Towards activity-level interpretation in forensic explosives investigations: Studying perchlorate trace transfer from...

Flash powder, typically a 70:30 w/w mixture of potassium perchlorate and aluminium powder, is currently the most frequently encountered explosive material in the Netherlands, accounting for hundreds of attacks per year. Perchlorate is a characteristic analytical marker that, even when present at low concentrations, can indicate an individual's handling of flash powder. To enable explosives experts to move beyond mere identification towards activity-level evaluations, the Netherlands Forensic Institute has initiated research on the transfer, persistence, prevalence, and recovery (TPPR) of perchlorate. This study focuses on the transfer and persistence of perchlorate in the immediate post-contact period. Various modes of contact between human hands and objects were mimicked, resulting in over 2000 individual transfer moments. Perchlorate is readily transferred from and to hands when shaking hands or handling objects. Cotton retains perchlorate the most, followed by human skin, and finally smooth surfaces. With each subsequent contact, the transferred amount of perchlorate decreases by approximately one order of magnitude. The presented experimental setup is useful for collecting data that will enable experts to assign probabilities to TPPR events and subsequently calculate the evidential strength given activity-level propositions. Empirical transfer data by contact type can support the assignment of probabilities in similar cases with varying initial amounts. To further strengthen activity-level evaluations of perchlorate traces, both the long-term persistence of perchlorate and realistic initial quantities in different situations should be assessed.