Amine-silica composites have been prepared following the grafting technique, tethering diethylenetriamine alkoxysilane (DT) onto SBA-15 pore walls, obtaining a series of materials with increasing DT content. A novel application of Si-29 CP/MAS NMR to estimate the amount of nitrogen loaded into the materials has shown to give comparable results to the elemental analysis technique. The amine-silica composites have been tested for CO2 capture using a pure gas stream and an ultra-dilute mixture with 400 ppm CO2 that simulates atmospheric air. In both cases, the net CO2 uptake (mmol CO2/g ads) and the amine efficiency (mol CO2/mol N) increased with the amount of nitrogen up to a certain point where the materials were too saturated. Regarding the comparison of CO2 adsorption results obtained for both streams, it was found that the CO2 uptakes in simulated air were increasingly similar to those achieved in pure CO2 as the DT content of the amine-silica composites was increased.