Whilst 1 H and C NMR spectroscopy can be useful probes of binding modes in amine-appended MOFs, the similar local environments surrounding the C atom makes unambiguous assignment of adsorption products difficult. There is a need to develop new techniques to unequivocally characterise these materials. From the adsorption mechanisms shown in Figure , it can be seen that each binding mode has two unique oxygen sites and the type of oxygen site present varies drastically depending on the binding mechanism. As such, O NMR spectroscopy should be a highly diagnostic probe to study these materials. As discussed previously, the low natural abundance and quadrupolar nature of spectra. Furthermore, the quadrupolar interaction introduces a second-order broadening which cannot be removed by MAS. This coupling can be quantified by its magnitude (CQ), defined as CQ= eQVzz/h, and the asymmetry (hQ), defined as hQ = Vxx-Vyy/Vzz, where e is the electronic charge, Q is the nuclear quadrupolar moment, h is Planck's constant and Vxx, Vyy and Vzz are the principle components of the electric field gradient tensor. The 17 O diso, CQ and hQ can be determined by fitting the 17 O MAS spectrum. For spectra containing many overlapping O signals, fitting the MAS spectrum can be challenging. This can be overcome by using two-dimensional MQMAS 53-55 or STMAS experiments. The use of O NMR spectroscopy, therefore, provides an additional set of parameters compared to C NMR spectroscopy, which can be used to classify CO2 adsorption products. One drawback of this method is the relatively high cost of O-enriched CO2. This, therefore, precludes the use of in situ experiments where a flow system is used. Using an ex situ MAS method, acquisition of a high-resolution O MAS NMR spectrum for amine-functionalised MOF materials can be achieved in approximately 30 minutes by dosing the sample at ~1 bar. This translates to a cost of £50 per sample, although this value could be reduced by optimising the design (i.e., reducing the volume of gas in the system) of the experimental set up shown in Figure .