Nitrogen isotopic analysis of nitrate in aquatic environment using cadmium–hydroxylamine hydrochloride reduction

Abstract

Rationale:The nitrogen isotopic ratio of nitrate (δ 15 N-NO 3 − value) is a critical parameter for understanding nitrogen biogeochemical cycling in aquatic systems.Current approaches to the determination of δ 15 N-NO 3 − values involve timeintensive handling procedures, the use of toxic chemicals and complicated microbial incubation.Methods: A chemical reduction method for measuring the δ 15 N-NO 3 − values of aquatic samples was established. Nitrate was first quantitatively reduced to nitrite in a column filled with copper-coated cadmium granules, and the produced nitrite further reduced to nitrous oxide gas with hydroxylamine hydrochloride. The nitrogen isotope ratio of the produced nitrous oxide was measured using a continuous-flow isotope ratio mass spectrometer coupled with a purge and cryogenic trap system. Results: The optimized experimental conditions were: solution acidity, H + concentration of 0.46 M, pH = 0.34; dosage of hydroxylamine, molar ratio of NH 2 OH to NO 2 − of 4; reaction temperature, 45 C; and reaction time, 14-16 h. No salt effect was found for this method. The reproducibility of the δ 15 N-NO 3 − value for the laboratory standard was better than 0.3‰ for long-term measurements (20 nmol NO 3 − requirement). Conclusions:This method provides a reliable approach for the determination of δ 15 N-NO 3 − values at natural abundance. It provides (1) high measurement accuracy,(2) ease of operation, (3) environmental-friendly procedure (less toxic regents used), and (4) suitability for both freshwater and saline water samples.

High‐precision measurement of phenylalanine and glutamic acid δ15N by coupling ion‐exchange chromatography and purge‐and‐trap continuous‐flow isotope ratio mass spectrometry

“…For example, IC separation can be replaced by a previously developed HPLC separation method without derivatization 34,41–45 . Azide conversion of nitrite to N 2 O can be replaced by the denitrifier method 28,30 or hydroxylamine method 32 …”

“…The acceptance of CSIA‐δ 15 N‐AA is also hindered due to the lack of multipoint isotopic reference standards and a consistent calibration method among different labs 27 . In labs measuring δ 15 N in nitrate and ammonium, multiple internationally recognized nitrate and ammonium isotopic standards covering a wide range of naturally occurring δ 15 N values are used to calibrate unknowns 28–33 . Published δ 15 N of individual AAs ranges between −35‰ and +45‰; it is thus necessary to have isotopic standards span in this range to improve the estimation of trophic positions 27 .…”

Response of Nitrate Processing to Bio-labile Dissolved Organic Matter Supply Under Variable Oxygen Conditions in a Sandy Beach Seepage Face

“…The isotopic composition of NO 3 − (δ 15 N/ 18 O-NO 3 − ) was determined in the collected samples to understand reaction pathways and rates in place (Jiang et al, 2019(Jiang et al, , 2021b. The isotopic ratios were measured by the bacterial reduction method (Sigman et al, 2001) using a Thermo-Fisher Delta V advantage analyzer coupled with a PreCon system (more details in Jin et al, 2020…”

Nitrogen in Atmospheric Wet Depositions Over the East Indian Ocean and West Pacific Ocean: Spatial Variability, Source Identification, and Potential Influences

“…Both δ 15 N-NO 3 − and δ 18 O-NO 3 − were determined via transforming NO 3 − to N 2 O using a denitrifier with the reference of USGS-32, USGS-34, and IAEA-N3 (Sigman et al, 2001) after removing NO 2 − (Weigand et al, 2016). The produced N 2 O was concentrated in a Thermo-Fisher Precon system at liquid nitrogen environment, and the signal of N 2 O was detected in a Thermo-Fisher Delta V Advantage Isotope Mass Spectrometer (Jin et al, 2020 (Zhang et al, 2007). The detection limit was ca.…”