Determining Nitrogen‐15 in Ammonium by Producing Nitrous Oxide

Standard procedures for determining 15 N in the NH 4 + fraction of 2 M KCl soil extracts convert NH 4 + to N 2 prior to isotope‐ratio mass spectrometry. We developed a new gas‐phase method for determining 15 N in NH 4 + by producing N 2 O. Producing N 2 O means that atmospheric contamination by N 2 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 NH 4 + by alkaline OBr − . The reaction is catalyzed by Cu 2+ and is pH‐dependent. The best practical conditions for producing N 2 O were when the NH 3 ‐absorbing solution contained 0.005 M H 2 SO 4 with 0.5 m M CuSO 4 , and when the molarity of the NaOH in the NaOBr was 10 M . Background N (0.20 µmol test −1 ) significantly diluted the theoretical enrichments for amounts of N <10 µmol test −1 . Corrected enrichment values were accurate for amounts of NH 4 + ‐N ≥1 µmol test −1 . For amounts of NH 4 + ‐N ≥1 µmol 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.