Observations and sources of carbon and nitrogen isotope ratio variation of pentaerythritol tetranitrate (PETN)

Abstract

Isotope ratio analysis allows forensic investigators to discriminate materials that are chemically identical but differ in their isotope ratios. Here we focused on the discrimination of pentaerythritol tetranitrate (PETN), an explosive with military and civilian applications, using carbon (δ(13)C) and nitrogen (δ(15)N) isotope ratios. Our goal was to understand some of the factors influencing the isotope ratios of commercially manufactured PETN. PETN was isolated from bulk explosives using preparative HPLC, which reduced chemical and isotopic within-sample variability. We observed isotope ratio variation in a survey of 175 PETN samples from 22 manufacturing facilities, with δ(13)C values ranging from -49.7‰ to -28.0‰ and δ(15)N values ranging from -48.6‰ to +6.2‰. Both within-sample variability and variation of PETN within an explosive block were much smaller than between-sample variations. Isotopic ratios of PETN were shown to discriminate explosive blocks from the same manufacturer, whereas explosive component composition measurements by HPLC were not able to do so. Using samples collected from three industrial PETN manufacturers, we investigated the isotopic relationship between PETN and its reactants, pentaerythritol (PE) and nitric acid. Our observations showed that δ(13)C values of PETN were indistinguishable from that of the reactant pentaerythritol. Isotopic separation between nitric acid and PETN was consistent within each sampled manufacturer but differed among manufacturers, and was likely dependent upon reaction conditions. These data indicate that δ(13)C variation in PETN is dependent on δ(13)C variation of PE supplies, while δ(15)N variation in PETN is due to both nitric acid δ(15)N and reaction conditions.

Keywords: Explosive; Isotope fractionation; Nitric acid; PETN; Pentaerythritol.