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Adaptation of micro-diffusion method for the analysis of15Nnatural abundance of ammonium in samples with small volume


Former lab graduate student Shasha Zhang published a paper entitled "Adaptation of micro-diffusion method for the analysis of15Nnatural abundance of ammonium in samples with small volume" on "Rapid Communications in Mass Spectrometry".

Zhang SS, Fang YT, Xi D (2015) Adaptation of micro-diffusion method for the analysis of 15N natural abudance of ammonium in samples with small volume. Rapid Communications in Mass Spectrometry 29: 1297-1306.


RATIONALE: There are several preparation methods for the measurement of the nitrogen (N) isotopic com position of ammonium (NH4+) in different types of samples (freshwater, saltwater and soil extracts). The diffusion method is themost popular and it involves NH4+in solutions being released under alkaline conditions and then immediately trappedby an acidified filter. However, the traditional preparation is designed for samples with large volume and relatively highN 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 (N2O)with hypobromite and hydroxylamine in sequence. The produced N2O was analyzed by a commercially available purgeand 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 thetemperature to 50 °C and the incubation time to 11 days did not improve the overall performance. There were nosignificant differences in the overall performance during diffusion with NH4+concentrations from 15 to 60 μM. The blanksize was relatively large, and the N contamination might come from the reagents especially KCl salts.

CONCLUSIONS: The method presented here combine s micro-diffusion and hypo bromite oxidation and hydroxylamine reduction. It is suitable for samples with small volume and low NH4+ concentrations. Our study demonstrates that theNH4+ concentrations in samples can be as low as 15 μM, and a volume of 5 mL is sufficient for this method. We suggestthat this method can be used for the routine determination of15N/15N for either natural abundance or15N-enrichedNH4+.