Forensic utility of isotope ratio analysis of the explosive urea nitrate and its precursors

Abstract 

Urea nitrate (UN) is an improvised explosive made from readily available materials. The carbon and nitrogen isotope composition of UN and its component ions, urea and nitrate, could aid in a forensic investigation. A method was developed to separate UN into its component ions for δ¹⁵N measurements by dissolving the sample with KOH, drying the sample, followed by removal of the urea by dissolution into 100% methanol. UN was synthesized to assess for preservation of the carbon and nitrogen isotope compositions of reactants (urea and nitric acid) and product UN. Based on nitrogen isotope mass balance, all UN samples contained varying amounts of excess nitric acid, making the ionic separation an essential step in the nitrogen isotope analysis. During UN synthesis experiments, isotopic composition of the reactants is preserved in the product UN, but the urea in the product UN is slightly enriched in ¹⁵N (<1‰) relative to the reactant urea. Published isotopic compositions of UN reactants, urea and nitric acid, have large ranges (urea δ¹⁵N=−10.8 to +3.3‰; urea δ¹³C=−18.2 to −50.6‰; and nitric acid δ¹⁵N=−1.8 to +4.0‰). The preservation of isotopic composition of reactants in UN, along with a significant variability in isotopic composition of reactants, indicates that isotope ratio analysis may be used to test if urea or nitric acid collected during an investigation is a possible reactant for a specific UN sample. The carbon and nitrogen isotope ratios differ significantly between two field-collected UN samples, as well as the lab-synthesized UN samples. These observed variations suggest that this approach is useful for discriminating between materials which are otherwise chemically identical.

Keywords: Urea nitrate, Home-made explosive, Isotope ratio mass spectrometry, Improvised explosive, Carbon isotopes, Nitrogen isotopes