oa Simultaneous δ15N and δ18O Nitrate Determination in Natural Waters via ORBITRAP-MS

Stable isotopes of nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) in nitrate are powerful tools for tracing nitrogen sources and transformations in aquatic systems. This study presents a rapid and precise method for simultaneous δ¹⁵N and δ¹⁸O determination in nitrate using high-resolution Orbitrap mass spectrometry, applied to water samples from the Rio Negro, Amazon. Samples were concentrated via SpeedVac and analyzed in negative ion mode with alternating injections of the USGS 32 standard to correct for instrumental drift. δ¹⁵N values ranged from +22‰ to +48‰, and δ¹⁸O values from ∼20‰ to ∼80‰, indicating diverse nitrate sources and biogeochemical processes. Despite no prior purification, the method showed strong reproducibility and analytical precision close to the theoretical shot-noise limit. The isotopic signatures suggest influences from microbial processes and anthropogenic inputs. Ongoing experiments involve solid-phase extraction and evaluation of potential isotopic fractionation during sample preparation to enhance the accuracy and reliability of the method in complex natural waters.