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

Using N-15 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 N-15-nitrate ((NO3-)-N-15) 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 (NO3-)-N-15 concentrations in a small volumes of water from isotope-enrichment experiments after converting the dissolved inorganic N to N-2. The nitrates (NO3- + NO2-) were reduced to NH4+ with zinc powder, and the ammonium (NH4+) was then oxidized to N-2 by hypobromite iodine solution. The resulting N-29(2) and N-30(2) were measured via MIMS. This optimized protocol provides a sensitive (similar to 0.1 mu M) and precise (relative standard deviation = 0.1-4.37%) approach to quantify (NO3-)-N-15 concentrations (0.1-500 mu M) in water samples over a wide range of salinities (0-35%) and in 2 M KCl solution with excellent calibration curves (R-2 >= 0.9996, p < 0.0001). The method was combined with (NO3-)-N-15 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.