The 28th Norwegian intercomparison study for analysis of fresh water, designated 20-28, was
organized in January – April 2020 with 42 participants. Two of the participants were unable to report
results in time for the deadline. The study 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 Kvisla and Viksjøen 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 85 % of participants' results were acceptable, a result which is comparable to the latest
intercomparison studies (Table 1). 20 parameters had acceptable results above 80 %. 8 parameters
had 70-79 %, 2 had 60-69 % and the final 2 had 50% or less acceptable results.
Some laboratories' results are dominated by large systematic or random deviations. As in previous
intercomparison studies, some results have been reported in the wrong unit of measurement. This
shows how all parts of the chain of analysis needs to have quality control if reliable data are to be
achieved. Systematic deviations are particularly prominent in some instrumental analyses. In these
cases, the goal of the troubleshooting should be to clarify whether the error is constant and/or
dependent on the concentration, to obtain an indication of what the cause of the error could be
(Appendix A). In some cases, the analysis methods haven't been sufficiently sensitive for the
concentration level of the analytes. When considering their results, laboratories must consider the
low level of concentration of analytes in some of the samples. Internal quality control [Hovind et al.
2006] is necessary for the running evaluation of the laboratories' own methods and routines. If
possible, the accuracy of the results should be controlled by certified reference materials (CRM's), or
alternatively by reanalyzing samples from intercomparison studies they have previously participated
in.
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