Automated rapid triple-isotope (N, O, O) analyses of nitrate by Ti(III) reduction and NO laser spectrometry.

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