Automated rapid triple-isotope (<i>δ</i>¹⁵N, <i>δ</i>¹⁸O, <i>δ</i>¹⁷O) analyses of nitrate by Ti(III) reduction and N₂O laser spectrometry

The nitrogen and oxygen (<i>δ</i>¹⁵N, <i>δ</i>¹⁸O, <i>δ</i>¹⁷O) stable isotopic compositions of nitrate (NO3–) are crucial tracers of nutrient N sources and dynamics in aquatic and atmospheric systems. Methods to reduce aqueous NO3– 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 <i>δ</i>¹⁵N, <i>δ</i>¹⁸O, and <i>δ</i>¹⁷O 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).