Firearms: Identification of Handling of Firearms/Trace Metal Detection

Latent fingerprints and invisible impressions of firearms, left on the holders' palms, are both distinct cases of Locard's exchange principle: “Every contact leaves traces”. Both are typical examples of circumstantial evidence that can prove a previous contact between an individual person and an object. The physiological and environmental factors, which are involved in latent fingerprint retrieval, have been studied extensively for nearly 100 years. Forensic scientists, however, have paid much less attention to the detection of firearms impressions on the hands, and only recently has it attracted considerable attention. Most handguns leave invisible impressions containing di- and trivalent iron on the hand. These impressions can be visualized by spraying with the chemical reagent 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (PDT). Even brand new weapons leave such marks. From recent experience it is evident that clear, well-defined impressions, which are developed on the hands by the PDT reagent, can unequivocally prove recent contact with specific firearms, and even faint, nonspecific marks can at least allude to such contact.