A Novel Membrane Inlet Mass Spectrometer Method to Measure ¹⁵NH₄⁺ for Isotope-Enrichment Experiments in Aquatic Ecosystems

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 ¹⁵N-ammonium (¹⁵NH₄⁺) ratios in small-volume ¹⁵N-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 ¹⁵NH₄⁺ in experimental waters previously enriched with ¹⁵N compounds. Dissolved reduced ¹⁵N (dominated by ¹⁵NH₄⁺) is oxidized with hypobromite iodine to nitrogen gas (²⁹N₂ and/or ³⁰N₂) and analyzed by membrane inlet mass spectrometry (MIMS) to quantify ¹⁵NH₄⁺ concentrations. The N isotope ratios, obtained by comparing the ¹⁵NH₄⁺ 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 (R² = 0.9994, p < 0.0001) over a range of salinities and ¹⁵N/¹⁴N 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.