大气气溶胶硝酸盐中稳定氮氧同位素比值测定

Determination of Stable Nitrogen and Oxygen Isotope Ratios in Atmospheric Aerosol Nitrates

改进了两步化学还原法的pH值、反应时间、镉粉种类、NaCl浓度等条件,测定了大气气溶胶硝酸盐的氮、氧同位素比值。将样品溶液中的硝酸盐通过两步还原为氧化亚氮气体(N_2O),将产生的N_2O通过自动进样器以及预浓缩装置进入MAT253稳定同位素比质谱仪测定其氮、氧同位素比值。借助已知氮、氧同位素比值的国际标准样品建立了N_2O和NO^-_3的氮、氧同位素比值的标准曲线,通过公式即可换算样品中硝酸盐的氮、氧同位素比值。方法最小进样量只需0.8μg(以N计),氮、氧同位素的标准偏差分别为0.08‰和0.24‰。本方法原理明确,操作简便、快速,灵敏度与精度符合环境微量样品测试要求。将此方法应用于东北大气颗粒物样品的测定中,所测氮氧同位素比值特征可以有效区分硝酸盐来源与生成机制的季节差异。此方法适用于大气气溶胶样品以及雨水、雪水等大气低浓度环境样品的测定。

Several conditions of the two-step chemical reduction method for simultaneous determination of both the stable nitrogen and oxygen isotope compositions in atmospheric nitrate(NO-3) particles, such as the pH, reaction time, type of cadmium powder and concentration of NaCl were optimized. The nitrite in the samples was reduced to nitrous oxide(N2O) through the improved method and the produced N2O was measured for their isotope compositions(δ15N and δ18O) by coupling an autosampler and a pre-concentration device with the MAT253 isotope ratio mass spectrometer. The standard curves of nitrogen(R2=0.9999) and oxygen(R2=0.9996) isotope ratios of N2O and NO-3 were established by measuring the international standards with known isotope ratios, and the nitrogen and oxygen isotope ratios of nitrates of the sample could be therefore converted. The principle of this method was clear, and only required very small amounts of samples(i.e., 0.8 μg N). The standard deviations of δ15N and δ18O measurements from the present method were 0.08‰ and 0.24‰, respectively. This research method was not only suitable for atmospheric aerosol samples, but also for the other environmental samples such as rainwater and snow samples with low concentrations. This study provided a novel method for better understand emission sources and transformation pathways of atmospheric nitrate.