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 analyse d 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 nitr on. The nitron-nitrate formed was used to produce NO2- thermal ions fr om a double-filament arrangement in the ion source of a quadrupole the rmionic mass spectrometer and from a triple-filament arrangement in th e source of an NBS-type magnetic-field thermionic mass spectrometer. B oth measurement procedures are discussed and compared. The IDMS experi ments were performed using the N-15-enriched nitrate species-specific spike isotopic reference material, IRMM-629, which is traceable to the international ST unit system in the shortest possible way. It was sho wn 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 uncert ainty evaluation for each step. Expanded uncertainties (coverage facto r k=2) of 2%-5% were obtained.