RATIONALE To perform the δ15 N isotopic analysis of ammonium (NH4 + ) with an elemental analyzer (EA) coupled to an isotope-ratio mass spectrometer, it is necessary to isolate the NH4 + prior to the analysis. Existing methods are work-intensive and time-consuming. For broader applicability in the environmental sciences, it is desirable to simplify the sample preparation process. The method used here is based on the insolubility of ammonium tetraphenylborate ((C6 H5 )4 BNH4 ) in water. Its suitability for the stable isotope measurement of δ15 N values for NH4 + has already been proven (Howa et al. Rapid Commun Mass Spectrom. 2014;28:1530-1534). However, there are no studies on the usability of the method for the analysis of ammonium-containing water samples. In this study, the method was tested for its applicability to natural groundwater samples. METHODS To achieve the necessary high NH4 + concentrations for complete (C6 H5 )4 BNH4 precipitation, the water samples were first freeze-dried and then prepared for the analysis. To precipitate the NH4 + , sodium tetraphenylborate ((C6 H5 )4 BNa) was added to the samples. The precipitate was then separated from the water by filtration using a membrane filter and analyzed using an EA interfaced with an isotope ratio mass spectrometer to determine the nitrogen isotope ratio. RESULTS A consistent 15 N enrichment of +0.65‰ was found in the measured isotope ratios. No significant effect due to other ions in the water samples could be identified. There was no significant difference between the isotope ratios of the natural groundwater samples determined from the tetraphenylborate (TPB) method and those determined from the diffusion method (DM). This indicates that the TPB method can easily be used for natural groundwater samples. CONCLUSIONS The TPB method in combination with freeze-drying is an efficient technique for the precise δ15 N measurement of NH4 + in natural groundwater samples. The laboratory effort required for sample preparation is very low, and the repeatability of the measurements is very high.

中文翻译：

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冷冻干燥的天然地下水样品中铵盐的四苯基硼酸钠沉淀的δ15N分析。

理由要使用与同位素比质谱仪耦合的元素分析仪（EA）对铵（NH4 +）进行δ15N同位素分析，必须在分析之前分离NH4 +。现有的方法是工作量大且费时的。为了在环境科学中具有更广泛的适用性，希望简化样品制备过程。此处使用的方法基于四苯基硼酸铵（（C6 H5）4 BNH4）在水中的不溶性。已经证明其适用于稳定同位素测量NH4 +的δ15N值（Howa等人，Rapid Commun Mass Spectrom。2014; 28：1530-1534）。但是，尚无关于该方法用于分析含铵水样品的可用性的研究。在这项研究中，测试了该方法对天然地下水样品的适用性。方法为了获得完全（C6 H5）4 BNH4沉淀所必需的高NH4 +浓度，首先将水样品冷冻干燥，然后准备进行分析。为了沉淀NH4 +，将四苯硼酸钠（（C6 H5）4 BNa）添加到样品中。然后通过使用膜滤器的过滤将沉淀物与水分离，并使用与同位素比质谱仪连接的EA进行分析以确定氮同位素比。结果在测得的同位素比率中，发现15 N的恒定富集+ 0.65‰。由于水样中的其他离子，没有明显的影响。用四苯硼酸酯（TPB）法测定的天然地下水样品和用扩散法（DM）测定的天然地下水样品的同位素比率之间没有显着差异。这表明TPB方法可轻松用于天然地下水样品。结论TPB法与冷冻干燥相结合是一种用于精确测量天然地下水样品中NH4 +的δ15N的有效技术。样品制备所需的实验室工作量非常低，并且测量的可重复性非常高。