Forensic utility of a nitrogen and oxygen isotope ratio time series of ammonium nitrate and its isolated ions

Abstract

Ammonium nitrate (AN) based fertilizers are inexpensive and easily obtained, characteristics that often lead to their use in homemade explosive devices. The stable nitrogen and oxygen isotope ratios (15N/14N and 18O/16O, expressed as δ15N and δ18O) of AN have the potential to aid in forensic investigations by providing supplemental properties for sample-to-sample comparison in materials which are otherwise chemically identical. The forensic utility of stable isotope analyses depends on demonstrated variation between different sources and minimal variation within a source. To test the variability within a single manufacturer (here considered a source), a total of 26 samples representing two production time periods and two product lines were analyzed for bulk δ15N and δ18O. Additionally, because AN is known to have a modest isotopic range, a potassium nitrate precipitation method was developed to separate the component ions (NO3- and NH4+) for individual δ15N analysis and increased discriminatory power. The average δ15N and δ18O of bulk AN (− 0.10‰ and + 22.8‰, respectively) is similar to the isotopic signature of atmospheric N2 and O2, the starting reactants in AN production. The bulk δ15N, δ18O, and NO3- δ15N show average values from both product lines that differ by 1.5‰, 2.0‰, and 2.6‰, respectively, between the production periods of June and November 2015. Conversely, the NH4+ δ15N remained relatively consistent over time. Furthermore, whereas samples in the two product lines produced on the same day in June are isotopically similar, there are isotopic differences between samples in the two product lines manufactured within 6h of each other in November. The observed variability could be useful in comparing AN from two or more bombs, or a bomb and a stash of AN in a suspect's possession, but the observed lot-to-lot differences within one manufacturer could complicate attribution efforts. In contrast, the NH4+ δ15N values, which appear to be the most consistent over time within this factory, need to be further explored as a potentially reliable signal.