Two-dimensional nitrogen isotope analysis of ammonium nitrate

Stefan Bindereif1, Thomas Holdermann2, Sandra Merten2, Sabine Schneiders2, Gerhard Gebauer3
1 BayCEER - Laboratory of Isotope Biogeochemistry, University of Bayreuth, 95447 Bayreuth, Germany; Federal Criminal Police Office, Forensic Science Institute, 65203 Wiesbaden, Germany
2 Federal Criminal Police Office, Forensic Science Institute, 65203 Wiesbaden, Germany
3 BayCEER - Laboratory of Isotope Biogeochemistry, University of Bayreuth, 95447 Bayreuth, Germany

P 6.10 in Paving the way for research: Databases, instruments, networks
& Open Session

Introduction

Stable isotope analysis is a suitable method for tracking and discriminating sources of explosives and related substances. Compared to other explosive nitro-organic compounds, the natural abundance range of 15N in ammonium nitrate (AN) is rather narrow (Benson et al. 2009). For this reason, the ability to discriminate among AN samples is highly reliant upon accurate measurements and/or multi-element isotope analysis. Previous studies suggest that separate analysis of component ions would provide additional discriminatory power (Howa et al. 2014; Aranda et al. 2011). This study aimed to validate a recommended method for separate AN component ion analysis by Howa et al. Therefore, various nitrate and ammonium salts, extracts from fertilizers and evidence objects were examined.

Material and Methods

Isolation of ammonium ions in dissolved AN was achieved by precipitation using sodium tetraphenylborate solution. For the analysis of fertilizers, AN was extracted prior to the precipitation. Both AN and ammonium tetraphenylborate were tested for their δ15N values using elemental analysis (EA) coupled to an isotope ratio mass spectrometer (IRMS). The δ15N values of nitrate ions were calculated using mass balance.

Results

In this study the precipitation method sufficiently separated the AN component ions without influencing the nitrogen isotope ratios of the ammonium ions. There was no correlation between δ15N values of ammonium and nitrate within the 21 fertilizer AN samples. In difference to bulk AN analysis, separate analysis of AN component ions revealed potential isotope ratio variation in both ions.

Conclusions

In conclusion, isolation of component ions using sodium tetraphenylborate proves a valid technique for stable isotope analysis of AN containing substances. It is simple to perform and does not require specialized equipment. Furthermore the procedure adds forensic value, as the bulk nitrogen isotope analysis might obscure potential isotope ratio variation.



Keywords: stable isotope analysis, explosives, ammonium nitrate, component ions, fertilizers, evidence objects
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