Scientists have for the first time provided insight into how bomb residue transfers to the hands of suspects, which could lead to stronger evidence in court.
The study, which was led by a team from King's College London and published today in the journal Science & Justice, investigated how explosive materials commonly found in pipe bombs and improvised explosive devices (IEDs) transferred to the palms and fingers of individuals who handled them. The researchers learned that the amount that was handled and the unique characteristics of the handler were important determining factors. Surprisingly, the brand or type of material appeared not to be a decisive influencing factor.
The study is the first of its kind to explore how contextual factors influence explosive material residue transfer to hands, and could be used to help forensic experts in cases involving explosive materials.
Lead author, Dr Matteo Gallidabino, Lecturer in Forensic Chemistry at King's College London, said: "These findings could mark a breakthrough in forensic investigations into individuals who are suspected of handling explosive materials. Currently, forensic experts can identify the presence of explosive material-related molecules on hands, but interpretation rarely goes deeper than that because of a lack of contextual data.
"This study is a first step towards understanding how a range of contextual factors influence how residue is transferred. Developing this understanding is critical if we want forensic evidence to truly be helpful in understanding what happened in a case. The 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."
Often used in the construction of IEDs, smokeless powders (SLPs) are a common and easily available material. In some countries, they can be bought over the counter by people seeking to manually load their own gun cartridges. 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 was involved. However, due to a lack of data on background levels and persistence of powder, as well as secondary transfer – by way of actions such as shaking hands or touching surfaces - assessment of whether a suspect actually handled an explosive remains limited.
The research team conducted controlled experiments where volunteers handled different SLP samples containing common additives – namely 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. This was followed by swabs being rubbed on their hands to collect a sample.
The researchers then used a streamlined 'filter and shoot' method – a simple and fast sample preparation technique used in analytical chemistry - that was optimised for the experiment, before analysing the samples using gas chromatography-mass spectrometry.
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. The researchers were surprised by this finding, indicating that the type or brand of SLP is not a key influencing factor.
The unique characteristics of the handler also influenced residue transfer. These included physical characteristics – such as skin properties, and behaviours – such as how the powder was handled 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.
The results indicate the importance of taking contextual factors into account when investigating explosive residue traces in real cases.
The study also introduces a faster and more robust sampling method – the optimised filter and shoot technique – that could be used for future research studies.
Dr Gallidabino calls for future research to look at a wider range of chemical substances associated with explosives, as well as to study related retention and persistence of SLP traces on hands.
