细菌反硝化法与化学转化法测试硝酸盐氮氧同位素对比研究

A Comparative Study of Nitrogen and Oxygen Isotopes of Nitrate by Bacterial Denitrifier and Chemical Conversion

利用细菌反硝化和化学转化法将标准样品、海水、湖水和自来水样品中硝酸盐转化为N 2O,并用气体稳定性同位素质谱测定其氮氧同位素组成,对两种转化方法获得氮氧同位素比值校准曲线的相关参数、方法精密度、准确度和检出限等指标进行对比。结果表明,两种方法将硝酸盐还原为N 2O的转化率均大于90%;细菌反硝化法和化学转化法的氮同位素比值校准曲线斜率分别为0.959和0.481,接近各自理论值1和0.5;两种方法氧同位素比值校准曲线斜率分别为0.908和0.697,与理论值1之间存在一定偏差;两种方法测定氮同位素比值标准偏差均小于0.5‰,氧同位素比值标准偏差均小于1‰;细菌反硝化法和化学转化法测试硝酸盐氮氧同位素方法检出限分别为4.4 nmol和7.54 nmol;两种方法均能较为准确地测定三种类型样品(海水、湖水和自来水)中硝酸盐的氮同位素组成,其结果可以相互验证,差值在0.11‰~0.77‰之间;氧同位素比值差值在3.67‰~5.75‰之间,结果表明,两种方法在硝酸盐还原为N 2O的过程中氧元素产生不同程度的同位素分馏效应,化学转化法产生的氧同位素分馏强度较大,优选细菌反硝化法。

In this study,nitrate in standard samples,seawater and lake water and tap water samples were converted to N 2O using bacterial denitrification and chemical conversion,the nitrogen and oxygen isotope composition were determined by Isotope Ratio Mass Spectrometry(IRMS),and the parameters of the nitrogen and oxygen isootope calibration curve,the precision,accuracy and detection limit of the two conversion methods were obtained.The results show that:(1)The yield rates of nitrate converting to N 2O were all greater than 90%.(2)The slope of the nitrogen isotope calibration curve for the bacterial denitrification and the chemical conversion were 0.959 and 0.481,respectively,Close to the respective theoretical values of 1 and 0.5;The slope of the oxygen isotope calibration curve were 0.908 and 0.697,respectively,there is a certain deviation from the theoretical value of 1.(3)The standard deviations of nitrogen isotopes determined by the two methods were less than 0.5‰,The standard deviations of oxygen isotopes were all less than 1‰.(4)The detection limit of nitrate nitrogen and oxygen isotope test by bacterial denitrification and chemical conversion is 4.4 nmol and 7.54 nmol,respectively.(5)Both methods can more accurately determine the nitrogen isotopic composition of nitrate in the three types of samples(sea water,lake water and tap water).The results were consistent by the two methods.The difference of nitrogen isotope results by the two methods were between 0.11‰and 0.77‰.The oxygen isotope difference were from 3.67‰to 5.75‰,It was shown that oxygen isotopic fractionation were occur when the nitrate were reduced to N 2O,and the degree of oxygen isotopic fractionation were difference in the two conversion methods.