High-field nuclear magnetic resonance (HF NMR) spectroscopy is a powerful analytical tool for reaction monitoring, as it provides structural and chemical information about a sample in a quantitative and noninvasive manner. However, the operation of conventional NMR spectrometers imposes substantial restrictions on its applicability for in situ reaction monitoring. Reactions carried out in metallic containers cannot be addressed by HF NMR (typically, NMR sample containers are made of glass, quartz, plastic or ceramic), and heterogeneous samples generally yield low-resolution spectra due to magnetic susceptibility induced field gradients . Nevertheless, NMR spectroscopy is extensively applied for analyses of reactions conducted in NMR tubes , and for offline analysis of aliquots from chemical reactors . More advanced approaches such as rapid injection and stopped-flow analysis have been proposed to study rapid kinetics and have allowed direct observation of reactive intermediate species. However, these approaches are generally incompatible with operando analyses . Recently, online NMR reaction monitoring was introduced, which involves transfer of a reaction mixture from a reactor to the NMR magnet for the detection of the flowing sample without the need for discrete sampling or exposing the reaction mixture to the external atmosphere . At the same time, the studies of heterogeneous reactors and reacting systems require different experimental approaches.