Transfer, persistence, contamination and background levels of inorganic gunshot residues

Abstract

This paper summarises the available literature data for the evidential evaluation topics of transfer (196 experiments), persistence (63 time series), contamination (1515 samples), and background prevalence (2158 samples) of inorganic gunshot residues (IGSR). In-house data on IGSR transfer, and the prevalence and persistence of IGSR on different types of glove are also contributed.Combining new and previously published data in a meta-analysis, we report the following findings: The median transfer rate of IGSR was 11 % and the probability distribution of contact transfer was modelled using a Beta distribution. The half-life of IGSR on hands was estimated at 52 min. On gloves, decay followed a two-phase process with the slower process proceeding at a decreased rate compared to on hands (t1/2,slow = 77 min). The occurrence of characteristic IGSR on the hands of police officers was modelled using a generalised Pareto model (GPM). Combining the prevalence and the transfer probability models, a product probability distribution model was established. The product model estimates the probability of finding any amount of IGSR post-arrest on previously clean hands, estimating 8 % probability of non-zero transfer. Characteristic IGSR detected on the general public (1 % positives), on at-risk individuals (2 % positives), and in public places (0 % positives) showed low background levels. The likelihood of finding any amount of IGSR on the general public (at-risk included) was modelled using a GPM giving a 1.3 % probability of finding at least one, and 0.2 % probability to find more than three characteristic IGSR on the general public.