Rationale: To perform the δ N isotopic analysis of ammonium (NH ) with an elemental analyzer (EA) coupled to an isotope-ratio mass spectrometer, it is necessary to isolate the NH 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 ((C H ) BNH ) in water. Its suitability for the stable isotope measurement of δ N values for NH 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 NH concentrations for complete (C H ) BNH precipitation, the water samples were first freeze-dried and then prepared for the analysis. To precipitate the NH , sodium tetraphenylborate ((C H ) 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 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 δ N measurement of NH in natural groundwater samples. The laboratory effort required for sample preparation is very low, and the repeatability of the measurements is very high.
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