Introduction of isotope ratio mass spectrometry (IRMS) for the forensic analysis of explosives

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The key role of a forensic scientist is to assist in determining whether a crime has been committed and, if so, assist in the identification of the offender. It is a commonly held belief that a particular item can be conclusively linked to a specific person, place or object. Unfortunately, this is often not an achievable outcome. An evaluation was undertaken to determine if isotope ratio mass spectrometry (IRMS) could assist in the investigation of complex forensic cases by providing a level of discrimination not achievable utilising traditional forensic techniques. The focus of the research was on ammonium nitrate (AN), a common oxidiser used in improvised explosive mixtures. A secondary objective was to adapt the methods and protocols developed for AN for the analysis of other threat explosives, namely triacetone triperoxide (TATP) and pentaerythritol tetranitrate (PETN). The potential of IRMS was demonstrated through the successful development and validation of a method for the measurement of bulk nitrogen isotope ratios in AN samples and the subsequent development of an AN classification scheme based on nitrogen stable isotopes to assist in determining the potential manufacturer (in the case of an Australian source). Although the discrimination was limited, the classification scheme could be used as an investigative aid. A comparison of nitrogen isotope ratios from intact AN prill samples with those from post-blast AN prill residues highlighted that the nitrogen isotopic composition of the prills is not maintained; hence, this is a limitation of the technique for explosives analysis. Combining oxygen and hydrogen stable isotope ratios permitted the differentiation of AN prills from three different Australian manufacturers. Groups corresponding to source were also identified in the overseas AN prill samples. When these values were combined with the nitrogen isotope values, there was some level of discrimination (within the scope of the samples analysed) between prills manufactured in Australia and those manufactured overseas. The IRMS procedures developed through this research were successfully applied to the analysis of both TATP and PETN. Preliminary results of a limited sample set demonstrated that TATP sources may be discriminated utilising carbon and hydrogen isotopes alone, and in combination with oxygen isotopes. Preliminary results for PETN samples demonstrated that different sources can be discriminated based on carbon and nitrogen isotope ratios. A laboratory inter-comparison for carbon and nitrogen bulk stable isotope ratios across seven Australian and New Zealand IRMS laboratories was conducted and provides an initial snapshot of the potential for traceability. A Microsoft Access 2.0 database was developed for the IRMS data and its successful operation demonstrated. This research highlights the significant value of IRMS in complex forensic investigations, particularly with respect to explosives analysis. Further research is justified to continue the path towards broader forensic casework application of the technique.
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