Determining Nitrogen-15 in Ammonium by Producing Nitrous Oxide
- R. J. Laughlin,
- R. J. Stevens and
- S. Zhuo
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 <10 µmol test−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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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