Elemental determination of bullets using atomic absorption spectroscopy

Firearm identification is an important aspect of crime scene investigation due to the frequency of weapons used in various forms of crime. Issues can arise during firearm analysis because of the subjective and non-scientific basis of the expert’s opinion to justify the identification or elimination of certain firearm involvement. When a bullet or casing is found at a scene, without access to the suspect’s weapon the only mode of analysis is via microscopic examination. Implementation of atomic absorption spectrometry (AAS) analysis of the projectiles found at the scene can identify specific ratios of elements found in the exterior alloy, which has the potential to be linked to the manufacturer when these ratios are different compared to other brands. The U.S. Integrated Ballistics Identification System (IBIS) database exists for comparison of identification markers present on projectiles by observation only to determine if a bullet was fired from the same gun. This database only provides the firearm and toolmark examiner with pictures for split image microscope analysis, which fails to provide chemical characterization to enhance evidence validity. A database that contains results from bullets analyzed via AAS can provide examiners with scientific data that their testimonies currently fail to demonstrate. Circumstances when projectiles or firearm is not present at the scene leads to limitations of the investigation due to undeveloped methodology to determine the manufacturer in which the bullet belongs to. The main objective of this research was to evaluate the usefulness of atomic absorption spectroscopy (AAS) to quantify elemental composition from striations on surfaces that the bullet encountered. The metal deposits contained within these transfer marks can be used to make associations between bullets or lead fragments and samples of bullets related to a suspect(s). For this project, AAS instrumentation can analyze the elemental composition and generate ratios specific to brands. Results from this study examined 10 brands of 0.22 caliber ammunition and showed lead and antimony ratios of most manufacturers were distinguishable. However, it was found that few brands were similar in ratios that were indistinguishable between one another due to high variation of lead values that is possibly dependent on grain size and lead jacket. It was also discovered that lead concentrations were at least 10 times greater than antimony concentrations in all manufacturers. RWS (40 GR), BPR (37 GR), Armscor Precision (36 GR), Aguila Original (40 GR), CCI Stinger (32 GR), Aguila Super Caliber (20 GR), Aguila Rifle Match (40 GR), Federal Premium (40 GR), La Norma USA (40 GR), and HVHP Sellier & Bellot (36 GR) were among the brands tested. Averages of 25 samples per box were calculated from 5 bullets and 5 sample repeats that determined lead concentration differences greater than 100 ppm in comparison to some manufacturers. RWS (40 GR) average lead concentration is 188 ppm whereas Aguila Rifle Match (40 GR) average lead concentration is 290 ppm. Greater than 100 ppm variance is also highlighted within HVHP Sellier & Bellot (36 GR) with an average lead concentration of 99 ppm. Antimony concentrations varied between each manufacturer, however differences did not exceed 22 ppm as this element was detected at lower concentrations ranging from 5-27 ppm across all brands. The greatest variation between boxes tested was Federal Premium (40 GR) with an average antimony concentration of 5 ppm and BPR (37 GR) with an average antimony concentration of 27 ppm. Forensic casework can greatly benefit from AAS analysis based on this data as it shows distinguishability can be achieved from only 10 brands tested. This has the potential to be useful in criminal investigations when corroborating microscopy is applicable or when metal deposits are discovered on scene in the absence of ballistics evidence. Determination of brands used from a fired weapon can assist in identification of a suspect in various scenarios such as if that individual is in possession of the particular ammunition manufacturer and if corroborating microscopy is applicable to the investigation in the presence of ballistic evidence. In the absence of all ballistic evidence, data shows it could be possible to distinguish the brand and type of bullet from contact with surfaces. Currently, methods have not been developed for bullet identification in this circumstance.

Embargo status: Restricted until 09/2023. To request the author grant access, click on the PDF link to the left.