What is lock?

TL;DR: The lock keeps the magnetic field constant over time to maximize sensitivity and 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 add the results of multiple repetitions of the exact same experiment to generate usable spectra. We call these repetitions “scans” and the whole process “signal averaging.” Signal averaging works, because true signals have the same amplitude and frequency in every repetition so they add perfectly from scan to scan, while the amplitude of noise at a given frequency will vary randomly leading to imperfect addition. Since the signal increases linearly with the number of repetitions and noise grows with the square root of the number of repetitions, the signal-to-noise ratio doubles for every four-fold increase in the number of scans.

However, this only works if each scan is exactly the same. If the magnetic field drifts between scans, the signals will no longer have the same frequency in each repetition and therefore not add perfectly. This reduces the effectiveness of signal averaging and generates broadened, “blurry” signals. This is where the “lock” system comes in:

1. Your NMR sample tube typically contains your compound dissolved in a “deuterated solvent” (like CDCl3 or D2O) that has deuterium (²H) atoms instead of normal hydrogen (¹H). Deuterium is an NMR-active nucleus with a resonance frequency that is about one sixth of ¹H.
2. 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.
3. If the magnetic field starts to drift, the deuterium signal will shift from its expected position.
4. The lock system detects this shift and automatically makes tiny adjustments to a small electromagnet inside the big magnet to bring the magnetic 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 to ensure as close to perfect signal averaging as possible.

It is possible to run samples unlocked in non-deuterated solvents. The magnetic-field drift will be small enough over a few minutes to have only minimal effect on quick experiments. On the other hand, longer experiments on nuclei with sharp lines, like an hour-long ¹³C, may be completely useless without locking.