The QUASIMEME laboratory performance studies (1993–1995): Overview of the nutrients section

The QUASIMEME Project (1993–1996) was established to assist European laboratories to improve the data they produce in marine monitoring programmes. Through laboratory performance studies (with six-monthly reports), workshops and expert visits the programme was fully interactive. There were five rounds of laboratory performance studies. For the nutrient section, in which about 50 laboratories took part, the reference materials distributed to the participants consisted of standard solutions of nutrients and seawater samples stabilized by autoclaving. The material included low and high concentrations typical of those encountered in coastal seawater; at least two samples with different concentrations were distributed in each round. Robust statistics were used to determine the means and standard deviations for each set of results. For inorganic nutrients, the assessment of the data for bias and precision was based mainly on a Z- and P-scoring system in which targets of ±6% were allocated to the high concentrations, likewise ±12.5% to the low concentrations. This overview discusses overall performance separately for nitrate plus nitrite, nitrite, ammonia, phosphate, total nitrogen and total phosphorus, and classifies the performance of individual laboratories in each round, while maintaining their anonymity. Performance for nitrate plus nitrite and nitrite improved steadily and these determinands are now fully under control; at the end of the programme, standard deviations (SD) for nitrate plus nitrite were 0.2 μmol l−1 at low concentration and 0.6 μmol l−1 (4%) at high concentration, and for nitrite they were 0.03 μmol l−1 and 0.06 μmol l−1 (5%) respectively. Phosphate showed a somewhat stable level of performance with SD of 0.06 μmol l−1 and 0.10 μmol l−1 (10%) at low and high concentrations respectively, but this could be improved. Ammonia proved the most difficult to determine, and in spite of a substantial improvement at the beginning of the exercise, this determinand is not under control in many laboratories. At low concentrations, ammonia shows a positive bias of 0.2 μmol l−1 and a SD of 0.3 μmol l−1, while at high concentrations SD reaches 0.5 μmol l−1 (20%). For total nitrogen (TN) and total phosphorus (TP), the exercises show that only two thirds of the participants produced consistent data for TN, and less than half of them produced consistent data for TP.