The forensic exploitation of fingermark chemistry: a review

There is evidence that the use of fingerprints for the identification of an individual, either for civil or criminal purposes, has been considered in some form for over 2000 years (Barnes, 2011). The comparison of a mark left by an individual at a crime scene with sets of reference prints taken under controlled conditions is a cornerstone of forensic investigation, first being proposed in 1880 (Faulds, 1880). Following the generation of classification systems for fingermark patterns (Galton, 1892) and filing systems enabling databases to be searched (Henry, 1901), fingerprint comparison and identification has been successfully employed for over 120 years. The fact that fingerprint patterns are ‘unique’ (to the best of scientific knowledge) and persistent throughout life makes them a powerful identification tool. Indeed, the term ‘fingerprint’ is widely used across many other branches of science to describe something that is characteristic and easily distinguished from (e.g.) spectra of other nominally similar substances. In criminal investigations the focus is on locating marks that may have been left by the suspect at the crime scene. These can be of three principal types; a positive mark where material is transferred from the fingertip to the surface, a negative mark where the fingertip removes material (e.g. dust) from the surface, or a ‘plastic’ mark where the fingertip leaves a permanent impression in a soft substance such as putty. The classification of a positive mark can be further divided into a ‘patent’ mark where the material transferred is readily visible to the eye (e.g. mud, ink), or a ‘latent’ mark where the material transferred cannot typically be readily seen by eye and needs further enhancement to be seen. The chemistry of the material transferred from the finger to the surface is important to the visualisation of the contact trace. This is because chemical substances present can be utilised by a range of processes that either convert a latent mark into one that is visible or assist in further enhancing the pre-existing detail in a patent mark. The chemical development of fingermarks was observed as early as the 1860s (Quinche & Margot, 2010) and was already being explored in a more focused way in the 1920s (Mitchell, 1920), with a range of chemical processes targeting different constituents being proposed for use. This review will focus on the material that is ultimately transferred from the fingertip to the surface to form a fingermark. It will consider how this highly complex chemistry can be utilised by a wide range of chemical reagents, and by advanced analytical techniques in combination with imaging capabilities, to reveal the fingermark ridge detail and additional information contained within it.