Analysis of macroscopic gunshot residues by Raman spectroscopy to assess the weapon memory effect.

Gunshot residues (GSR) are valuable evidence which provide the forensic analyst with useful information about a crime scene when proper analytical methods are used. Nowadays, the method of choice for analyzing GSR is scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM/EDX). However, SEM/EDX presents limitations when the GSR identification of "non-toxic" ammunition types is performed. To overcome this drawback, Raman spectroscopy has been recently proposed as a complementary technique to SEM/EDX. However, for the time being, it can only be used in a limited number of casework (e.g. examining the macroscopic GSR produced at short distance over victim's clothes) and further research to know when this technique could support SEM-EDX results is required. In the present work, the memory effect of the weapon, which plays an important role to link the GSR found and the ammunition fired, is studied. Twenty shots were fired at close distance (~30 cm) at paper targets using the same weapon with two different types of ammunition. The first, third, ninth, and twentieth shots were fired with the first ammunition and the shots among them using the second ammunition. The macroscopic GSR produced by the first ammunition were analyzed using Raman spectroscopy. First, the spectra obtained were visually differentiated by taking into account the band at about 1342 cm(-1). This first approach shown that in the first shot were no GSR particles from the second ammunition, but 1.5-7.5% of analyzed particles corresponded to the second ammunition in the third, ninth, and twentieth shots. Additionally, the same differentiation was then performed by discriminant analysis using the spectral range from 1800 to 800 cm(-1). Although using this second approach only one GSR was identified as the second ammunition, was remarkable that after the shots with different ammunition the GSR obtained shows greater variability. The results obtained suggest that the memory effect of the weapon has not a significant influence when the organic analysis of macroscopic GSR on targets by Raman spectroscopy is performed.