Measuring SI-traceable nitrate concentrations in water by a primary method of measurement, isotope dilution mass spectrometry

Simulated rain-, tap-, river- and sparkling mineral-water were analysed for their nitrate content, using isotope dilution mass spectrometry (IDMS). After the removal of nitrite with amidosulfuric acid, nitrate was isolated selectively from the matrix by precipitating it with nitron. The nitron-nitrate formed was used to produce NO2− thermal ions from a double-filament arrangement in the ion source of a quadrupole thermionic mass spectrometer and from a triple-filament arrangement in the source of an NBS-type magnetic-field thermionic mass spectrometer. Both measurement procedures are discussed and compared. The IDMS experiments were performed using the 15N-enriched nitrate species-specific spike isotopic reference material, IRMM-629, which is traceable to the international SI unit system in the shortest possible way. It was shown how isotope dilution combined with negative thermal-ionisation mass spectrometry yielded accurate and traceable values for nitrate along an unbroken chain of a transparent procedure with full orthodox uncertainty evaluation for each step. Expanded uncertainties (coverage factor k = 2) of 2%–5% were obtained.