Certified reference materials (CRMs 479 and 480) for the quality control of nitrate determination in freshwater

Monitoring of freshwater and groundwater is routinely performed to control the level of contamination by a wide variety of trace and major constituents, including nitrates. In order to verify the quality of such determinations, the Standards, Measurements and Testing Programme (formerly BCR) has started a project, the first step of which was to evaluate the performance of European laboratories in an interlaboratory study and the second phase was a certification campaign of nitrate in two artificial freshwater samples. This paper presents an overview of the results of the interlaboratory evaluation and gives details of the preparation of the candidate certified reference materials (CRMs), the verification of their homogeneity and stability, the techniques used in the certification and the certified values. Two CRMs were produced: CRM 479 containing a low nitrate content (214 +/- 4 mu mol kg(-1) or 13.3 +/- 0.3 mg kg(-1)) and CRM 480 containing a high nitrate content (885 +/- 13 mu mol kg(-1) or 54.9 +/- 0.8 mg kg(-1)).