Ammonium nitrate (AN) based fertilizers are inexpensive and easily obtained, leading to their use in bombs and homemade explosive devices. Differentiation among various sources of AN may be useful as an investigative lead in bomb characterization. AN is manufactured from NH3 and HNO3, which are produced using atmospheric N2 and O2 under high temperatures and pressures. Therefore, N and O isotope ratios of AN (15N/14N and 18O/16O), expressed in delta notation, are similar to those of atmospheric N2 and O2 (δ15N ≡ 0‰, δ18O ≈ +23.5‰). However, inconsistencies in manufacturing can cause small but measureable variations in isotope ratios, permitting differentiation among AN samples that are otherwise chemically similar. For any characteristic to be useful for differentiation, the between “source” variations must exceed the within “source” variations, noting that “source” could be a manufacturing lot, distributor, manufacturer, or region. To assess the isotopic variability within and among sources, a time series of 26 samples from a single manufacturer and 21 other distinct AN products were analyzed. A total of 49 AN samples were analyzed for bulk δ15N and δ18O values, followed by δ15N analyses of the NO3- ion and calculation of the NH4+ ion δ15N value via mass balance. The bulk isotope values yielded a modest range of δ15N (-4.6 to +4.4‰) and δ18O (+20.9 to +25.2‰), with the mean δ15N and δ18O values close to atmospheric signatures at -0.2 and +23.1‰, respectively, as expected. These variations permit differentiation among some of the AN samples. The difference between δ15NNO3 and δ15NNH4 varies between -0.4‰ to +6.4‰, indicating that δ15N values from the two component ions of AN can provide additional means of discrimination of samples that otherwise display similar bulk isotope values. The 26 samples representing a time series of AN production within a single factory exhibited variations of δ15NBulk, δ15NNO3, and δ18O values of ~1.5‰, 2.5‰, and 2‰, respectively, between the months of June and November, while the δ15NNH4 remained constant. These results are consistent with a change in the isotope composition of the HNO3 used in production, which is sourced from outside the factory, whereas the NH3 is produced onsite. The temporal changes in δ15N and δ18O values and the variability among the entire collection of samples demonstrates that isotope ratios of AN are useful at differentiating among distinct lots of AN, but not in identifying a manufacturer.

Isotope composition of manufactured bulk ammonium nitrate and its isolated ions as a method of differentiating forensic samples

253rd National Meeting (2017)

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