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.