Adaptation of micro‐diffusion method for the analysis of 15N natural abundance of ammonium in samples with small volume. (7th June 2015)

• Record Type: Journal Article
• Authors: Zhang, Shasha; Fang, Yunting; Xi, Dan
• Is Part Of: Rapid communications in mass spectrometry. Volume 29:Number 14(2015)
• Page Start: 1297
• Page End: 1306
• Publication Date: 2015-06-07
• Subjects: Mass spectrometry -- Periodicals; 543.65
• DOI: 10.1002/rcm.7224
• Languages: English
• ISSNs: 0951-4198

Abstract

Abstract : Rationale: 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 + . Copyright © 2015 John Wiley & Sons, Ltd.