Stability and preservation of primary calibration solutions of nutrients

In the absence of commercially available calibration solutions in a seawater matrix, the accurate determination of concentrations of nutrients (nitrate, nitrite, ammonia, phosphate, silicate) relies primarily on laboratory calibration solutions, especially primary salt solutions. Their stability is therefore a main factor for quality assurance of data. Since the existing information on nutrient calibration solution stability does not refer to published data, stability tests performed in our laboratory are presented. Various conditions, including bottle type, storage time and effect of chemical preservatives have been investigated. Preservatives appear to be either unnecessary or even a source of adverse effects. Potential problems of combined nutrient calibration solutions are discussed. The main suggestions for managing primary calibration solutions for most nutrient purposes are: (i) to prepare one-salt solutions, (ii) to add no preservatives, (iii) to store solutions indifferently in glass (except silicate) or plastic bottles, chilled or at room temperature, (iv) to handle with care to prevent evaporation and contamination, (v) to renew solutions at least every 6-12 months (for nitrite: every 1-2 months).