Automated rapid triple-isotope ( δ 15 N, δ 18 O, δ 17 O) analyses of nitrate by Ti(III) reduction and N 2 O laser spectrometry.

The nitrogen and oxygen ( δ ¹⁵ N, δ ¹⁸ O, δ ¹⁷ O) stable isotopic compositions of nitrate ( N O 3 - ) are crucial tracers of nutrient N sources and dynamics in aquatic and atmospheric systems. Methods to reduce aqueous N O 3 - to N ₂ O gas (microbial or Cd method) before ¹⁵ N and ¹⁸ O isotope analyses require multi-step conversion or toxic chemicals, and ¹⁷ O in N ₂ O 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 N ₂ O triple-isotope laser analyzer based on off-axis integrated cavity output spectroscopy. The automation yielded accurate and precise results for routine determinations of δ ¹⁵ N, δ ¹⁸ O, and δ ¹⁷ O values for aqueous nitrate in environmental waters. Systematic corrections were required for cavity pressure, N ₂ O 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 NO ₃ ^- are required but have been, until now, cost-prohibitive and time-consuming (e.g. atmospheric N pollution).