Premium
Nitrogen isotopic analysis of nitrate in aquatic environment using cadmium–hydroxylamine hydrochloride reduction
Author(s) -
Jin Jie,
Jiang Shan,
Zhang Jing
Publication year - 2020
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.8804
Subject(s) - chemistry , nitrite , hydroxylamine , nitrogen , nitrate , environmental chemistry , isotopes of nitrogen , nitrous oxide , cadmium , hydroxylamine hydrochloride , inorganic chemistry , organic chemistry
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 time‐intensive 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.