RAPID, SENSITIVE AND HIGHLY SELECTIVE N-15 ANALYSIS OF N-15 ENRICHED NITRITE IN WATER SAMPLES AND SOIL EXTRACTS BY NITRIC-OXIDE PRODUCTION AND CF-QMS MEASUREMENT

Nitrite is a very important intermediate in many microbiological N tra nsformations in soils and water. The stable isotope N-15 is often used to investigate these processes. The determination of N-15 in low conc entrations of nitrite in the presence of large concentrations of nitra te is very difficult. Methods used so far for the isotope analysis of nitrite are unsatisfactory, because the nitrite must be calculated as the difference between nitrate plus nitrite and nitrate alone. More us eful are methods by which the nitrite is selectively converted into a chemical form that is suitable for N-15 analysis and that is free from interference From other N species, particularly nitrate. Using this p rinciple in the present study we developed a method where the nitrite is reduced to nitric oxide by iodide in acid medium. This reaction is fast and quantitative, and the N-15 abundance of NO can be precisely m easured by continuous Row mass spectrometry. This method is used for s amples from tracer experiments with artificially enriched nitrogen 15. Therefore, the use of simple quadrupole mass spectrometers directly l inked to the reaction unit is possible with sufficient precision (Reac tion-Continuous Flow Quadrupole Mass Spektrometry-RCFQMS). Using the t echnique developed sample volumes up to 10ml containing at least 1.0 m u g nitrite-N (0, 1 mu lg/ml) with a N-15 abundance of greater than or equal to 0.42 at.% gave a precision of RSD less than or equal to +/- 3%.