Description of GSR
In addition to gunshot residue (GSR), discharge residues from firearms are also termed ‘firearm discharge residues’ and cartridge discharge residues’. They are complex heterogeneous mixtures.
There are two types of GSR, being organic and inorganic. The focus of this post is on inorganic GSR, therefore organic GSR (OGSR) is mentioned only to avoid confusion in the mind of the reader that there is but one type of GSR.
OGSR is burnt and un-burnt particles ejected from a firearm. It originates from the powder consisting mostly of propellants such as nitro-glycerine and nitrocellulose as well as stabilisers such as diphenylamine and methyl centralite. These organic analytes are vaporised during the firing process and re-condense.
Elemental composition of GSR
Inorganic GSR (GSR) emanating from ‘traditional’ ammunition contains a number of elements -
and may also contain -
The introduction of ‘clean’, ‘lead-free’, ‘green’, or ‘non-toxic’ ammunition over the last 30 years means that not all elements may be present.
Production of GSR
Ammunition cartridges consist of -
Essentially GSR is material that is not burnt or only partially burnt during the firing of a cartridge in a firearm. The firing of a cartridge in a firearm generates hot gases and high pressures causing GSR to escape from openings in the firearm (for example the barrel end, ejection ports, space between the barrel and cartridge holder in revolvers.
GSR may originate from the primer, propellant, the cartridge casing, or bullet. Most frequently, analysis focuses on the remains from the cartridge’s primer.
In order to answer the investigator’s question as to who fired a firearm, or was in the vicinity, when a firearm was discharged, something must be known about the persistence and transferability of GSR; that is, how long after a shooting is GSR detectable on different substrates, and the possibility of GSR being transferred from one substrate to another and how easily that might occur.
GSR may be present on a person or thing for a number of reasons either by primary transfer or secondary transfer. Primary transfer occurs at the time when the firearm is discharged: It may be transferred to the shooter and to persons and objects close-by. Secondary transfer occurs after primary transfer when GSR is relocated to a different person or item.
Expelled GSR is likely to be locatable on the face, hair, hands, and clothing of the shooter as well as other persons within the vicinity of the weapon when it was fired (primary transfer). The hands of the shooter are where the greater amount of GSR is likely to be found. However, time after the shooting event is a significant factor as to whether or not GSR is able to be located on a shooter because its persistence is affected by activity such as wiping of hands, placing hands in pockets, and other activity. Because GSR particles are shed quite quickly samples should be collected from skin surfaces within three hours; because GSR located as little as four to six hours after shooting is unlikely to a give meaningful result. GSR embedded in clothing remains for longer periods, even after it is no longer detectable on a shooter’s hands.
GSR by its nature is transferable between people and articles (secondary transfer).
Since 1933 several collection and analysis methods relating to GSR have been employed. This post focuses on analysis using scanning electron microscopy coupled with X-ray spectroscopy (SEM/EDX) which is the contemporary method; therefore only the manner of collecting GSR to allow for that means of analysis is presented. The most popular method of collection is lifting micro-traces from a substrate upon which GSR has been deposited by multiple pressings of adhesive material until the tackiness of the adhesive has gone. The adhesive material is attached to the head (smooth and flat) surface of a SEM stub as that does not require carbon coating, one that is polished smooth and flat, and does not contain elements of high atomic number.
Both skin and clothing are suitable for the stub method of collection. Specifically, in relation to the hands of the shooter one stub is pressed along the thumb and forefinger and a second along the palm of each hand (resulting in four stubs).
Sticky tape, specialised Micro vacuum cleaners, vacuum lifting onto a filter disc for clothing, plastic shafted, alcohol moistened cotton swabs are other means of collection.
Collection by tape and aluminium stub for use in a SEM is superior to that of liquid adhesives and swabs.
During the analytical phase it is necessary to be able to visually distinguish unburnt gunpowder particles from GSR in order to avoid confusion and slewing of results.
There are a number of ways to analyse GSR; these include microscopy, chemistry, and spectroscopy. This post focuses on the SEM/EDX method.
GSR originating from the cartridge’s primer is analysed using SEM/EDX; this is the preferred and most common method having been employed in criminal matters since the 1980s. This method enables morphology examination at the same time as identification of chemical composition; importantly no damage is caused to the sample which does not require complicated preparation.
Modern SEM-EDX equipment is able to detect submicron particles – as low as 1 µm diameter. Time taken to perform analysis has been greatly reduced by the replacement of manual searching and analysis with an automated computer system and the use of silicon drift detectors (SDD) to the order of four to ten times faster.
In order to be assured that SEM/EDX analysis has is being performed properly the performance of individual machines must be measured by testing artificial GSR thus fulfilling the requirements of ISO 5725 and 13528.
A limitation on the use of SEM-EDX arises when lead-free or nontoxic ammunition is encountered; false negatives are likely to arise. To overcome this problem the components of GSR can be analysed using a modified Griess Test, gas chromatography coupled with mass spectrometry (GC/MS), and gas chromatography coupled with thermal energy analysis (GC/TEA) providing chemical identification; capillary electrophoresis has also been employed; as has high performance liquid chromatography (HPLC).
GSR can be forensically useful in a number of ways, for instance differentiation of entry and exit wounds, estimation of the shooting distance, establishing the kind of ammunition used, tracing the trajectory of a projectile, and relating an individual to a shooting incident.
Environmental GSR and GSR-like material
Before the degree of usefulness of detected and analysed GSR can be gauged it is necessary to have information about the presence of GSR and ‘environmental GSR’ in the relevant place or places.
In one study samples of air were collected before a firework display was conducted, it was found that neither GSR nor GSR-like particles were detected. Following the conduct of a firework display a number of particles (8.5% of the total collected) were classified as GSR-like having regard to their composition, shape, and morphology.
GSR-like particles, by chemical composition, have been found in pyrotechnic devices, brake pads from vehicles and clutch parts, yet these are non-spherical in shape and therefore may be distinguishable from GSR. Remains from airbag igniters are also distinguishable from GSR due to the presence of numerous different particles. However some GSR-like particles, although non-spherical in shape, are said to be “indistinguishable from some irregular particles originating from ascertained GSR”.
Whilst the appearance of a particle may be GSR-like, cross-sectioning using focused ion beam milling (FIB) and analysis by electron beam allows the inner morphology and elemental profile to be ascertained and can give some indication of how the particle was formed; that might allow for differentiation between GSR and GSR-like material.
© Hadyn R Green, January 2014
After this was posted I received the following query -
‘I would like to ask you a question: is there any chance that I fire a 22 caliber gun (pistol or revolver, don´t know if there is any difference!), not wash my hands or do anything at all and the GSR test outcome is negative? Is there any chance that I could get a false negative just for the caliper of the gun? Kind regards.’
And my response was -
|Yes a false negative is possible in the situation you describe but it does not have anything to do with the calibre of the gun.
A false negative can arise for a number of reasons, the most common of which I have listed below.
1. There was never anything that could be collected, e.g. despite the GSR issuing from the weapon it did not attach to the subject.
In review. It can be said that, ‘Absence of evidence is not evidence of absence.’ Meaning that just because GSR is neither found or apparent that cannot lead to the conclusion that the suspected shooter was not the shooter.
Hope this helps.