利用次溴酸盐氧化结合盐酸羟胺还原法测定大气气溶胶样品铵态氮同位素

硝酸铵、硫酸铵和硫酸氢铵等铵盐作为PM_2.5中主要的二次无机气溶胶污染物,在灰霾形成过程中发挥了重要作用.关于其来源及转化过程的研究备受关注.本文在前人研究基础上,基于稳定同位素分析技术改进了样品进样量,增加了调节浸提液pH的步骤,给出了一种能够快速、准确测量大气样品中铵态氮同位素的化学转化方法.即向含有0.25 μg·mL^-1铵态氮的4 mL大气气溶胶滤膜样品浸提液中加入碱性次溴酸盐溶液,使铵盐(NH₄⁺)氧化为亚硝酸盐(NO₂^-);然后通过调节pH值,在 pH<0.3 的条件下,使用盐酸羟胺将亚硝酸盐(NO₂^-)还原为氧化亚氮气体(N₂O),最终通过Precon-GasBench-IRMS系统的连用实现大气气溶胶样品中的铵态氮同位素的测定.该方法所需进样含量低,避免了剧毒易制爆试剂的使用,重复测试精确度可达到0.2‰(n=10).通过调节大气气溶胶滤膜样品浸提液的pH可提高还原效率(约100%).测量精确度和准确度满足了大气气溶胶中颗粒态铵的测定,有助于进一步开展大气铵盐的来源、化学转化过程、传输沉降等研究.

Hypobromite oxidation combined with hydroxylamine hydrochloride reduction method for analyzing ammonium nitrogen isotope in atmospheric samples.

Ammonium salts, including ammonium nitrate, ammonium sulfate and ammonium hydrogen sulfate, are the main components of secondary inorganic aerosols and play an important role in the formation of haze events. The sources and transformation processes of atmospheric ammonium have received more and more attention. In this study, we modified the previous stable isotope analysis technique by improving the injection volume and adding a pH adjustment step, which gave a rapid and accurate measurement of ammonium nitrogen isotope ratio in atmospheric aerosol samples. Firstly, we added alkaline hypobromite to the extracted solution of the atmospheric aerosol filter samples (0.25 μg·mL^-1 ammonium nitrogen in 4 mL) to oxidize ammonium (NH₄⁺) to nitrite (NO₂^-). Then, after adjusting the pH, nitrite (NO₂^-) was reduced to nitrous oxide (N₂O) by hydroxylamine hydrochloride under pH <0.3. Finally, nitrous oxide (N₂O) was analyzed by Precon-GasBench-IRMS system to measure ammonium nitrogen isotope ratio. Our approach required low amount of NH₄⁺ and avoided the use of highly toxic and explosive reagents. Meanwhile, the precision of our method could reach as high as 0.2‰ (n=10). This method could increase the NH₄⁺ reduction efficiency to 100% at a condition of pH <0.3 and satisfy the demands of precision and accuracy for determination of ammonium nitrogen isotope in atmospheric aerosol samples. This method would help us better understand the sources, evolutions, chemical and deposition processes of atmospheric ammonium.