High-precision determination of nitrite, nitrate, phosphate, and silicate for the characterization of MOOS-4 certified reference material for nutrients in seawater

High-precision determination of inorganic nutrients (i.e., nitrite, nitrate, phosphate, and silicate) in seawater is paramount for understanding variations in marine biogeochemical cycles. Historically, the lack of consistency between nutrient data sets has been the Achilles' heel for large-scale oceanographic studies and the regular use of certified reference materials (CRMs) was identified as a solution to improve data quality. In this study, the preparation and certification of NRC MOOS-4 nearshore seawater CRM for nutrients are presented. The discussion is focused on the optimization of the analytical methods that were developed in decade-long research. On the one hand, the classical spectrophotometric methods were studied with attention to potential systematic biases not normally considered in routine analysis. On the other hand, higher-order (isotope dilution) methods with better performance characteristics were developed for accurate quantitation of nitrite, nitrate, and silicate. This rigorous method development work resulted in exceptional agreement between nutrient data obtained by traditional and modern analytical methodologies and allowed improvement of measurement uncertainties with respect to previous MOOS-3 CRM. As a result, MOOS-4 could be certified with expanded (k = 2) uncertainties between 0.8% and 2.6%, consistent with the requirements of the oceanographic community (i.e., c(NO₂^-) = 1.676 ± 0.013 µmol/L, c(NO₃^-) = 17.14 ± 0.44 µmol/L, c(PO₄^3-) = 2.835 ± 0.031 µmol/L, c(SiO₂) = 7.16 ± 0.13 µmol/L). In line with the FAIR principles for scientific data management, all measurement data for value assignment and software used for data analysis was made available in the Electronic Supporting Material.