A modified trace metal detection test for secondary imprints on porous substrates: A preliminary study

In the investigation of criminal cases involving metallic weapons such as firearms and knives, the trace metal detection test (TMDT) and the transfer detection technique are two effective and on-the-spot methods to link a suspect and a suspected metallic weapon. In general, these tests need to be conducted on suspects' hands or done by lifting trace metals from their hands within 3 days of the crime being committed. However, if no suspects are arrested within this period, neither of these two techniques is applicable. This paper presents preliminary development of a modified TMDT to overcome the intrinsic disadvantages of conventional TMDTs. The method primarily focuses on the secondary imprints on porous substrates that are transferred unconsciously from the palms of criminals after handling galvanized weapons. The modified TMDT was established by studying the effect of various factors on secondary imprints on porous substrates. We also tested the effectiveness of the modified TMDT for common porous substrates and galvanized weapons and its relative sensitivity in a depletion series. Additionally, the storage conditions of the developed secondary imprints as physical evidence were studied under different time lapses and light conditions. Finally, we proposed an improved procedure for detecting metal traces formed in the use of metallic weapons and subsequent activities at a crime scene. The modified TMDT provides a novel method for investigators to demonstrate the relationship between a suspect and a metallic weapon, thus reducing the heavy reliance of conventional TMDTs on suspects and the time limit available to visualize or lift metal traces. For this reason, it can be used as a complementary and remedial method for conventional TMDTs, especially when suspects are not arrested within 3 days of the commission of a crime. Furthermore, the improved procedure can serve as a guide for investigators to apply the TMDT series properly to solving the cases involving metallic weapons.