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δ 15 N analysis of ammonium in freeze-dried natural groundwater samples by precipitation with sodium tetraphenylborate

δ 15 N analysis of ammonium in freeze-dried natural groundwater samples by precipitation with sodium tetraphenylborate

Patricia Stock, Andreas Hösch, Diana Burghardt

Rapid Commun Mass Spectrom. 2019 Nov 30;33(22):1730-1738.

PMID: 31295375

Abstract:

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.

Chemicals Related in the Paper:

Catalog Number	Product Name	Structure	CAS Number	Price
AP143668	Sodium tetraphenylborate		143-66-8	Price

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