Source determination of homemade ammonium nitrate using ATR-FTIR spectroscopy, trace elemental analysis and chemometrics

Abstract

Ammonium nitrate (AN) is one of the most significant chemicals used within industry, but has also seen frequent use by terrorist and extremist groups as it can be easy to source, prepare and implement in improvised explosive devices (IEDs). AN typically exists as fertiliser or explosive grade but can also be prepared from a range of commercial products. For the first time, the synthesis, characterisation and source determination of AN prepared from alternative products and mixtures has been reported. Analysis with attenuated total reflectance fourier-transform infrared spectroscopy (ATR-FTIR) revealed that AN samples could be separated into two distinct classes termed as pure form, which included different grades of AN, or as homemade, which included AN prepared from commercial products. Linear discriminant analysis (LDA) was used to develop classification models that predicted the correct source of 100 % of pure form samples and 80 % of homemade samples within an independent dataset. Trace elemental analysis using inductively coupled plasma mass spectrometry (ICP-MS) proved to have greater discriminatory power as full separation of samples was achieved by principal component analysis (PCA). The concentrations of ten elements were used to discriminate between products including B, Ti, Mn, Ni, Cu, Se, Rb, Sr, Ba and U. Furthermore, LDA revealed the model generated was able to correctly match all validation samples to their source. This work contributes to the forensic intelligence of AN and demonstrates how ATR-FTIR and ICP-MS can be used to potentially identify and discriminate between different grades, precursors and brands.