Wassenaar Leonard I, Douence Cedric, Altabet Mark A, Aggarwal Pradeep K
Isotope Hydrology Section, International Atomic Energy Agency, Vienna, Austria.
Department of Estuarine and Ocean Sciences, University of Massachusetts Dartmouth, New Bedford, MA, USA.
Rapid Commun Mass Spectrom. 2018 Feb 15;32(3):184-194. doi: 10.1002/rcm.8029.
The nitrogen and oxygen (δ N, δ O, δ O) isotopic compositions of NO and NO are important tracers of nutrient dynamics in soil, rain, groundwater and oceans. The Cd-azide method was used to convert NO or NO to N O for N and triple-O isotopic analyses by N O laser spectrometry. A protocol for laser-based headspace isotope analyses was compared with isotope ratio mass spectrometry. Lasers provide the ability to directly measure O anomalies which can help discern atmospheric N sources.
δ N, δ O and δ O values were measured on N/O stable isotopic reference materials (IAEA, USGS) by conversion to N O using the Cd-azide method and headspace N O laser spectrometry. A N tracer test assessed the position-specific routing of N to the α or β positions in the N O molecule. A data processing algorithm was used to correct for isotopic dependencies on N O concentration, cavity pressure and water content.
NO /NO nitrogen is routed to the N position of N O in the azide reaction; hence the δ N value should be used for N O laser spectrometry results. With corrections for cavity pressure, N O concentration and water content, the δ N , δ O and δ O values (‰) of international reference materials were +4.8 ± 0.1, +25.9 ± 0.3, +12.7 ± 0.2 (IAEA NO ), -1.7 ± 0.1, -26.8 ± 0.8, -14.4 ± 1.1 (USGS34) and +2.6 ± 0.1, +57.6 ± 1.2, +51.2 ± 2.0 (USGS35), in agreement with their values and with the isotope ratio mass spectrometry results. The O excess for USGS35 was +21.2 ± 9‰, in good agreement with previous results.
The Cd-azide method yielded excellent results for routine determination of δ N, δ O and δ O values (and the O excess) of nitrate or nitrite by laser spectrometry. Disadvantages are the toxicity of Cd-azide chemicals and the lack of automated sampling devices for N O laser spectrometers. The N-enriched tracer test revealed potential for position-specific experimentation of aqueous nutrient dynamics at high N enrichments by laser spectrometry, but exposed the need for memory corrections and improved spectral deconvolution of O.
一氧化氮（NO）和二氧化氮（NO₂）的氮和氧（δ¹⁵N、δ¹⁸O、δ¹⁷O）同位素组成是土壤、雨水、地下水和海洋中养分动态的重要示踪剂。采用叠氮化镉法将NO或NO₂转化为N₂O，以便通过N₂O激光光谱法进行氮和三重氧同位素分析。将基于激光的顶空同位素分析方案与同位素比率质谱法进行了比较。激光能够直接测量¹⁷O异常，这有助于辨别大气氮源。
使用叠氮化镉法和顶空N₂O激光光谱法将N/O稳定同位素参考物质（国际原子能机构、美国地质调查局）转化为N₂O，测量δ¹⁵N、δ¹⁸O和δ¹⁷O值。一项¹⁵N示踪剂试验评估了氮在N₂O分子中α或β位置的位置特异性路由。使用一种数据处理算法来校正同位素对N₂O浓度、腔压和含水量的依赖性。
在叠氮化反应中，NO₂/NO氮被路由到N₂O的¹⁵N位置；因此，δ¹⁵N值应用于N₂O激光光谱分析结果。校正腔压、N₂O浓度和含水量后，国际参考物质的δ¹⁵N、δ¹⁸O和δ¹⁷O值（‰）分别为+4.8±0.1、+25.9±0.3、+12.7±0.2（国际原子能机构NO₃），-1.7±0.1、-26.8±0.8、-14.4±1.1（美国地质调查局34）和+2.6±0.1、+57.6±1.2、+51.2±2.0（美国地质调查局35），与它们的值以及同位素比率质谱分析结果一致。美国地质调查局35的¹⁷O过剩为+21.2±9‰，与先前结果高度一致。
叠氮化镉法在通过激光光谱法常规测定硝酸盐或亚硝酸盐的δ¹⁵N、δ¹⁸O和δ¹⁷O值（以及¹⁷O过剩）方面取得了优异结果。缺点是叠氮化镉化学品有毒，且N₂O激光光谱仪缺乏自动采样装置。¹⁵N富集示踪剂试验表明，通过激光光谱法在高¹⁵N富集条件下对水体养分动态进行位置特异性实验具有潜力，但也表明需要进行记忆校正和改进¹⁷O的光谱去卷积。
