Reference material for nutrients in seawater: stability of nitrate, nitrite, ammonia and phosphate in autoclaved samples

Reference material is increasingly used as a basis for quality control and quality assurance in general. Results of autoclaving applied to produce reference material for nutrients in seawater are presented and discussed. When preliminary experiments had shown satisfactory behavior of nitrate, nitrite and ammonia when autoclaved together in seawater samples, a 27 month experiment was undertaken with these three nutrients and phosphate at three concentration levels. Autoclaving was therefore used to stabilize the samples for the ICES 5th intercomparison exercise and checked over a 19 month period. Nitrogenous nutrients exhibited the greatest inter-sample homogeneity. Among them, nitrate and nitrite were extremely stable throughout the whole experiment with overall standard deviations lower than 0.3% (range 5–50 μmol l−1) and 0.8% (range 0.5–5 μmol l−1), respectively. For these two nutrients, autoclaving appears to be a prime method for production of reference material in seawater. Ammonia exhibited a slight increase over time, up to 0.05–0.07 μmol l−1 per year, attributed to air contamination when samples were insufficiently protected from the laboratory atmosphere. Phosphate increased by 0.02–0.07 μmol l−1 per year due to leaching from the container glass. Despite these slight adverse effects, autoclaving may also be used for ammonia and phosphate with shorter periods of storage under adequate, controlled conditions. The phosphate problem, not pertaining to the stabilization process itself, can probably be solved by using a more inert container.