A substance's spectrum is its fingerprint. Usually, to record one, you place the sample in a strong magnetic field, like in an MRI scanner. But there is a way to do without a magnet altogether: detect the signal from atomic nuclei at zero field. Until now, this method suffered from poor sensitivity, and its complex spectra were hard to predict.

The authors of the new study took ordinary liquids, the kind sitting on a lab shelf, and for the first time recorded their spectra at zero field using carbon-13 at its natural abundance (just 1.1% of all carbon atoms). No special sample preparation or expensive signal enhancement was needed. The instrument, a compact rubidium vapor magnetometer, proved so sensitive that it even distinguished molecules with two rare carbon-13 atoms, which make up only 0.0121% of the sample. The spectral lines were narrow (less than 250 millihertz), and the instrument ran stably for over a week.

In parallel, the scientists learned to predict these spectra using quantum-chemical calculations that account for molecular vibrations. The prediction accuracy is on the order of a few hertz. Comparing calculation with experiment proved useful in itself: discrepancies showed where a molecule forms hydrogen bonds, how it hydrates in water, or forms ion pairs in a concentrated salt solution.

This approach paves the way for cheap and flexible analysis of substances: you can look inside metal containers, and you don't need a bulky superconducting magnet. And the calculations help not only identify a molecule but also understand who it interacts with in solution.