When a gun is fired, small particles are generated during the explosion which may be deposited on objects in the immediate vicinity of the weapon (e.g. body, clothing, etc.) These small particles (typically only 0.5 -10 microns, or 0.005 to 0.01mm) are called gunshot residue (GSR). The particles are formed from the bullet and/or the primer. Bullet particles are primarily composed of lead (Pb); primer residue of Sb (antimony) and Ba (barium) +/- Pb.
An electron microscope is capable of imaging such small grains and, if fitted with appropriate xray detectors, can also determine their chemistry thus making it a useful tool in identifying potential GSR.
Samples are collected using a double sided tape which is dabbed around the clothing and exposed skin of the “suspect” (eg hands and sleeves of clothing). This sample is then given a fine coat of carbon to make it electrically conductive (necessary for electron microscope work).
Once inside the high vacuum chamber of the electron microscope the sample can be viewed in two main ways. One type of image (secondary electron) shows the surface relief or morphology (shape) of the grains, the second (backscatter electron) shows the difference in chemistry (with atomically heavier elements appearing brighter). Backscatter electron images are particularly useful in looking for evidence of GSR because the characteristic elements of interest (Pb, Sb and Ba) are all relatively atomically heavy compared with the other particles the sampling tape may pick up (dust, dirt, fibres, flakes of skin etc). Therefore, when choosing what to particles to analyse for possible GSR, one can target (either manually or using automated software) the grains that appear bright in the backscattered electron images.
The following examples were collected using a Jeol JXA8200 electron microprobe at the Advanced Analytical Centre, JCU. The chemistry of the identified particles were determined using energy dispersive xray spectrometry.
Secondary electron image of bullet fragment (left) and energy dispersive spectrometry (EDS) showing xray peaks for lead (Pb) and antimony (Sb).
Secondary electron image of bullet fragment (left) and energy dispersive spectrometry (EDS) showing xray peaks for barium (Ba),lead (Pb) and antimony (Sb).
It should be noted that these samples were viewed and analysed manually for illustrative purposes only. In order for this to be accepted in a court of law as evidence there are strict guidelines that need to be followed in terms of exactly how and by whom samples are taken and a thorough chain of evidence has to be established from sampling through to analysis by an accepted laboratory. Typically this type of work is undertaken using automated, unattended software controls to eliminate human error and or bias.