4. Conclusion

This article describes various elements of metrology in the context of a large-scale monitoring program such as that required by the Water Framework Directive. It therefore mixes legislative considerations with technical aspects that underline the complexity of a global approach needed to demonstrate traceability of chemical monitoring data on an EU scale. Compared with physical metrology, the difficulties are high, as such an approach involves many chemical specificities (e.g. operational steps such as sampling, sample pre-treatment, different types of matrices, thousands of substances, need for reference materials to validate methods, etc.) not faced by physical measurements. A "simple" translation of the principles of physical metrology to chemical metrology is therefore impossible. This was intensively discussed by chemical experts during the years 1995-2005