title: A novel membrane inlet mass spectrometer method to measure ¹⁵NH4₄⁺ for isotope-enrichment experiments in aquatic ecosystems.
abstract: Nitrogen (N) pollution in aquatic ecosystems has attracted much attention over the past decades, but the dynamics of this bioreactive element are difficult to measure in aquatic oxygen-transition environments. Nitrogen-transformation experiments often require measurement of (15)N-ammonium ((15)NH4(+)) ratios in small-volume (15)N-enriched samples. Published methods to determine N isotope ratios of dissolved ammonium require large samples and/or costly equipment and effort. We present a novel ("OX/MIMS") method to determine N isotope ratios for (15)NH4(+) in experimental waters previously enriched with (15)N compounds. Dissolved reduced (15)N (dominated by (15)NH4(+)) is oxidized with hypobromite iodine to nitrogen gas ((29)N2 and/or (30)N2) and analyzed by membrane inlet mass spectrometry (MIMS) to quantify (15)NH4(+) concentrations. The N isotope ratios, obtained by comparing the (15)NH4(+) to total ammonium (via autoanalyzer) concentrations, are compared to the ratios of prepared standards. The OX/MIMS method requires only small sample volumes of water (ca. 12 mL) or sediment slurries and is rapid, convenient, accurate, and precise (R(2) = 0.9994, p < 0.0001) over a range of salinities and (15)N/(14)N ratios. It can provide data needed to quantify rates of ammonium regeneration, potential ammonium uptake, and dissimilatory nitrate reduction to ammonium (DNRA). Isotope ratio results agreed closely (R = 0.998, P = 0.001) with those determined independently by isotope ratio mass spectrometry for DNRA measurements or by ammonium isotope retention time shift liquid chromatography for water-column N-cycling experiments. Application of OX/MIMS should simplify experimental approaches and improve understanding of N-cycling rates and fate in a variety of freshwater and marine environments.
authors.affiliation: State Key Laboratory of Estuarine and Coastal Research and ‡Department of Geography, East China Normal University , 3663 North Zhongshan Road, Shanghai 200062, China.
authors.forename: Guoyu
authors.initials: G
authors.lastname: Yin
authors.forename: Lijun
authors.initials: L
authors.lastname: Hou
authors.forename: Min
authors.initials: M
authors.lastname: Liu
authors.forename: Zhanfei
authors.initials: Z
authors.lastname: Liu
authors.forename: Wayne S
authors.initials: WS
authors.lastname: Gardner
mesh_terms: D064751:Ammonium Compounds / Q000032:analysis*; D002681:China; D005618:Fresh Water / Q000737:chemistry; D019015:Geologic Sediments / Q000737:chemistry; D013058:Mass Spectrometry / Q000295:instrumentation / Q000379:methods*; D008567:Membranes, Artificial; D009566:Nitrates / Q000737:chemistry / Q000378:metabolism; D009584:Nitrogen / Q000032:analysis*; D009587:Nitrogen Isotopes / Q000032:analysis*; D009930:Organic Chemicals / Q000032:analysis; D054712:Salinity
chemical_list: D064751:Ammonium Compounds; D008567:Membranes, Artificial; D009566:Nitrates; D009587:Nitrogen Isotopes; D009930:Organic Chemicals; D009584:Nitrogen