Determining nitrogen-15 in ammonium by producing nitrous oxide

• Published in: Soil Science Society of America journal Vol. 61
• Main Authors: Laughlin R.J, Stevens R.J, Zhuo S
• Format: Journal Article
• Language: English
• Published: 01.03.1997
• Subjects: ammonia; ammoniac; ammonium compounds; amoniaco; analisis del suelo; analyse de sol; azote; bromides; bromure; bromuros; catalizador; catalyseur; catalysts; cobre; compose de l' ammonium; compuestos de amonio; contenido de nitrogeno; copper; cuivre; ensayo; gases; gaz; hidroxido sodico; hydroxyde de sodium; isotope radioactif; nitrogen; nitrogen content; nitrogeno; nitrous oxide; oxidacion; oxidation; oxido nitroso; oxydation; oxyde nitreux; radioisotopes; radioisotopos; sodium hydroxide; soil testing; teneur en azote; testage; testing
• ISSN: 0361-5995; 1435-0661
• DOI: 10.2136/sssaj1997.03615995006100020013x

Standard procedures for determining 15N in the NH4+ fraction of 2 M KCl soil extracts convert NH4+ to N2 prior to isotope-ratio mass spectrometry. We developed a new gas-phase method for determining 15N in NH4+ by producing N2O. Producing N2O means that atmospheric contamination by N2 at natural abundance is not a problem. The method is more sensitive than other methods at lower enrichments, and as accurate and precise as other methods. Nitrous oxide is produced as a by-product during the oxidation of NH4+ by alkaline OBr-. The reaction is catalyzed by Cu2+ and is pH-dependent. The best practical conditions for producing N2O were when the NH3-absorbing solution contained 0.005 M H2SO4 with 0.5 mM CuSO4, and when the molarity of the NaOH in the NaOBr was 10 M. Background N (0.20 pmol test-1) significantly diluted the theoretical enrichments for amounts of N < 10 micromoles test-1. Corrected enrichment values were accurate for amounts of NH4+-N greater than or equal to 1 micromole test-1. For amounts of NH4+-N greater than or equal to 1 micromole test-1, precision was determined with a CV of 5.6% at 0.10 atom % excess and with a CV of 0.3% at 10.00 atom % excess. The method is suitable for any matrix and has the potential for better performance if the background contamination could be lowered.