Forensic Applications of Double-Shot Pyrolysis Gas Chromatography with Mass Spectrometry

Abstract

Polymers are present in many rubber and plastic products that are used daily. The probability of discovering polymeric evidence at crime scenes is high. Polymeric evidence can provide class evidence for crime investigations by comparing samples recovered from suspects with scene samples, establishing a connection between the suspect and incident scene or among scenes. The main chemical composition variation in polymeric products across brands arise from their differences in additive compositions. Brand-specific additives can be potential chemical markers for discrimination between samples. Double-Shot Pyrolysis Gas Chromatography/Mass Spectrometry (DS-PyGC/MS) is a derivative analytical method of Single-Shot PyGC/MS (SS-PyGC/MS), which is the current analytical method routinely used for polymeric evidence analysis in most forensic laboratories. DS-PyGC/MS involves thermal desorption GC/MS and pyrolysis GC/MS, while SS-PyGC/MS only involves pyrolysis GC/MS. The pyrolyser furnace operates at a lower temperature during the thermal desorption step, allowing the low volatility compounds (i.e., additives) to be thermally desorbed and detected, and minimises interference from the larger polymer compounds, offering increased discrimination potential. This thesis is a proof-of-concept study investigating whether DS-PyGC/MS can detect more differences in the chemical composition (i.e., better in discrimination) of the chosen polymeric products across brands compared to SS-PyGC/MS. Tyre rubbers, disposable gloves, road cones, cling films, and shotgun wads were the selected substrates as they are frequently recovered at crime scenes in New Zealand. Qualitative results showed that tyre rubbers and road cones were the ideal substrates for DS-PyGC/MS analysis. Brand-specific compounds and additives were detected exclusively from thermal desorption GC/MS for these two substrates. For example, vulcanisation accelerator and antioxidant compounds were detected in tyre rubbers, while fluorescent orange pigment and plasticisers were detected in road cones. For disposable gloves, more compounds were detected through thermal desorption GC/MS, but not many additive compounds were detected. The thermal desorption results for cling films and shotgun wads were poor, no compounds were detected other than alkane and alkene chains. DS-PyGC/MS has the potential to identify more variations in the chemical compositions for some polymeric products across brands. It is a promising method for forensic comparison analysis of polymeric evidence.