Assessing the performance of international laboratories analysing the stable isotope composition (δ15 N, δ18 O, δ17 O) of nitrate in environmental waters.

Background: 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 ‰ range of δ values for δ15 N, δ18 O and δ17 O. One blind duplicate evaluated reproducibility and the effect of water δ18 O. Laboratories used diverse methods to convert nitrate to N2 O, N2 , CO or O2 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 δ15 N and δ18 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 δ17 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 δ17 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 δ15 N spanning -20 ‰ to +80 ‰ and new materials for δ17 O.