大气浓度下N_2O中氮稳定同位素比值测定的GasBench-IRMS系统改造

Modification of GasBench-IRMS for Automatic Determination of Nitrogen Stable Isotope Ratio in N_2O at Natural Abundance

为实现在线自动测定大气环境N_2O中氮稳定同位素比值,本工作对多用途在线气体制备和导入系统(GasBench)进行了改造。增加了化学阱和2个冷阱,与八通阀构成预浓缩装置,以期达到纯化和浓缩大气样品中N_2O的目的,同时更换大体积进样瓶及其配套样品盘,用于增加样品进样量。优化了质谱测定条件、样品收集和浓缩时间等关键参数,并进行了系统的线性、稳定性测试以及方法准确性验证。结果表明,在本方法条件下,当离子流强度在280~3 000mV时,系统线性关系良好,δ15 NAir值随离子流强度的增加而变化不大;不同压力条件下,δ15 NAir值的S.D<0.04‰,达到仪器稳定性指标的要求。利用该方法测量配制的400mg/L N_2O标准样品气,δ15 NAir平均值为7.253‰,接近标准气的氮稳定同位素标准值,说明该方法准确性良好,可用于大气浓度下N_2O中氮同位素比值测定。

GasBench was modified to determinate nitrogen stable isotope ratio in N2O at atmospheric concentration automatically.For purifying and concentrating N2O at atmospheric,apre-concentration device was added.The chemical trap,two cold traps and the eight-way valve were involved.Meanwhile,the sample vials and their matching sample panes were replaced to increase the sample injection.Parameters such as test conditions,collection and concentration time were optimized,and the linearity,stability and method validation were also evaluated.The results suggested that after the modification,the linearity of the system was good,the value of δ^15 NAirdid not change a lot when the ion intensity was at the range of 280-3 000 mV;the δ^15 NAirvalues of the modified systemwas stable（S.D〈0.04‰）under different pressure conditions.A standard sample gas of 400 mg/L N2O were prepared and its average value of δ^15 NAir was 7.253‰,close to the standard value of the standard gas,which indicated that the method had good accuracy,and could be used to determine the nitrogen isotope ratio of N2 O at atmospheric concentration.