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Determining Nitrogen-15 in Ammonium by Producing Nitrous Oxide

Dep. of Agriculture for Northern Ireland, Agricultural and Environmental Science Division, Newforge Lane, Belfast BT9 5PX, UK

S. Zhuo

Inst. of Soil Science, Chinese Academy of Science, Nanjing, China

*Corresponding author (stevensj{at}dani.gov.uk).

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 <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.

Received for publication December 6, 1995.

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