A Novel System for Simultaneous Real-Time Determination of ¹⁵N-Enriched Ammonia, Hydroxylamine, Nitrite, and Nitrate

Ammonium (NH₄⁺), hydroxylamine (NH₂OH), nitrite (NO₂^-), and nitrate (NO₃^-) account for the most important reactive nitrogen (N) species in the N cycle, playing a key role in N elimination and N retention, as well as the production of nitrogenous trace gases. However, it is still challenging to fulfill simultaneous real-time determination of all four N compounds enriched in ¹⁵N. This study successfully established a novel system by coupling an automatic simultaneous sample preparation unit to a membrane inlet mass spectrometer (4n-ASSP-MIMS) for rapid online ¹⁵N fraction analysis of all four key compounds in the N cycle. The limit of detection (LOD) was 20.22, 0.38, 0.17, and 0.25 μM for NH₄⁺, NH₂OH, NO₂^-, and NO₃^-, respectively, and relative error for ¹⁵N fraction determination was within 5% when ¹⁵N enrichment was higher than 5 atom %. This 4n-ASSP-MIMS system provides a first online analytical tool to capture the real-time isotopic signals during biogeochemical transformations of NH₄⁺, NH₂OH, NO₂^-, and NO₃^-, using the ¹⁵N tracing technique. Application of this system will dramatically advance our understanding of mechanisms involved in the N cycle.