Characterization of gunshot residues by surface analysis techniques

Resumen

Nowadays, the detection and identification of gunshot residues (GSR) from firearm discharges provide valuable investigative information that is generally used to determine whether a person has discharged a firearm and therefore to confirm an alibi, differentiate homicide from suicide and even to determinate a bullet entrance hole or estimate the firing distance. The research work described in this thesis aims to develop an alternative approach, for the unequivocal identification of GSR, what would provide unequivocal information about the chemical composition of individual GSR particles from different ammunition types: lead-based, lead-free and heavy metal free ones. In the first stage of this work, a method based on laser ablation inductively coupled plasma-mass spectrometry (LA-ICPMS) is presented. This instrumental combination was applied for the inorganic characterization of GSR particles on tape-lift collection devices with carbon adhesive as sampling surface. The laser ablation pattern and ICPMS conditions were optimized for the identification of the elemental profile of GSR particles. Once the validity of the method was verified, a study on the nature and composition of the GSR particles was conducted for samples taken under normal shooting conditions, not wearing gloves on the shooter hand, using different firearms, calibers and ammunitions. The usefulness of this methodology was also verified for the analysis of GSR from lead-free and heavy metal free ammunitions and the results were confirmed by fs-LA-ICPMS analysis. In the second part, two surface analysis techniques, Fourier Transform Infrared Spectrsocopy (FTIR) and Raman Spectroscopy, are presented for the analysis of organic GSR complementary to the inorganic analysis by LA-ICPMS. A modification of the tape-lift sample collection device was required in order to be able to simultaneously measure inorganic and organic compounds. The optimization of the parameters for the detection and identification of organic GSR is discussed. Both FTIR and Raman spectroscopy ere applied to the analysis of gunpowders and GSR from different lead-based, lead-free and heavy metal free ammunitions taken directly from the skin. The obtained results were confirmed by the development of a third methodology based on liquid chromatography quadrupole-time-of-flight mass spectrometry (LC/Q-TOF) which was applied to the analysis of gunpowders and GSR of the studied ammunitions.