The first practical consideration is which nucleus to study. Many studies directly probe the bound CO2 molecules by C NMR spectroscopy. As a spin 1/2 nucleus 13 C can readily be studied with a wide range of NMR techniques, though the low natural abundance of 13 C (1.11 %) may require the use of signal enhancement techniques, especially since T1 relaxation times are often long for C. The use of C-enriched CO2 is generally very helpful for increasing the signal to noise ratios and enabling more advanced experiments. Beyond 13 C NMR, O NMR may be used to directly study the CO2 molecules. The very low natural abundance of this isotope (0.037 %) generally means that enrichment is likely to be essential for most experiments. Since 17 O is a spin 5/2 quadrupolar nucleus, solid-state experiments require the use of high magnetic fields and multi-dimensional experiments to resolve closely related chemical environments (see later). Beyond directly studying the captured CO2, insight into the capture behaviour can be gained by studying additional NMR-active nuclei present in the capture material. For example, N NMR experiments have been used in amine-based materials, where reaction generally occurs at the nucleophilic nitrogen centres. A critical further consideration for NMR spectroscopy studies of carbon dioxide capture is how to prepare and study the sample in the presence of CO2. NMR experiments of carbon dioxide capture may be categorized according to whether they are performed with sample magic angle spinning (MAS), or with static sample conditions (Figure ). Experiments may then be further categorised into ex situ and in situ experiments, where ex situ refers to experiments on samples that are dosed with CO2 and sealed in the laboratory prior to NMR studies, while in situ refers to dosing the sample with gas inside the magnet while simultaneously studying it with NMR methods (Figure ). Each of these approaches has advantages and disadvantages, and the optimum method will depend on the exact materials studies and the desired information. Importantly, safety must always be the top priority. CO2 is a colourless and odourless gas at ambient conditions. Exposure to CO2 may cause headaches, dizziness, confusion and loss of consciousness. At high levels of exposure, death by asphyxiation could occur (note that CO2 is heavier than air, and can accumulate in the laboratory in some scenarios). Figure shows a common ex situ approach for studying solid-adsorbents for CO2 capture, which we term "ex situ MAS". In this approach, a MAS rotor is packed with the solid adsorbent and is transferred to a gas-dosing manifold, with the rotor left open. The sample is then evacuated before dosing CO2 gas. Following a period of equilibration, the sample is sealed, for example by using a mechanical plunger, and is then removed for MAS magnetic resonance experiments. An advantage of this approach is that standard MAS rotors and probes are used, which can greatly improve spectral resolution for solid samples. If the gas-dosing manifold is designed to have a low volume, isotopically enriched gases can also be readily used (e.g. 13 CO2) to improve signal intensities. A clear disadvantage of this approach is that it is time-consuming to study multiple gas-dosing conditions (e.g. different gas-dosing pressures, different gas-dosing temperatures). A further disadvantage is that the dosed gas may escape from the MAS rotor over time (depending on how strong the adsorption is, and how leak-tight the rotor is). In cases where bespoke rotor dosing equipment is not available, simple alternatives such as a gas-filled glovebag or chamber may also be employed, provided appropriate safety measures are taken to avoid CO2 exposure. The ex situ MAS approach has been applied to a range of solid-adsorbent materials to study carbon capture modes. A number of studies have explored carbon capture chemistry in amine-functionalised silicas, 20-26 metal-organic frameworks, 8,27-32 amine-functionalised metal-organic frameworks, zeolites, polymers, and porous carbons. The disadvantages of the ex situ MAS approach can be partly resolved by the "in situ MAS" approach (Figure ). Here MAS NMR experiments are performed on the solid adsorbent sample with the simultaneous delivery of CO2 gas to the sample via a hole in one of the rotor caps. This approach has the advantages that the sample is studied directly under operating conditions, and that the gas dosing conditions (sample temperature and partial pressure of CO2) can be varied continuously.