The risk of inter-stub contamination during SEM/EDS analysis of gunshot residue particles

Detecting gunshot residue (GSR) particles on samples collected from individuals or their belongings can connect them to a shooting event. Scanning electron microscopy combined with energy-dispersive X-ray spectrometry (SEM/EDX) is currently the most common forensic method for detecting and characterizing GSR. At the forensic laboratory of the Israel Police, one inch (25 mm) diameter sticky stubs are used to collect samples from suspects' hands, hair, clothes and vehicles. To maximize testing capacity, stubs of samples collected from several different cases and persons may be analyzed side by side in a single run. This has raised concern in court that a clean sample taken from an innocent person may be contaminated during the analysis by GSR particles from an adjacent sample transferred inside the SEM chamber. several experiments were conducted where stubs that were known to contain GSR particles were run adjacent to stubs that were known to be clean. Not a single event of GSR particle transfer was detected, even when a clean stub was surrounded on all sides by stubs containing a total of over 100,000 particles. Thus, the probability of transfer of a single particle is at most 1:100,000. Since the total number of GSR particles found per run is usually three orders of magnitude lower than 100,000, we conclude that the risk of inter-stub contamination is highly negligible.