We’ve all seen the classical TV crime drama clip where the over-worked genius detective throws a couple of bullets under the comparison microscope, lines up a set of striations and declares that the two bullets were fired from the same gun or maybe they came from the same box of bullets. Whilst this may be the crux that solves the case in fiction, and very occasionally in reality, linking bullets is typically not so simple. A more accurate method of connecting objects such as projectiles is to study them at an elemental level or, in the case of this research, at an isotopic level.

Elements exist as a number of different stable isotopes (atoms of the same element differing in the number of neutrons present in the nucleus). Lead, a common component in bullets, exists as four isotopes in nature; ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb. When lead occurs naturally in ore (a type of rock containing minerals and metals), different sources of lead will vary in their isotopic compositions. Further dissimilarity arises through recycling of lead products, meaning that lead from numerous sources may be mixed together into a new product. This variation can be utilised to distinguish between lead bullets from different batches or conversely establish that two bullets are likely to have originated from the same source.

The research we’re talking about here, led by a team at the University of Oslo in Norway, used an analytical technique called MC-ICP-MS to analyse the lead isotopic compositions of a range of bullets, cartridge cases and firearm discharge resides.

What’s MC-ICP-MS, I hear you ask?

MC-ICP-MS stands for multiple-collector inductively coupled plasma mass spectrometry. Put simply, a conventional ICP-MS involves the introduction of the sample as a fine aerosol, using an inductively coupled plasma source to ionise the sample, after which the newly ionised components are separated based on their different mass-to-charge ratios. The ions impact with a dynode of an electron multiplier, resulting in the release of an electron for each ion strike. This can then be amplified until an intensity significant enough for measurement is achieved. The signal is ultimately proportional to the ion concentration, therefore allowing for the amount of a substance present to be determined. Multiple detectors (such as MC-ICP-MS) use multiple detectors to simultaneously measure separated isotopes.

ICP-MS Schematic (http://www.spectro.com)

Okay, that concludes our technical talk! But now just what did this research find, and why is it useful?

After extracting lead from a wide range of bullet samples using nitric acid and subjecting the specimens to MC-ICP-MS, researchers could examine the distribution of isotopic ratios in bullets across a variety of manufacturers. Not only did it seem possible to distinguish between bullets from different manufacturers based on lead isotopic composition, but also between boxes of bullets from the same manufacturer produced at different times. In many instances fired bullets will become disfigured upon impact, making microscopic examination difficult if not impossible. But by studying the bullet at an isotopic level and even determining a kind of isotopic fingerprint, analysts may be able to distinguish between bullets produced in different regions of the world, by different manufacturers, and even between individual batches from the same company. The ability to do this could prove invaluable to forensic investigators.

Though naturally there was a certain amount of uncertainty associated with the work, the use of isotope ratios in the study of bullets proves promising. The idea of utilising isotopic ratios to distinguish between bullets is not a new concept, with researchers investigating the theory as early as 1975.  But as analytical techniques progress and improve, forensic scientists are able to obtain much more from their evidence, bettering the criminal justice system one isotope at a time.

References

Sjastad, K-E. et al. Lead isotope ratios for bullets, a descriptive approach for investigative purposes and a new method for sampling of bullet lead. Forensic Sci. Int, 244 (2014), pp. 7-15.

Perkin Elmer. The 30-Minute Guide to ICP-MS. [Online][Accessed 20 November 2014] Available from: http://www.perkinelmer.co.uk/PDFs/Downloads/tch_icpmsthirtyminuteguide.pdf