Scientists at King’s College London have taken a first step toward understanding how a range of contextual factors influence how bomb residue transfers to the hands of suspects, which could eventually lead to stronger evidence in court.
The new research showed the amount of explosive material handled and the unique characteristics of the handler are important determining factors. However, the brand or type of material appeared not to have any influence.
“Developing this understanding is critical if we want forensic evidence to truly be helpful in understanding what happened in a case,” said lead author Matteo Gallidabino, lecturer in forensic chemistry at King’s College London.
Often used in the construction of IEDs, smokeless powders (SLPs) are a common and easily available material. The powder residues can be detected by hand swabs applied to suspects following a bombing incident. Currently, analysts primarily focus on determining whether explosive-related molecules are present, and when possible, what type of explosive. However, due to a lack of data on background levels and persistence of powder, as well as secondary transfer, assessment of whether a suspect actually handled an explosive remains limited.
In the new study, published in Science & Justice, Gallidabino and team had volunteers handl different SLP samples containing common additives, specifically diphenylamine (DPA), dibutyl phthalate (DBP) and ethyl centralite (EC). The volunteers were instructed to wash their hands thoroughly with soap and water before rubbing samples of SLP between their palms for 30 seconds. They were then asked to clap their hands to dislodge loose particles before the researchers swabbed the volunteers’ hands for samples.
According to the study, the results showed that the quantity transferred to hands ranged from in the billionths to in the millionths of a gram. DBP was the most concentrated additive in all SLP samples, followed by DPA and EC—a pattern also seen in the hand residues. However, the specific concentrations of these additives (which varied across SLPs) were not mirrored in the hand residues, indicating that the type or brand of SLP is not a key influencing factor.
Importantly, the study also revealed how strong a role the specific handler plays in residue transfer. Unique physical characteristics such as skin properties were found to influence residue transfer, as well as individual behaviors such as how a handler held the powder and the force applied.
“While not directly measurable, these factors caused the final amount transferred to differ from person to person. In addition, the amount of SLP found on the hands correlated with the total mass of SLP handled,” said Gallidabino. “These results provide a foundation that can help experts better interpret residue in cases involving the use of energetic materials, such as terrorist attacks, leading to stronger and more reliable conclusions in court.”
The research team says future work should examine a wider range of chemical substances associated with explosives, as well as related retention and persistence of SLP traces on hands.