TLDR: The lock keeps the magnetic field constant over time to prevent distortions in your spectra.
If you have ever taken a photo at night with your phone camera, you have likely seen a warning message telling you to hold the phone steady. This happens because in low light conditions the image sensor has to be turned on for a longer period of time to collect enough light to form an image. If you move your phone while the sensor is on, every pixel on the sensor ends up collecting light from different places and you get a blurry image.
NMR is one of the less sensitive analytical techniques, so we often need to use of signal averaging to generate usable spectra. When we add together multiple repetitions of the exact same experiment, true signals have the same amplitude and frequency in every repetition, while the amplitude of noise will vary randomly. Since the signal grows linearly with the number of repetitions, while noise grows with the square root of the number of repetitions, the signal-to-noise ratio increases as more experiments are added together.
However, this only works if each repeat of the spectrum is exactly the same. If the magnetic field drifts between scans, the signals will appear at slightly different positions each time. When you try to add these spectra together, instead of getting nice sharp peaks, you’ll get broad, smeared-out signals. This is where the “lock” system comes in:
- Your NMR sample tube typically contains your compound dissolved in a “deuterated solvent” (like CDCl3 or D2O). This solvent has deuterium (²H) atoms instead of normal hydrogen. Deuterium is an NMR-active nucleus with a resonance frequency that is about one sixth of 1H.
- Inside the big spectrometer, there is a small, deuterium-specific spectrometer, which runs a deuterium spectrum every few milliseconds and compares the chemical shift of the solvent to an internal reference.
- If the magnetic field starts to drift, the deuterium signal will shift from its expected position.
- The lock system detects this shift and automatically makes tiny adjustments to a small electromagnet inside the big magnet to bring the field back to where it should be.
When setting up an NMR experiment in a deuterated solvent, we talk about “finding the lock” or “locking the sample” – this just means establishing this deuterium signal reference point before starting the actual experiment.