IJUP08 - Universidade do Porto

Firing distance estimation through the analysis of the gunshot
residues deposit pattern around de bullet entrance hole
J. Tuna 1 , A. Santos 2 , A. Almeida 1 and T. Magalhães 2
1 REQUIMTE, Serviço de Química-Física, Faculdade de Farmácia, Universidade do Porto, Portugal.
2 Serviço de Medicina Legal, Faculdade de Medicina, Universidade do Porto, Portugal. Serviço de
Patologia Forense, Instituto Nacional de Medicina Legal, I.P. – Delegação do Norte.
The determination of the shooting distance is of utmost importance in criminal
investigation of firearm incidents since it can decisively contribute for the differential
medico-legal diagnosis among a simple accident, a suicide or a homicide [1]. One of the
main evidences in these cases is the presence of residues (gunshot residues – GSR) in the
skin of the victim, at the impact zone and around it. Such GSR are essentially particles
resulting from the condensation of vapors of the “primer” that were jointly transported
with the projectile to the impact surface. Regarding metallic elements, GSR mainly contain
lead (Pb), antimony (Sb) and barium (Ba).
In a forensic ballistics perspective, it is understandable that the quantity (the deposit
density) and the distribution pattern of GSR around the bullet entrance hole is somewhat
related with the firing distance, a fact that has been explored by some investigators.
In a previous work [2], we studied the 6.35-mm
pistol, the gun most frequently involved in firearms
incidents in Portugal. Test shots were made against
a target of cotton tissue, and the amounts of Sb, Ba
and Pb deposited in quadrangular pieces (1 x 1 cm)
of the target, cut from four radial positions around
the bullet entrance hole (Fig. 1), were determined
by Inductively Coupled Plasma-Mass Spectrometry
Fig. 1 – Sampling positions for
GSR analysis.
(ICP-MS). The data obtained were used to establish a mathematical model for estimating
the firing distance. In the experimental conditions used, it was possible to estimate it on the
interval [20-80] cm. The best regression curve which adjusted to the data was a linear
multiple regression between the firing distance (d) and the logarithm of the mass (m) of
each element in samples C (i.e., radial distance between 3.5 cm and 4.5 cm), expressed in
µg/g of target tissue: d = a + b1X1 + b2X2 + b3X3, where X1 = ln m (Sb), X2 = ln m (Ba) and
X3 = ln m (Pb). The accuracy of firing distance estimation using only 1 or 2 elements was
not significantly different from the one obtained with the 3 elements. Using a similar
experimental protocol, we have now extended the study to the 7.65 mm pistol, another gun
also commonly involved in firearm incidents in Portugal, and the results were quite
similar. However, for this pistol, the best radial position for samples collection seems to be
position B.
References:
[1] DiMaio, V.J.M. (1999), Gunshot Wounds: Practical Aspects of Firearms, Ballistics, and Forensic
Techniques, CRC Press, Boca Raton.
[2] Santos, A., Magalhães. T., Vieira, D.N., Almeida, A.A. and Sousa, A.V. (2007), Firing distance
estimation through the analysis of the gunshot residue deposit pattern around the bullet entrance hole by
inductively coupled plasma-mass spectrometry: an experimental study, The American Journal of Forensic
Medicine and Pathology, 28(1), 24-30.
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