Pasteurization: A reliable method for preservation of nutrient in seawater samples for inter-laboratory and field applications

Following previous work, the production of reference material for nutrients in seawater, using pasteurization as a preservation method, was carried out seven times between 2006 and 2010 in the framework of inter-laboratory exercises. The preparation of samples from natural seawater allowed to become depleted in nutrients then spiked, bottled and pasteurized, is described. Five main nutrients are involved in this study: ammonium, nitrite, nitrate, phosphate and silicate. Bottles are in glass for the ammonium samples and in plastic for the other nutrients. Pasteurization was performed at 80 ± 3 °C for 2 h. Samples were controlled for homogeneity and stability at 3–5 month interval, before and after the period allocated for each exercise and a third time between 8 to 40 months storage. Homogeneity, not altered by sample aging, remains within a few nanomoles per litre for nitrite (< 1.4 μmol/L) and phosphate (< 3 μmol/L) and within a few tens of nanomoles per litre for ammonium (< 4.5 μmol/L), nitrate (< 35 μmol/L) and silicate (< 30 μmol/L). Except for ammonium, stability data are close to homogeneity data at the low level and within 0.3–0.5% at the high level. Ammonium shows a slight drift due to atmospheric contamination through the plastic cap, in the order of 0.1 μmol/L per year, which allows nevertheless an acceptable performance over an 1–2 month analytical delay. for ammonium, pasteurization does not produce detectable concentration changes, thus it can be applied to field samples for long term storage at ambient temperature.