As it was discovered in the 18th century, there was a fraction of air that did not support combustion. That fraction was indeed Nitrogen. Nitrogen is a non-reactive gas at normal temperatures and the combustion technology used to oxidize the Nitrogen sample is different from that of Carbon analysis. To analyze Nitrogen quite often the sample is heated to very high temperatures of 3000o C (5432o F) in the atmosphere of inert gas, like Helium (He). Nitrogen is released from the sample into the Helium atmosphere. This changes physical qualities of the atmosphere, primarily the rate of thermal conduction. The change in the rate is measured to derive the amount of Nitrogen released during the process.
All elemental analysis techniques have to take into consideration impurities introduced by the equipment, such as furnace, crucible, detectors, as well aas those of the inert gas, like Helium in our case. To compensate the effect of those impurities on the concentrations of detected Nitrogen compounds, a series of tests could be run with and without the sample.See various Nitrogen Analysis Resources here.
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