Simultaneous 15N Online Analysis in NH4+, NO2–, NO3–, and N2O to Trace N2O Production Pathways in Nitrogen-Polluted Aqueous Environments

Engineered nitrogen (N) removal processes in water treatment plants and N-transformation reactions in polluted environments represent prominent sources of the potent greenhouse gas, nitrous oxide (N2O). The relevance of microbial and abiotic formation pathways can be assessed by using 15N tracer techniques. While 15N–N2O analysis with optical analyzers is straightforward, the quantification of atom % 15N of inorganic N compounds, such as ammonium (NH4+), nitrite (NO2–), and nitrate (NO3–), requires discrete sample analyses that are time-consuming and labor-intensive. In this study, we developed an automated sample preparation unit, coupled to a membrane inlet quadrupole mass spectrometer, for the online, quasi-simultaneous analysis of atom % 15N in NH4+, NO2–, and NO3–. This technique was designed and validated for 15N-spiking applications at moderate (100–200 μmol L–1, 1 atom % 15N) to high (2–3 mmol L–1, 33 atom % 15N) dissolved inorganic N concentrations typically encountered in sewer systems or contaminated watersheds. The high potential of the developed system, in combination with 15N–N2O analysis by Fourier-transform infrared spectroscopy, to constrain N transformations and sources of N2O was demonstrated in a feasibility study, where nitrifier denitrification was identified as the primary N2O formation pathway during the partial NH4+ oxidation to NO2– in a lab-scale sequencing batch reactor.