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. The isotope dilution exper iments were carried out using the Institute for Reference Materials an d Measurements's N-15-enriched nitrate spike reference material IRMM-6 27. 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-c ost, easy-to-handle thermionic quadrupole mass spectrometer equipped w ith a secondary electron multiplier coupled to an ion counter, The pro cedure developed was applied to the measurement of nitrate in a certif ied reference material (simulated rainwater, CRM 409 from Community Bu reau of Reference), in sparkling mineral water, and in tap water, Resu lts were compared with those obtained using ion chromatography. Good a greement (within 1%) was found between the concentration determined by isotope dilution mass spectrometry, the values from ion chromatograph y, and the certified value, The procedure developed allowed accurate a nd traceable determinations of nitrate in water samples, with an expan ded uncertainty (coverage factor k = 2) of 2-5%, and the detection lim it was found to be 2 mu mol kg(-1).