Determination of the Combined Nitrogen‐15 Content of Dinitrogen and Nitrous Oxide in Air

Abstract

AbstractA method for measuring the total 15N content of an air sample containing 15N‐N2O and 15N‐N2 has been developed. A high voltage arcing process is used to rapidly decompose N2O into N2 and NOx, to randomly distribute the N isotopes between the N2 and NOx pools, and to fix a sample of N2 as NOx. A routine mass spectrometric 15N analysis based on the 29:28 ion signal ratio of either the N2 pool or the NOx sample after conversion to N2 will be equivalent to the 15N content of the original sample. Two methods of 15N analysis were shown to be equally accurate and precise. The air sample may be arced and the N2 analyzed by direct inlet into the mass spectrometer while cryogenically trapping the NOx, or the NOx may be reduced to an NH+4‐N salt that can be stored and analyzed later. If only a small air sample is available, 10 mL of air (at ambient pressure and temperature) can be mixed with ultrapure O2 to a N2/O2 ratio of 9:91 in a 90‐mL arc vessel and arced at atmospheric pressure for at least 150 s to produce sufficient NOx to yield 1 mg of fixed N for mass spectrometric analysis. The arc method measures the combined 15N content of the major gas species produced by denitrification by a single analysis and may be applicable for field studies.