The need for reference materials when monitoring nitrate intake.

Whether dietary exposure to nitrate metabolites is detrimental or beneficial to human health has long been a matter of controversy. In spite of no consistent epidemiological evidence, nitrate metabolites are associated with the formation of carcinogenic-nitrosamines and gastric cancer. Furthermore, recent studies demonstrate that ingested nitrate plays a role in host defence against gastrointestinal pathogenic bacteria. Analytical values of nitrate content in foods are essential for estimating nitrate intake. The analytical process is of paramount importance for assessing human nitrate exposure and for establishing a link between these exposures and the current and future observed health effects. Therefore, the quality assurance of the measurement process is crucial to obtaining reliability, comparability and traceability of results. Certified Reference Materials (CRM) should play a role in the consistency of the measurement process. However, the availability of nitrate CRMs is still poor. When food monitoring is demanded, an approach could be to use In House Reference Materials (IHRM), prepared at a high metrological level, and all preparation steps should be quality driven. IHRMs were prepared, and available CRMs were used to provide traceability of the process. The homogeneity of IHRM was evaluated using an appropriate statistical design. The stability was monitored using an isochronous method. The material shelf life and storage conditions are presented. HPLC was optimised for the determination of nitrates in four vegetable categories. When a suite of IHRMs were used, the response of the HPLC system was linear over the range 1 to 8 mg L(-1). The detection limit for these compounds was 0.2 microg L(-1) and the determination limit 1.2 microg L(-1). The relationship between measurement uncertainty and critical points of the analytical process is presented. The differences in observed relative uncertainty between food categories could reflect current limitations in the food matrix reference materials.