Tu Ying, Fang Yunting, Liu Dongwei, Pan Yuepeng
Key Laboratory of Forest Ecology and Management, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110016, China.
Qingyuan Forest CERN, Chinese Academy of Sciences, Shenyang, 110016, China.
Rapid Commun Mass Spectrom. 2016 May 30;30(10):1213-1222. doi: 10.1002/rcm.7551.
The azide method for measuring the stable isotope ratios of nitrate (NO ) is easy to set up. However, the method requires spongy cadmium (Cd) or activated Cd powder which are not easy to prepare, and a toxic azide buffer is used. We aimed to use Cd powder directly to simplify preparation and to substantially reduce the azide dose.
The reaction conditions were optimized in order to maximize the NO reduction yield. The original azide buffer was diluted by 10- to 10000-fold with or without addition of sodium acetate to reduce O-exchange between nitrite (NO ) and H O. The isotope ratios of the produced nitrous oxide (N O), used to examine the overall reaction performance, were measured using a purge and cryogenic trap system coupled to an isotope ratio mass spectrometer.
It was found that Cd powder could be directly used to reduce NO to NO . A 100-fold diluted azide buffer could be used to reduce NO to N O when only the δ N value was measured, and the diluted azide buffer with sodium acetate when both δ N and δ O values were measured. Using the modified method, the standard deviations of the δ N and δ O measurements of international NO standards were 0.1 to 1.0‰ and often better than 0.3‰ (3 replicates).
Compared with the original azide method, the techniques described here can reduce preparation time by using Cd powder without activation in the first reaction step and substantially (by >60-fold) reduce the dose of extremely toxic reagents containing azide by incorporating sodium acetate in the second reaction step. Our modified method is suitable for samples with small volume (5 mL), being different from previous methods in which 50 or 70 mL samples were used. Copyright © 2016 John Wiley & Sons, Ltd.
叠氮化物法用于测量硝酸盐（NO₃⁻）的稳定同位素比率，该方法易于设置。然而，此方法需要不易制备的海绵镉（Cd）或活性镉粉，并且使用了有毒的叠氮化物缓冲液。我们旨在直接使用镉粉以简化制备过程，并大幅减少叠氮化物的用量。
对反应条件进行了优化，以最大化NO₃⁻的还原产率。原始的叠氮化物缓冲液在添加或不添加乙酸钠的情况下稀释10至10000倍，以减少亚硝酸盐（NO₂⁻）与H₂O之间的氧交换。使用吹扫和低温捕集系统与同位素比率质谱仪联用，测量用于检查整体反应性能的生成氧化亚氮（N₂O）的同位素比率。
发现镉粉可直接用于将NO₃⁻还原为NO₂⁻。当仅测量δ¹⁵N值时，100倍稀释的叠氮化物缓冲液可用于将NO₂⁻还原为N₂O；当同时测量δ¹⁵N和δ¹⁸O值时，则使用添加了乙酸钠的稀释叠氮化物缓冲液。使用改进后的方法，国际NO₃⁻标准品的δ¹⁵N和δ¹⁸O测量的标准偏差为0.1至1.0‰，通常优于0.3‰（3次重复）。
与原始的叠氮化物法相比，本文所述技术通过在第一步反应中使用未活化的镉粉可减少制备时间，并通过在第二步反应中加入乙酸钠，大幅（>60倍）减少含叠氮化物的剧毒试剂的用量。我们改进后的方法适用于小体积（5 mL）样品，这与之前使用50或70 mL样品的方法不同。版权所有© 2016约翰威立父子有限公司。
