Hesse Tobias, Niemann Felix, Khaliq Shaista, Köster Daniel, Enss Julian, Feld Christian K, Nachev Milen, Kerpen Klaus, Jochmann Maik A, Schmidt Torsten C
Instrumental Analytical Chemistry, University of Duisburg-Essen, Essen, Germany.
Probenahmedienst Feststoffe, Ressourcen- und Qualitätsmanagement, Landesamt für Natur, Umwelt und Verbraucherschutz NRW, Duisburg, Germany.
Rapid Commun Mass Spectrom. 2025 Feb 15;39(3):e9950. doi: 10.1002/rcm.9950.
The analysis of nitrogen isotopes in aqueous dissolved nitrate is an effective method for identifying pollution sources and offers the potential to study the nitrogen cycle. However, the measurement of nitrogen isotope ratios of nitrate still requires extensive sample preparation or derivatization.
In this study, a modified commercially available liquid chromatography-isotope ratio mass spectrometer (LC-IRMS) interface is presented that enables automated measurement of δN signatures from nitrate by online reduction of nitrate in two consecutive steps. First, vanadium(III) chloride is used as a reducing agent to convert NO to NO under acidic conditions. The mix of nitrogen oxides is then transferred into a stream of helium and reduced to nitrogen (N) analysis gas via a hot copper reactor. Prior to the online conversion of aqueous nitrate into elemental nitrogen, the sample was chromatographically separated from potential matrix effects on a PGC column.
Precision was achieved at a level below 1.4‰ by injecting 10 μL of 50 mg L N, using five different nitrate standards and reference materials. These materials spanned a range of more than 180‰ in δN. To demonstrate the applicability of the method, we measured water samples from an enrichment experiment, where isotopically enriched ammonium chloride was administered into a small river over the course of 2 weeks. In contrary to our expectation, the δN values of river nitrate showed values between +0.4 ± 0.4‰ and +4.1 ± 0.3‰, varying over a small range of 3.7‰.
Our study showed that the measurement of nitrate nitrogen isotope ratios with a modified LC-IRMS system is possible but that further modifications and improvements would be necessary for a robust and user-friendly instrument.
分析水溶硝酸盐中的氮同位素是识别污染源的有效方法，并且具有研究氮循环的潜力。然而，硝酸盐氮同位素比值的测量仍需要大量的样品制备或衍生化处理。
在本研究中，我们展示了一种经过改进的商用液相色谱-同位素比率质谱仪（LC-IRMS）接口，该接口能够通过连续两步在线还原硝酸盐来自动测量硝酸盐的δN特征。首先，使用氯化钒（III）作为还原剂，在酸性条件下将NO转化为NO。然后将氮氧化物混合物转移到氦气流中，并通过热铜反应器还原为氮气（N）分析气体。在将水溶硝酸盐在线转化为元素氮之前，样品在PGC柱上进行色谱分离，以消除潜在的基质效应。
通过注入10μL 50mg L⁻¹ N的五种不同硝酸盐标准品和参考物质，实现了低于1.4‰的精密度。这些物质的δN范围超过180‰。为了证明该方法的适用性，我们测量了富集实验中的水样，在为期2周的时间里，向一条小河中投放了同位素富集的氯化铵。与我们的预期相反，河水中硝酸盐的δN值在+0.4±0.4‰至+4.1±0.3‰之间，在3.7‰的小范围内变化。
我们的研究表明，使用改进的LC-IRMS系统测量硝酸盐氮同位素比值是可行的，但要使仪器稳健且用户友好，还需要进一步的改进和完善。
