三价钛还原法分析硝酸根氮氧同位素的方法研究

Determination of the nitrogen and oxygen isotopic composition of dissolved nitrate by Ti^(3+) reduction method

本研究利用三价钛还原法将标准样品、海水、湖水和雨水中硝酸根转化为N_(2)O测试其氮氧同位素值,并优化实验条件(NO^(-)_(3)浓度、试剂量、反应时间和温度等)。结果表明:4 cm^(3)硝酸根水样(浓度约为20μmol/dm^(3)),加入100 mm^(3)TiCl_(3)溶液(浓度为150 g/dm^(3)),25℃反应24 h,NO_(3)^(-)还原为N_(2)O的平均转化率为86.3%(n=5)。5种丰度硝酸根氮氧同位素标样校准曲线斜率分别为0.947和0.617,相关系数R 2分别为0.999和0.994;方法检出限的NO_(3)^(-)浓度为2.5μmol/dm^(3),其δ^(15)N的标准偏差小于0.7‰(n=5),δ^(18)O_(VSMOW)值的标准偏差小于2.5‰(n=5);20μmol/dm^(3)硝酸根样品δ^(15)N的标准偏差小于0.5‰(n=5),δ^(18)O_(VSMOW)值的标准偏差小于1.0‰(n=5)。该方法测定3种不同类型水样δ^(15)N的标准偏差分别为0.3‰、0.6‰和0.5‰;δ^(18)O_(VSMOW)的标准偏差分别为2.1‰、2.0‰和1.6‰。三价钛还原法分析水体中硝酸根氮氧同位素操作步骤简单,测试效率高,但氧同位素结果需要系统校正。

This study uses the trivalent titanium reduction method to convert nitrate ions in standard samples of seawater,lake water,and rainwater into N_(2)O to test their nitrogen and oxygen isotope values,and optimize experimental conditions(NO_(3)^(-) concentration,reagent amount,reaction time,and temperature,etc.).Results show that when 4 cm^(3) nitrate water sample with concentration approximately 20μmol/dm^(3),added with 100 mm^(3) TiCl_(3) solution 150 g/dm^(3) and reacted at 25℃for 24 hours,the average conversion rate of NO_(3)^(-) reduction to N_(2)O is 86.3%(n=5).The slopes of the calibration curves for 5 nitrate nitrogen and oxygen isotope standards are 0.947 and 0.617,respectively,and the correlation coefficients R^(2) are 0.999 and 0.994,respectively.The detection limit of the method is 2.5μmol/dm^(3) for NO_(3)^(-),of which the standard deviation ofδ^(15)(N)is less than 0.7‰(n=5),the standard deviation ofδ^(18)O_(VSMOW)value is less than 2.5‰(n=5),standard deviation ofδ^(15)(N)is less than 0.5‰(n=5),and standard deviation ofδ^(18)O_(VSMOW)value is less than 1‰(n=5).Thus,three types of water samples measured by the method have resulted in standard deviation of 0.3‰,0.6‰,and 0.5‰forδ^(15)(N),respectively and standard deviations of 2.1‰,2.0‰and 1.6‰forδ^(18)O_(VSMOW),respectively.The Ti 3+reduction method is simple and efficient for the determination of nitrogen and oxygen isotope of NO_(3)^(-) in water,but the oxygen value needs to be corrected systematically.