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No. 20: Project ”hidden controls”

Lassen, P., Juhler, R.K. & Linneberg, M.S. 2013. Projekt ”skjulte kontroller”. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 46 s. - Teknisk rapport fra DCE - Nationalt Center for Miljø og Energi nr. 20. www.dmu.dk/Pub/TR20.pdf


The purpose of this project is to investigate the possibility of using spiked samples as ”hidden controls” in connection with the ordinary sampling in the national monitoring program. The design of the project is focused on an evaluation of the applicability of “hidden controls” based on an assessment of practical as well as calculation conditions. The number of analyses is low but considered sufficient for this evaluation. However, for single parameters the statistical data are insufficient for an evaluation of the laboratories’ analytical methods or other quality parameters. This will need much larger statistical data.

Groundwater samples were selected as a model and spiked with inorganic parameters. The samples and parameters were selected based on the criteria that the analyses should be performed by several Danish laboratories, and that the analytical parameters should be relatively stable. Nitrate, total phosphor (TP), chloride (Cl), sulphate, potassium (K) and sodium (Na) were selected for investigation. The project was conducted in collaboration with the Nature Agency’s former environmental centres in Roskilde, Aalborg and Nykøbing.

The samples were spiked in the field and sent to the laboratories together with the ordinary samples from the environmental centres. The spikening of the samples was performed by weighing sample and spike. Three additional samples were taken together with the sample from the environmental centre, two were spiked and one left unspiked; giving two spiked and two unspiked samples. The spike mixture was also analysed in order to evaluate the stability of the parameters. The analytical results were evaluated according to DS/ISO 5725.

In general the tested principle and the method seem to work well and the procedure can be used for “hidden controls” in a broader range in the monitoring programme. The project also showed that the procedure for spiking samples in the field was easy to handle.

For most of the parameters, the results seemed sufficiently robust. However, according to the results from the project it is recommended to modify the concept in order to be able to include nitrate, TP and natrium. The measurements of the spike mixture showed low stability of nitrate, and TP’s stability seemed to depend on the storage temperature. For Na where the laboratories in general measured lower concentrations than the calculated value, the results cannot be explained as the analysis of the spike mixture did not indicate instability. Especially nitrate is an important parameter in the monitoring programme, and further investigations of the stability are needed in order to include these parameters.

The project and the statistical analysis in the report show that “hidden controls” is a suitable tool to establish statistically based quality parameters in the national monitoring programme. There is, however, a need for adjustment of the concept and for a better statistical documentation before it can be used as standard procedure in the monitoring programme.