化学转化法测定水体中硝酸盐的氮氧同位素比值  

DETERMINATION OF NITROGEN AND OXYGEN ISOTOPE RATIO OF NITRATE IN WATER WITH A CHEMICAL CONVERSION METHOD

作　　者：王曦[1] 曹亚澄[1] 韩勇[1] 唐昊冶[1] 王如海[1] 孙晓丽[1] 孙玉芳[1] 

机构地区：[1]土壤与农业可持续发展国家重点实验室(中国科学院南京土壤研究所),南京210008

出　　处：《土壤学报》2015年第3期558-566,共9页Acta Pedologica Sinica

基　　金：土壤与农业可持续发展国家重点实验室分析技术方法研究项目(Y412010010)资助

摘　　要：采用化学转化法可同时测定各种水体样品中硝酸盐的氮、氧同位素比值。在弱碱性环境中,镀铜镉粒将硝酸盐还原为亚硝酸盐,然后在弱酸性缓冲体系中叠氮酸根将亚硝酸盐转化为氧化亚氮。通过带有预浓缩装置的同位素比值质谱计测定氧化亚氮的氮、氧同位素比值,并用公式将其换算为硝酸盐的氮、氧同位素比值。通过国际标准样品和实验室标准样品的多次检测证实化学转化法具有适用范围广、灵敏度高、操作简便、准确的特点,其转化过程仅需两步,且需样量仅为4.5μg NO3--N。δ15N和δ18O的测量精度分别为0.31‰和0.55‰,测定值与参考值的差异均小于1σ,测量的精确度和准确度均可满足样品分析的需求。A chemical method was developed for determination simultaneously of nitrogen and oxygen isotope ratios of nitrate in water samples. This method uses copper-plated cadmium granules to reduce nitrate into nitrite in a weakly alkaline environment and sodium azide to transform nitrite into nitrous oxide in a weakly acid buffer. Then the nitrous oxide is purged from the water sample and trapped cryogenically using an automated system with subsequent release into a gas chromatography column. The isolated nitrous oxide is then analyzed with a continuous flow isotope ratio mass spectrometer for N and 0 isotope ratios, which is then converted into those of nitrate using an equation. Meanwhile, the experimental conditions of chemical conversion have been optimized by some further experiments. It is found that pH of the solution, reaction time and concentration of nitrate all had some effects on processes of the reducing reaction; When pH of the solution was 8.0, N20 generation rate reached the highest; When duration of the reaction exceeded 80 min, the reaction was nearly 100% completed; And temperature of the reaction system did not have much obvious effect on completeness of the reaction, but the time of reaching equilibrium. On such a basis, optimal conditions for the reducing reaction are set as： 8.0 for pH, 80min for duration of the reaction and 25~C for temperature of the reaction system. To prepare a series of standard solutions to form a gradient in nitrogen and oxygen isotopic abundance, two standard sample solutions, USGS34 and USGS32, the same in concentration 20μ mol L-1, were blended in different volume ratios. The standard solutions were used in transformation experiments and eventually a conversion working curve was obtained. Besides, a number of validation experiments were carried out using USGS international standard samples and laboratory standard samples to verify precision and accuracy of the chemical conversion method. The conversion process has only two steps and needs only 4.5 μ g of NO-3-N, whil

关 键 词：硝酸盐 氧化亚氮 氮同位素比值 氧同位素比值 

分 类 号：X132[环境科学与工程—环境科学] O615.45[理学—无机化学]