Study and application of ammonium-zinc-cadmium-hydroxylamine hydrochloride conversion method for testing nitrate nitrogen isotopes

Studying the geochemical behavior of nitrogen is of great significance for understanding and predicting the structure and function of ecosystems. Nitrate is one of the important forms of nitrogen, and the accurate identification and tracement of its source and transformation process are helpful in understanding the dynamics of the regional nitrogen cycle. Traditional sample pretreatment methods for isotope analysis of nitrate, such as ammonium distillation, ammonia diffusion, and ion exchange have problems such as large sample requirements, high pretreatment requirements, and limited applicability. In this study, zinc-cadmium was utilized to reduce nitrate to nitrite and hydroxylamine hydrochloride to N₂O further, and its nitrogen isotope composition was tested. By testing the nitrogen isotopes of nitrate standard samples, the key indexes of the method, such as experimental conditions, conversion rate, precision and accuracy, were determined, and the calibration curves of nitrogen isotopes of different abundances were established. The experimental results showed that the conversion rates of nitrate were more than 90% when the ratio of NH₂OH to NO₃⁻ was larger than 4∶1 and the reaction temperatures were in the range of 37～42 ℃. The standard deviations of the nitrogen isotope measurements of different nitrate concentrations after blank correction were 0.21‰. The slopes of the calibration curve of nitrate nitrogen isotope specimens of five abundances after deducting the blanks were 0.481, and the correlation coefficients were 0.999, which showed good correlation, indicating that there was almost no nitrogen isotope fractionation during the conversion process. Results of nitrate nitrogen isotope tests in tap water, river water, and seawater showed that the standard deviations were 0.49‰, 0.45‰, and 0.56‰, respectively (n=5). The method utilizes hydroxylamine hydrochloride to replace the highly toxic and explosive sodium azide reagent, which is more environmentally friendly, and at the same time performs excellently in terms of precision and accuracy, which can meet the testing requirements for the isotopic analysis of nitrate nitrogen in different types of environmental water samples.