title: Adaptation of micro-diffusion method for the analysis of (15) N natural abundance of ammonium in samples with small volume.
abstract: BACKGROUND
  There are several preparation methods for the measurement of the nitrogen (N) isotopic composition of ammonium (NH4 (+) ) in different types of samples (freshwater, saltwater and soil extracts). The diffusion method is the most popular and it involves NH4 (+) in solutions being released under alkaline conditions and then immediately trapped by an acidified filter. However, the traditional preparation is designed for samples with large volume and relatively high N concentrations. The performance of diffusion for small-volume samples (e.g., a few milliliters) remains unknown.
  
  
  METHODS
  We examined the overall performance of micro-diffusion on 5 mL samples on varying the incubation time, temperature and initial NH4 (+) concentration. The trapped ammonia was chemically converted into nitrous oxide (N2 O) with hypobromite and hydroxylamine in sequence. The produced N2 O was analyzed by a commercially available purge and cryogenic trap system coupled to an isotope ratio mass spectrometer.
  
  
  RESULTS
  We found that diffusion can be complete with no more than 7 days of treatment at 37 °C. Increasing the temperature to 50 °C and the incubation time to 11 days did not improve the overall performance. There were no significant differences in the overall performance during diffusion with NH4 (+) concentrations from 15 to 60 μM. The blank size was relatively large, and the N contamination might come from the reagents especially KCl salts.
  
  
  CONCLUSIONS
  The method presented here combines micro-diffusion and hypobromite oxidation and hydroxylamine reduction. It is suitable for samples with small volume and low NH4 (+) concentrations. Our study demonstrates that the NH4 (+) concentrations in samples can be as low as 15 μM, and a volume of 5 mL is sufficient for this method. We suggest that this method can be used for the routine determination of (15) N/(14) N for either natural abundance or (15) N-enriched NH4 (+) .
authors.affiliation: State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110164, China.|Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang, 110016, China.|College of Resources and Environment, University of Chinese Academy of Sciences, Beijing, China.
authors.forename: Shasha
authors.initials: S
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110164, China.|Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang, 110016, China.
authors.forename: Yunting
authors.initials: Y
authors.lastname: Fang
authors.affiliation: State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110164, China.|Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang, 110016, China.|College of Resources and Environment, University of Chinese Academy of Sciences, Beijing, China.
authors.forename: Dan
authors.initials: D
authors.lastname: Xi
mesh_terms: D064751:Ammonium Compounds / Q000032:analysis*; D001959:Bromates / Q000737:chemistry; D004058:Diffusion; D065928:Forests; D019811:Hydroxylamine / Q000737:chemistry; D013058:Mass Spectrometry / Q000379:methods*; D009587:Nitrogen Isotopes / Q000032:analysis; D009609:Nitrous Oxide / Q000032:analysis; D018401:Sample Size; D012987:Soil / Q000737:chemistry*
chemical_list: D064751:Ammonium Compounds; D001959:Bromates; D009587:Nitrogen Isotopes; D012987:Soil; D019811:Hydroxylamine; C027664:hypobromous acid; D009609:Nitrous Oxide