Publication

Summary

The 25th Norwegian intercomparison study for analysis of fresh water, designated 16-25, was organized in October-November 2016 with 50 participants. It comprised analysis of three sample sets of four samples (A-D, E-H, I-L), and two sample sets of two samples (M-N and O-P), made by adding known amounts of stoichiometric material to water from Preståsen lake in Aurskog-Høland municipality, which had been filtered through membrane filter with pore size 0,45 μm. The program included 32 different parameters: pH, conductivity, turbidity, colour, UV-absorption, sodium, potassium, calcium, magnesium, hardness, alkalinity, chloride, sulfate, fluoride, total organic carbon, chemical oxygen demand (CODMn), phosphate, total phosphorus, ammonium, nitrate, total nitrogen, aluminium, lead, iron, cadmium, copper, chromium, manganese, nickel, zinc, antimony and arsenic. The analysis was largely carried out according to Norwegian standards or equivalent methods (Table B1). The median of the participants' results, after outliers have been omitted, is selected as the "true" value. The acceptance limit is normally set to ± 20 % of the average true value for the two samples that form a pair. The results are presented graphically in a Youden diagram, where a circle with the acceptance limit as radius is drawn. The result pairs within the circle are affected by a total error less than the limit and are hence considered acceptable (Appendix A). In total 86 % of participants' results were acceptable, a number which is somewhat higher than for the previous intercomparison studies (Table 1).