Determination of stable nitrogen isotopic ratios of nitrate ions in ammonium nitrate

Abstract

Ammonium nitrate (AN) is one of the most commonly used explosives in criminal cases. The comparison and source-tracing of AN is important for investigation of attribution and fingerprinting of an explosive used at different events. The stable isotope signature of AN is an important index for comparison and tracing. However, the characteristics of the stable nitrogen isotopic ratios of AN (δ¹⁵ N_NH4NO3 ) alone are not sufficient to achieve a fine comparison between different AN samples. To increase the comparison index and further improve the discriminability between stable nitrogen isotopic ratios of different ANs, a method of isolation and analysis of nitrate ions in AN was established using stable-isotope-ratio mass spectrometry (IRMS). The method was based on the principle that strong alkali react with AN to produce ammonia and nitrate. After the isolation, stable nitrogen isotopes of nitrate ions (δ¹⁵ N_NO3 ) were obtained using IRMS, and then the stable nitrogen isotopes of ammonium ions from AN (δ¹⁵ N_NH4 ) was calculated according to the principle of mass balance. The results show that the method is effective for the isolation of nitrate ions without notable isotope fractionation. The developed method was applied to analyze and discriminate AN samples from eight different cities in China. Three samples out of the initial eight AN samples with similar δ¹⁵ N_NH4NO3 values were further distinguished by their δ¹⁵ N_NH4 and δ¹⁵ N_NO3 values. The isolation and stable-nitrogen isotopic analysis method developed for nitrate ions in AN is simple and effective, thereby increasing the discriminability of the stable isotope ratios in AN.

Keywords: ammonium nitrate; discrimination; isolation; nitrate ions; stable isotope ratio mass spectrometry; stable nitrogen isotopic ratio.