Banged to rights: Firearms distinguished using gunshot residues on fabrics

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  • Published: Dec 15, 2011
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: Banged to rights: Firearms distinguished using gunshot residues on fabrics

Gunshot residues

About 550,000 people were killed in Brazil by firearms over a 15-year period, according to an official UNESCO document. So, it is not particularly surprising that Brazilian research groups have been investigating methods to analyse gunshot residues to aid criminal investigations.

One team has devised methods for identifying gunshot residues on the hands of suspects who have fired revolvers and pistols, with the result that they could clearly distinguish between shooters and non-shooters. In addition, they recognised a different pattern of residual elements originating from each type of weapon.

Now, the same team has applied similar techniques to identify gunshot residues on fabrics. Joao Carlos D. Freitas, Jorge E. Souza Sarkis, Osvaldo Negrini Neto and Sonia Bocamino Viebig from the University of Sao Paulo and the Institute of Criminalistics of Sao Paulo determined the relative proportions of antimony, barium and lead and displayed the data in a graphical manner which allowed easy visual comparison.

Their technique was based on high-resolution inductively coupled plasma mass spectrometry (ICPMS) using a sector field instrument. Although it is a destructive technique, only small portions of fabric were required and it was preferred due to the relatively rapid speed of analysis. The popular scanning electron microscopy/energy dispersive X-ray analysis, a non-destructive method, takes several hours to complete. Given the number of shooting incidents and the high demand for forensic tests, analysis time is an important factor to consider.

Multiple shootings

Three weapons representing different calibre ammunition were each fired into five fabrics under controlled conditions by different volunteers. The firing distance was 50 cm which is a typical range encountered in many cases reported by the Sao Paulo Scientific Police.

Pieces of flannel, microfibre, canvas, a commercial polyester and tricoline (a type of cotton) were shot with a 0.38 inch calibre revolver containing lead round nose cartridges, a 0.40 inch pistol with full metal jacket cartridges, and a 9 mm pistol containing round nose ammunition.

The fabrics were analysed before and after firing. A small area about 2.5 cm2 was cut out from the virgin fabric or around the bullet entrance using decontaminated equipment and mixed with 10% nitric acid to recover the gunshot residues.

The extracts were analysed by sector field ICPMS, measuring the specific isotopes antimony-121, barium-138 and lead-208. The absolute quantities of each element were not considered relevant because various factors can contribute to their variation, such as the background distribution already present on the fabrics before shooting.

The researchers found it more illuminating to compare the relative amounts of each element with respect to each other. They plotted these values in two-dimensional ternary graphs with the distribution of each element forming one side of the triangular plots.

Firearms distinguished using elemental distributions

All of the clean fabrics analysed before shooting contained small but significant amounts of each element ranging up to 13.6 µg/cm2. These were regarded as the blank values and represented the normal environmental background.

The levels of the three elements after shooting varied. This was what the team expected because the distribution of gunshot residue depends on a number of factors including the shooting angle, which varies with the shooter, the condition of the gun and the type and condition of the fabric. After all, this is what occurs in real-life situations.

Even with the same weapon and the same type of fabric, the amounts varied widely. For instance, the amounts of antimony, barium and lead on flannel fired at with the 0.38 revolver varied from 4.3-99, 9-109 and 127-3191 µg/cm2, respectively.

Despite this wide variation, the ternary graphs revealed some distinctive patterns. These allowed the blank samples to be clearly distinguished from the targets but there was a second distinguishing feature. The pattern for the revolver was much different to those of the pistols, having a tighter, less dispersed distribution. The pistol patterns were quite similar.

The researchers postulated that the differences might be related to the shot energy of each weapon but there are many other factors at play, as outlined earlier.

In criminal cases, the method could be used to compare the elemental distribution from around the bullet hole with those from other areas of the clothing as a preliminary screen. The distribution patterns were also very similar to those reported for gunshot residues on the hands of the shooters, so the method could help to link weapons with suspects.

The sector field ICPMS technique is recommended as a preliminary screening method that is capable of high throughput. However, it is limited by the high initial cost of the instrumentation, requiring some initial capital investment.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Gunshot residues on different types of fabrics have been analysed by high resolution mass spectrometry and the elemental data displayed on ternary graphs to reveal the type of weapon that was fired. The method could provide vital clues in forensic investigations where the firearm has not been recovered

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