Tracing threats Towards source- and activity-level interpretation in forensic explosives investigations

This thesis addresses the growing use of flash-powder explosives in the Netherlands by advancing forensic interpretation of perchlorate, a key chemical marker of flash powder, beyond mere detection towards source- and activity-level evaluation. The work presents the first systematic chemical profiling of potassium perchlorate for forensic use, combining trace elements and oxygen isotope ratios into a validated likelihood ratio system to quantify the evidential strength of chemical similarity between samples. An environmental background study of inorganic ions in the Netherlands shows that perchlorate concentrations are elevated after fireworks activity, and the sampling of hands from 297 individuals demonstrates that perchlorate detection is often associated with explosives-related professions. However, a key and unexpected finding is that cars frequently contain relatively high perchlorate levels. With maximised contact, driving a new car can also be a significant innocent source of contamination. Finally, transfer and persistence studies show that perchlorate is readily transferred between people and objects and can remain detectable long after flash-powder handling or manufacture. A probabilistic framework was developed that enables experts to assess the evidential significance of perchlorate quantities in the context of competing activity-level propositions, rather than leaving this to the court. As a result, courts gain stronger scientific support for making well-informed, evidence-based decisions.