Characterisation of gunshot residue particles using self-consistent ion beam analysis

Abstract

Individual particles of gunshot residue were studied with particle-induced X-ray emission and backscattering spectrometry using a 2.5 MeV H⁺ beam focussed to ∼4 μm and self-consistent fitting of the data. The geometry of these spherical particles was considered in order to accurately fit the corresponding particle spectrum and therefore to quantify the trace element composition of these particles. The demonstrable self-consistency of this method allows the compositions of most residue particles to be determined unambiguously and with a higher sensitivity to trace elements than conventional methods.

Introduction

Identification of gunshot residues found on suspects has challenged forensic scientists for many years [1]. Energy dispersive spectroscopy (EDS) on the scanning electron microscope (SEM) is used routinely by police authorities to identify gunshot residue, and it has been shown that SEM-EDS can be used to distinguish between some residues from different types of ammunition powders [2]. We have shown [11] that particle-induced X-ray emission (PIXE) with a microbeam is a more powerful technique to characterise such materials because of the much higher sensitivity to trace elements.

PIXE can be quantified absolutely without the use of standards, provided that the elemental composition of the material on the exit path of the X-rays is known, which is the case if the backscattered particle signal is collected and correctly analysed. However, until recently it has not been possible to analyse the photon and particle spectra self-consistently, that is, in an integrated code that uses the elemental specificity of the PIXE to overcome the poor mass resolution of the particle spectra, together with the compositional depth profile sensitivity of the particle spectra to correctly calculate the X-ray absorption on the exit path. In fact, the main PIXE codes cannot correctly calculate absorption where the same element is in multiple layers.

We use a new implementation of the DATTPIXE code [3] which has been incorporated into an open source C module LibCPIXE and integrated [4] into DataFurnace [5]; the latter is a fitting code using simulated annealing as a global minimisation algorithm as well as the local minimisation algorithms commonly used. DataFurnace has powerful facilities for handling multiple spectra simultaneously. The new PIXE/backscattering integrated code has been used to identify the corrosion product on a historic heliograph from 1827 [6], to quantify the pigment used on a 19th century reproduction of Frans Hals’ La Bohémienne (1630) [7], and to determine the O content of nitric acid treated carbon nanotubes [8]. It has also been used to determine the position and composition of micro-inclusions in unexpected carbonaceous inclusions in impact glasses from the meteoritic impact crater near Mt. Darwin, Tasmania [9].

When a firearm is discharged, microscopic particles of gunshot residue (GSR) are deposited on the hands, clothing, skin and hair of the shooter. These GSR particles are condensation products of the high pressure, high temperature reactions that take place when a gun is fired, and are made up of material from various components of the gun including the primer, cartridge casing and the projectile. The particles range in size from a few nm to tens of microns in diameter and usually contain Pb, Ba and Sb.

Gunshot residues are frequently used in police casework to prove that a suspect has fired a gun, because the residues can remain on the clothing of the shooter for several days after the incident. However, suspects are often arrested by police officers who themselves may be contaminated with gunshot residue particles either from previous case work or from their own firearms. This leads to uncertainty over the origin of gunshot residue found on a suspect and a risk of assuming a false positive. This problem has come into the spotlight in the UK in the recent high profile case of Barry George, who was convicted of the murder of the television presenter Jill Dando, in which one particle of gunshot residue was found on his clothing. Defence lawyers have appealed by arguing that the gunshot residue was most likely to be contamination either from the arresting officers or from the analysis procedure. This has highlighted the need to be able to discriminate one type of gunshot residue over another.

Police authorities routinely use SEM-EDS to characterise gunshot residues. Forensic service providers in the UK have developed a classification scheme for gunshot residues, putting them into five broad categories in accordance with their elemental composition [10]. It follows that if gunshot residue found on a suspect falls in to the same category as gunshot residue generated by a firearms officer, the evidence is inconclusive. Therefore there is a clear need to further subclassify gunshot residues.

We have recently demonstrated [11] that microbeam PIXE is capable of detecting trace elements in gunshot residues that cannot be detected by SEM-EDS. We have shown that some residues, which fall into the same broad category of GSR in SEM-EDS terms, can be further sub-categorized using PIXE. It was concluded that quantitative treatment of the PIXE data would strengthen the discriminating power of the technique. A distinct advantage of PIXE over SEM-EDS is of course the ability to determine the major, minor and trace element concentrations in absolute terms.

Additionally, to properly demonstrate the validity of the technique it will be necessary to evaluate the uncertainties involved. This will follow only from a quantitative treatment of the PIXE data. In this work, we will show how such a treatment can be effected if the backscattering and PIXE spectra are analysed self-consistently.