?

Magnets used to manufacture low-field and high-field NMR spectrometers are not perfect and the magnetic field that they generate is prone to drift for a variety of reasons. However, during an NMR experiment it is important to keep the magnetic field as stable as possible to prevent the signals from drifting. This is taken care of by the lock system.

So, what is field drift and how does it affect an NMR spectrum? Magnetic field drift is simply the change in the magnetic field over time. Generally speaking, the magnetic field will typically drift by 1 to 10 Hz per hour,^[1] but in this theoretical example we will use 6 Hz, which corresponds to 0.1 ppm in a 60 MHz ¹H NMR. In Figure 1 you can see the effect of the magnetic drift in a proton spectrum. Six identical spectra of ibuprofen are superimposed, but the green spectra were each shifted by 0.1 ppm (i.e., 6 Hz) to high frequency. (Sidenote: “downfield” is a historic term that originates from early CW NMR instruments and is not perfectly correct to use anymore as in modern FT NMR instruments the magnetic field is constant - check out Glenn Facey's blog post on The Scale on an NMR Spectrum)

This should give you an idea of how a field drift affects your spectrum if you acquire data unlocked. Please note that we exaggerated the effect here by a lot. 

Figure 1. Superimposed 60 MHz 1H NMR spectra of ibuprofen. Black line: original; green lines: shifted spectra.

What can we do about field drifts? This is where the lock comes into play - for preventing the magnetic field from drifting and keep it as stable as possible, the lock system is monitoring the resonance frequency of the signal in the lock channel; and if a shift is detected the magnetic field is adjusted accordingly so the resulting overlay of FIDs will display no shifted or broadened spectra.

In Figure 2 you can see the three different scenarios from left to right that can be observed in the lock channel. The lock signal is shown in dispersion mode, as the sign changes for drifts to high or low frequency, respectively.

Figure 2. Lock circuit feedback loop for the different magnetic field (B0) scenarios.[2]

Left:          There is no field drift, the lock signal appears exactly where it should be – “on-resonance”

Middle:   The field is too high and needs to be decreased, as the center of the lock signal has shifted to high frequency 

Right:       The field is too low and needs to be increased, as the center of the lock signal has shifted to low frequency

^[1] Webb, A. CHAPTER 1:The Principles of Magnetic Resonance, and Associated Hardware , in Magnetic Resonance Technology: Hardware and System Component Design, 2016, pp. 1-47.

^[2] Jacobsen, N. E. NMR Spectroscopy Explained: Simplified Theory, Applications and Examples for Organic Chemistry and Structural Biology; Wiley: New Jersey, 2007; p 79.