A Ti(III) reduction method for one-step conversion of seawater and freshwater nitrate into N O for stable isotopic analysis of N/ N, O/ O and O/ O.

Rationale: The nitrogen and oxygen (δ N, δ O, and δ O values) isotopic compositions of nitrate (NO ) are crucial tracers of nutrient nitrogen (N) sources and dynamics in aquatic systems. Current methods such as bacterial denitrification or Cd-azide reduction require laborious multi-step conversions or toxic chemicals to reduce NO to N O for N and O isotopic analyses by isotope ratio mass spectrometry (IRMS). Furthermore, the O composition of N O cannot be directly disentangled using IRMS because O contributes to mass 45 ( N).

Methods: We describe a new one-step chemical conversion method that employs Ti(III) chloride to reduce nitrate to N O gas in septum sample vials. Sample preparation takes only a few minutes followed by a 24-h reaction producing N O gas (65-75% recovery) which partitions into the headspace. The N O headspace was measured for N, O and O by IRMS or laser spectrometry.

Results: IRMS and laser spectrometric analyses gave accurate and reproducible N and O isotopic results down to 50 ppb (3.5 μM) NO -N, similar in precision to the denitrifier and Cd-azide methods. The uncertainties for dissolved nitrate reference materials (USGS32, USGS34, USGS35, IAEA-NO ) were ±0.2‰ for δ N values and ±0.3‰ for δ O values using IRMS. For laser-based N O isotope analyses the results were similar, with an δ O uncertainty of ±0.9‰ without any need for N correction.

Conclusions: Advantages of the Ti(III) reduction method are simplicity, low cost, and no requirement for toxic chemicals or anaerobic bacterial cultures. Minor corrections may be required to account for sample nitrate concentration variance and potential chemical interferences. The Ti(III) method is easily implemented into laboratories currently using N O headspace sampling apparatus. We expect that the Ti(III) method will promulgate the use of N and O isotopes of nitrate in important studies of nutrient dynamics and pollution in a wide range of aquatic ecosystems.