Assessing the performance of international laboratories analysing the stable isotope composition (δ15N, δ18O, δ17O) of nitrate in environmental waters

Rationale: Stable-isotope analyses of nitrate (NO3-) in various water sources are crucial for understanding nitrogen pollution and its impact on aquatic ecosystems. We evaluated the accuracy and precision of stable-isotope analyses of nitrate conducted by international laboratories.Methods: Six samples with nitrate (2 mg L-1 NO3--N) were sent to 47 laboratories. The NO3- had a 30-50 parts per thousand range of delta values for delta N-15, delta O-18 and delta O-17. One blind duplicate evaluated reproducibility and the effect of water delta O-18. Laboratories used diverse methods to convert nitrate to N2O, N-2, CO or O-2 for stable-isotopic measurements (microbial, cadmium, titanium and elemental analysis) and isotope-ratio mass spectrometry or laser-based technologies.Results: Thirty-six international laboratories (83 %) reported results; however, 23 % did not analyze the test samples due to technical difficulties. Of the reporting laboratories, 79 % and 84 % produced accurate delta N-15 and delta O-18 results falling within +/- 0.8 parts per thousand and +/- 1.1 parts per thousand of the benchmark values, respectively. Three laboratories produced only outliers. The duplicate revealed most laboratories gave internally reproducible results at appropriate analytical precision. For delta O-17, six laboratories reported results, but 67 % could not reproduce results within their claimed analytical measurement precision. One complication is a lack of nitrate reference materials for delta O-17.Conclusions: Analyst experience contributed to better performance, and underperformance was from compromised standards or inappropriate delta range of working reference materials. The stable isotope community must develop new nitrate reference materials for delta N-15 spanning -20 parts per thousand to +80 parts per thousand and new materials for delta O-17.