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

Authors

Xianbiao Lin
Kaijun Lu
Amber K. Hardison, Virginia Institute of Marine ScienceFollow
et al

Document Type

Article

Department/Program

Virginia Institute of Marine Science

Publication Date

7-2021

Journal

Ecological Indicators

Issue

107639

Abstract

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.

DOI

doi: 10.1016/j.ecolind.2021.107639

Keywords

REOX/MIMS, 15NO3−, Gross nitrification, Gross NO3−, immobilization, Isotope dilution method

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 International License.

Recommended Citation

Lin, Xianbiao; Lu, Kaijun; Hardison, Amber K.; and et al, Membrane inlet mass spectrometry method (REOX/MIMS) to measure 15N-nitrate in isotope-enrichment experiments (2021). Ecological Indicators.

doi: 10.1016/j.ecolind.2021.107639