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 p roduce reference material for nutrients in seawater are presented and discussed. When preliminary experiments had shown satisfactory behavio r of nitrate, nitrite and ammonia when autoclaved together in seawater samples, a 27 month experiment was undertaken with these three nutrie nts and phosphate at three concentration levels. Autoclaving was there fore used to stabilize the samples for the ICES 5th intercomparison ex ercise and checked over a 19 month period. Nitrogenous nutrients exhib ited the greatest inter-sample homogeneity. Among them, nitrate and ni trite were extremely stable throughout the whole experiment with overa ll standard deviations lower than 0.3% (range 5-50 mu mol l(-1)) and 0 .8% (range 0.5-5 mu mol 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, u p to 0.05-0.07 mu mol l(-1) per year, attributed to air contamination when samples were insufficiently protected from the laboratory atmosph ere. Phosphate increased by 0.02-0.07 mu mol l(-1) per year due to lea ching from the container glass. Despite these slight adverse effects, autoclaving may also be used for ammonia and phosphate with shorter pe riods of storage under adequate, controlled conditions. The phosphate problem, not pertaining to the stabilization process itself, can proba bly be solved by using a more inert container.