Determining Nitrogen‐15 in Ammonium by Producing Nitrous Oxide

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Abstract
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 µmol test−1) significantly diluted the theoretical enrichments for amounts of N −1. Corrected enrichment values were accurate for amounts of NH4+‐N ≥1 µmol test−1. For amounts of NH4+‐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.
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