Using a titanium tube allowed for reactions to be carried out at higher pressure than was possible with the glass tubes. As mentioned in the Introduction, this is possible using ZULF NMR due to the larger skin depth for conductive metals at lower frequencies. Titanium was chosen as a non-magnetic metal, but we anticipate that other metals may be appropriate (perhaps after degaussing). Considering that industrial reactors are typically operated at high temperatures and pressures that are only practically attainable in metal containers, ZULF NMR is uniquely capable of non-invasive reaction monitoring under realistic conditions. Metal reactors would be indispensable for studying such industrially important reactions as synthesis caused by bubbling a gas) due to magnetic susceptibility broadening, but this is not the case for zero-field NMR under similar conditions. Additionally, there is no detectable line broadening and signal loss from the use of a titanium tube instead of a 5 mm glass NMR tube. The rate of bubbling used in all experiments was 50 ml min -1 , and 'stopped-flow' means the bubbling was stopped prior to signal acquisition.