Automated rapid triple-isotope (δ15N, δ18O, δ17O) analyses of nitrate by Ti(III) reduction and N2O laser spectrometry.

The nitrogen and oxygen (δ¹⁵N, δ¹⁸O, δ¹⁷O) stable isotopic compositions of nitrate ($N{O}_3^{\text{-}}$) are crucial tracers of nutrient N sources and dynamics in aquatic and atmospheric systems. Methods to reduce aqueous $N{O}_3^{\text{-}}$ to N₂O gas (microbial or Cd method) before ¹⁵N and ¹⁸O isotope analyses require multi-step conversion or toxic chemicals, and ¹⁷O in N₂O cannot be disentangled by IRMS due to isobaric interferences. This technical note describes the automation of the stable-isotope analyses of nitrate by coupling the new Ti method with a headspace autosampler and an N₂O triple-isotope laser analyzer based on off-axis integrated cavity output spectroscopy. The automation yielded accurate and precise results for routine determinations of δ¹⁵N, δ¹⁸O, and δ¹⁷O values for aqueous nitrate in environmental waters. Systematic corrections were required for cavity pressure, N₂O concentration and water vapour content to obtain the highest precision for all three isotopic ratios. For the first time, an automated laser-based system facilitates routine low-cost triple isotope analyses in studies where high-temporal resolution isotope analyses of NO₃^- are required but have been, until now, cost-prohibitive and time-consuming (e.g. atmospheric N pollution).