Altabet Mark A, Wassenaar Leonard I, Douence Cedric, Roy Rupsa
School for Marine Science and Technology, University of Massachusetts at Dartmouth, New Bedford, MA, USA.
International Atomic Energy Agency, Water Resources Section, Vienna International Center, 1400, Vienna, Austria.
Rapid Commun Mass Spectrom. 2019 Aug 15;33(15):1227-1239. doi: 10.1002/rcm.8454.
The nitrogen and oxygen (δ N, δ O, and δ O values) isotopic compositions of nitrate (NO ) are crucial tracers of nutrient nitrogen (N) sources and dynamics in aquatic systems. Current methods such as bacterial denitrification or Cd-azide reduction require laborious multi-step conversions or toxic chemicals to reduce NO to N O for N and O isotopic analyses by isotope ratio mass spectrometry (IRMS). Furthermore, the O composition of N O cannot be directly disentangled using IRMS because O contributes to mass 45 ( N).
We describe a new one-step chemical conversion method that employs Ti(III) chloride to reduce nitrate to N O gas in septum sample vials. Sample preparation takes only a few minutes followed by a 24-h reaction producing N O gas (65-75% recovery) which partitions into the headspace. The N O headspace was measured for N, O and O by IRMS or laser spectrometry.
IRMS and laser spectrometric analyses gave accurate and reproducible N and O isotopic results down to 50 ppb (3.5 μM) NO -N, similar in precision to the denitrifier and Cd-azide methods. The uncertainties for dissolved nitrate reference materials (USGS32, USGS34, USGS35, IAEA-NO ) were ±0.2‰ for δ N values and ±0.3‰ for δ O values using IRMS. For laser-based N O isotope analyses the results were similar, with an δ O uncertainty of ±0.9‰ without any need for N correction.
Advantages of the Ti(III) reduction method are simplicity, low cost, and no requirement for toxic chemicals or anaerobic bacterial cultures. Minor corrections may be required to account for sample nitrate concentration variance and potential chemical interferences. The Ti(III) method is easily implemented into laboratories currently using N O headspace sampling apparatus. We expect that the Ti(III) method will promulgate the use of N and O isotopes of nitrate in important studies of nutrient dynamics and pollution in a wide range of aquatic ecosystems.
硝酸盐（NO ）的氮和氧（δ N、δ O和δ O值）同位素组成是水生系统中营养氮（N）来源和动态的关键示踪剂。目前的方法，如细菌反硝化或叠氮化镉还原，需要繁琐的多步转化或有毒化学物质将NO 还原为N O，以便通过同位素比率质谱法（IRMS）进行N和O同位素分析。此外，使用IRMS无法直接解析N O的O组成，因为O对质量数45（ N）有贡献。
我们描述了一种新的一步化学转化方法，该方法使用三氯化钛（Ti(III)）在隔膜样品瓶中将硝酸盐还原为N O气体。样品制备只需几分钟，随后进行24小时反应生成N O气体（回收率65 - 75%），该气体分配到顶空中。通过IRMS或激光光谱法测量顶空中的N O的N、O和O。
IRMS和激光光谱分析给出了准确且可重复的N和O同位素结果，低至50 ppb（3.5 μM）的NO -N，精度与反硝化器和叠氮化镉方法相似。使用IRMS时，溶解硝酸盐参考物质（USGS32、USGS34、USGS35、IAEA-NO ）的δ N值不确定度为±0.2‰，δ O值不确定度为±0.3‰。对于基于激光的N O同位素分析，结果相似，δ O不确定度为±0.9‰，无需进行N校正。
Ti(III)还原法的优点是简单、成本低，不需要有毒化学物质或厌氧细菌培养。可能需要进行一些小的校正，以考虑样品硝酸盐浓度变化和潜在的化学干扰。Ti(III)方法很容易应用于目前使用N O顶空采样装置的实验室。我们预计，Ti(III)方法将在广泛的水生生态系统中营养动态和污染的重要研究中推广硝酸盐的N和O同位素的应用。
