Forensic fingerprinting: The smoking gun

Smokin'

UK scientists have used capillary-scale ion chromatography and suppressed conductivity detection to analyse gunshot residues, sweat and latent fingerprints. The statistical techniques used to extract the data can reveal whether fingerprints were left by a smoker, based on thiocyanate levels, but more critically can reveal traces of gunshot residue in the fingerprint even where analysis of metallic content is not possible.

According to Elizabeth Gilchrist, Norman Smith and Leon Barron of the Department of Forensic Science & Drug Monitoring at King's College London, liquid chromatography instrumentation has tended towards miniaturisation. This means smaller samples can be handled and portable devices used, two frequent demands of forensic science where trace residues are common at crime scenes. By contrast, capillary-scale ion chromatography (IC) has been confined mainly to environmental analysis and bioanalysis.

The researchers point out that there are problems with current methods for gunshot residue classification, so those developing novel forensic investigative techniques are continually seeking new methods. They add that current methods, such as scanning electron microscopy (SEM), involve detecting metallic particles and are not sensitive enough to distinguish gunshot residues from vehicle brake-lining particles, for instance. As such, a focus on other components of gunshot residues, such as low molecular weight organic and inorganic ions might be a more productive.

Picking a packed particle polymer

The team tested particle-packed and polymer monolith capillary ion exchange resins to see which would be most efficient chromatographically and in terms of operating back-pressures and thermal selectivities. Van Deemter curves plotted for each approach showed the difference in efficiency. Van't Hoff data showed the effect of temperature on anion retention on monolithic and particle packed capillary ion exchange resins.

The researchers used a multistep hydroxide gradient and were able to separate numerous inorganic and organic anions in less than 23 minutes using an injection volume of a mere 0.4 microlitres. The samples having been obtained from a controlled firing experiment to generate gunshot residues and thus contaminated fingerprints from the person firing the gun. They found that the transfer of low molecular weight ions to fingerprints is minimal, which they suggest means that caution is needed when looking at markers.

Nevertheless, the KCL team demonstrated that the technique was sufficiently sensitive to reveal differences between the sweat of smokers and non-smokers. Non-smokers have elevated levels of thiocyanate and benzoate in their sweat, which the technique could easily detect. The team could also reveal when the donor of latent fingerprints had been in direct contact with a black powder substitute (Pyrodex) to show raised levels of benzoate, nitrate and perchlorate when compared to "control" fingerprints. Pyrodex is often used as a propellant in muzzle loading rifles, the team points out. Importantly, the technique could detect low molecular weight anions within trace forensic evidence at the femtogram to pictogram level.

Mass sensitivity

"The enhanced mass-sensitivity of this technique facilitated a conclusion that transfer of gunshot residue into fingerprint deposits is minimal after discharge of a firearm," the team says. Most forensic laboratories focus on metallic microparticles, this technique offers an entirely different method of linking a gun crime scene to the perpetrator.