Assessing the performance of international laboratories analysing the stable isotope composition (δ N, δ O, δ O) of nitrate in environmental waters.

Rationale: Stable-isotope analyses of nitrate (NO ) 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 NO -N) were sent to 47 laboratories. The NO had a 30-50 ‰ range of δ values for δ N, δ O and δ O. One blind duplicate evaluated reproducibility and the effect of water δ O. Laboratories used diverse methods to convert nitrate to N O, N , CO or O 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 δ N and δ O results falling within ±0.8 ‰ and ±1.1 ‰ of the benchmark values, respectively. Three laboratories produced only outliers. The duplicate revealed most laboratories gave internally reproducible results at appropriate analytical precision. For δ O, 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 δ O.

Conclusions: Analyst experience contributed to better performance, and underperformance was from compromised standards or inappropriate δ range of working reference materials. The stable isotope community must develop new nitrate reference materials for δ N spanning -20 ‰ to +80 ‰ and new materials for δ O.