Traceable Values for Nitrate in Water Samples by Isotope Dilution Analysis Using a Small Thermionic Quadrupole Mass Spectrometer

An isotope dilution mass spectrometric procedure was developed for the determination of nitrate in water samples. A 15-N enriched nitrate spike reference material IRMM-627 was prepared with a nitrogen isotope ratio n(14-NO3-)/n(15-NO3-) = 0.548+-0.012 and a nitrate concentration of c(NO3-)/mmol.kg-1 = 3.124 +-0.080 (expanded uncertainty, k=2). Nitrate was isolated from the matrix by precipitating it as nitron-nitrate from which emission of negative thermal NO2- ions was found to be best. The ions were produced in the ion source of a small, low-cost, easy-to-handle termionic quadrupole mass spectrometer equipped with a secondary electron multiplier coupled to an ion counter. The procedure developed was applied to the measurement of nitrate in a certified reference material (simulated rainwater, CRM 409 from Community Bureau of Reference), in sparkling mineral water and in tap water. Results were compared with those obtained using ion chromatography. Good agreement (within 1%) was found between the concentration determined by isotope dilution mass spectrometry, the values from ion chromatography and the certified value. the procedure developed allowed accurate and traceable determinations of nitrate in water samples with an expanded uncertainty (cov. factor k=2) of 2-5%.JRC.D-Institute for Reference Materials and Measurements (Geel