Membrane inlet mass spectrometry method (REOX/MIMS) to measure 15N-nitrate in isotope-enrichment experiments

• Main Authors: Gao, D. (Author), Gardner, W.S (Author), Gong, J. (Author), Hardison, A.K (Author), Lin, X. (Author), Liu, Z. (Author), Lu, K. (Author), Xu, X. (Author)
• Format: Article
• Language: English
• Published: Elsevier B.V. 2021
• Subjects: 15 > N; 15NO3−; ammonium; Aquatic ecosystems; Chlorine compounds; concentration (composition); Cost effectiveness; decision making; Decision making; Enrichment experiments; Gross nitrification; Gross NO3− immobilization; immobilization; Isotope dilution method; Isotope dilution methods; Isotope enrichments; Isotopes; mass spectrometry; Mass spectrometry; Membrane inlet mass spectrometry; nitrate; Nitrates; Nitrification; nitrogen cycle; Potassium compounds; REOX/membrane inlet mass spectrometry; REOX/MIMS; stable isotope; Stable isotopes
• Online Access: View Fulltext in Publisher

 Using 15N stable isotope as a tracer to quantify N transformation rates in isotope-enrichment experiments improves understanding of the N cycle in various ecosystems. However, measuring 15N-nitrate (15NO3−) in small volumes of water for these experiments is a major challenge due to the inconvenience of preparing samples by traditional techniques. We developed a “REOX/MIMS” method by applying membrane inlet mass spectrometry (MIMS) to determining 15NO3− concentrations in a small volumes of water from isotope-enrichment experiments after converting the dissolved inorganic N to N2. The nitrates (NO3− + NO2−) were reduced to NH4+ with zinc powder, and the ammonium (NH4+) was then oxidized to N2 by hypobromite iodine solution. The resulting 29N2 and 30N2 were measured via MIMS. This optimized protocol provides a sensitive (~0.1 μM) and precise (relative standard deviation = 0.1–4.37%) approach to quantify 15NO3− concentrations (0.1–500 µM) in water samples over a wide range of salinities (0–35‰) and in 2 M KCl solution with excellent calibration curves (R2 ≥ 0.9996, p < 0.0001). The method was combined with 15NO3− isotope-enrichment incubation experiments to measure gross nitrification and gross NO3− immobilization rates in various ecosystems. It was rapid, accurate, and cost-effective. Future applications of this efficient approach will inform scientists, modelers and decision makers about mechanisms, sources, fates, and effects of NO3− delivered to or produced in numerous aquatic and terrestrial ecosystems. © 2021 The Author(s)