An alternative approach is to use zero-to ultralow-field (ZULF) NMR , in which the NMR experiment is performed in the absence of a strong † external magnetic field. This is the regime in which, in contrast to HF NMR, intramolecular J-couplings (electron-mediated indirect nuclear spin-spin couplings) are the dominant interaction, and the nuclear Zeeman interaction may be treated as a perturbation. By operating under these conditions, there is negligible magnetic susceptibility broadening for heterogeneous samples (i.e. in biphasic or porous media) , and the low-frequency magnetic fields readily penetrate metals . As an example, the depth of penetration of an oscillating magnetic field (the skin depth) for, e.g., stainless steel at a frequency of 100 MHz is ~42 µm, but increases to ~42 mm at 100 Hz. Moreover, in ZULF NMR, narrow resonance linewidths can be achieved, allowing for precise measurements of J-couplings . Since J-couplings are strongly dependent on molecular structure, this allows for chemical fingerprinting and analysis despite the fact that chemical shift information is not available at these magnetic fields. The combination of narrow linewidths due to the absence of susceptibility broadening and the ability to measure inside metal containers makes ZULF NMR a promising technique for in situ chemical reaction monitoring under practical conditions.