Blood or not blood—That is the question. A non-destructive method for the detection of blood-contaminated fingermarks

Abstract

Working in crime scenes presents a challenge to the forensic scientist, as some surfaces, such as floors and walls, cannot be transferred to the lab for further development and must, therefore, be processed at the crime scene itself. Two main types of latent fingermarks may be encountered in crime scenes: amino acids based and blood contaminated. One of the most common reagents, which are able to develop both types of fingermarks on porous surfaces, is ninhydrin. As blood contaminated fingermarks may be crucial in connecting the suspect to the crime it is important to be able to distinguish between them and natural fingermarks. More than a decade of experience in crime scene investigations led to the understanding that there is a clear visual distinction between natural and blood contaminated fingermarks that are developed by ninhydrin. This study attempted to translate the visual difference into a mobile, non-destructive spectrophotometric method that can be used in crime scenes. Three independent spectrophotometric approaches were examined. The first showed a clear difference between the UV–vis spectra of the solution of blood and ninhydrin versus that of Ruhemann’s purple. The second introduced another method in the solid phase to better simulate a real exhibit found in crime scenes. Once establishing the scientific foundation for the visible difference, a third technique for colour measurements was used in order to provide a potentially fast, quantitative, accurate and non-destructive field test for blood determination at the crime scene.