Chemical Method for Nitrogen Isotopic Analysis of Ammonium at Natural Abundance

We report a new chemical method to determine the ¹⁵N natural abundance (δ¹⁵N) for ammonium (NH₄⁺) in freshwater (e.g., precipitation) and soil KCl extract. This method is based on the isotopic analysis of nitrous oxide (N₂O). Ammonium is initially oxidized to nitrite (NO₂^–) by hypobromite (BrO^–) using previously established procedures. NO₂^– is then quantitatively converted into N₂O by hydroxylamine (NH₂OH) under strongly acid conditions. The produced N₂O is analyzed by a commercially available purge and cryogenic trap system coupled to an isotope ratio mass spectrometer (PT-IRMS). On the basis of a typical analysis size of 4 mL, the standard deviation of δ¹⁵N measurements is less than 0.3‰ and often better than 0.1‰ (3 to 5 replicates). Compared to previous methods, the technique here has several advantages and the potential to be used as a routine method for ¹⁵N/¹⁴N analysis of NH₄⁺: (1) substantially simplified preparation procedures and reduced preparation time particularly compared to the methods in which diffusion or distillation is involved since all reactions occur in the same vial and separation of NH₄⁺ from solution is not required; (2) more suitability for low volume samples including those with low N concentration, having a blank size of 0.6 to 2 nmol; (3) elimination of the use of extremely toxic reagents (e.g., HN₃) and/or the use of specialized denitrifying bacterial cultures which may be impractical for many laboratories