A new “tailor-made” protocol for operational latent fingermarks’ development on unfired ammunition

Fingermarks’ (FM) development on unfired ammunition, found at shooting crime-scenes, provide key evidence in connecting a suspect to the crime. However, the standard FM enhancement technique for metal surfaces, ethyl cyanoacrylate (ECA) fuming, is often ineffective for this type of evidence, due to its high sensitivity to presence of residual water and salts from soil, leading to a non-selective polymerization on the entire surface. In this work, we have developed a new protocol for unfired brass ammunition suitable for standard operational conditions, and based on the “reverse-development” principle, to achieve higher sensitivity by targeting the surface, rather than the FM residues. This involves a pre-treatment stage using octyl cyanoacrylate (OCA) fuming, which provides a superhydrophobic protective layer to the FM ridges, followed by an electroless deposition reaction between a palladium complex (Na₂PdCl₄) and the brass surface. This long-chain cyanoacrylate derivative demonstrated excellent selectivity towards sebaceous residues in fingerprints, as its less reactive nature results in a polymerization that is unaffected by water, even after long exposure to moist soil (6 days, 80 % RH) prior to development. Moreover, the proposed process allowed the use of a concentrated Na₂PdCl₄ solution, without the fear of over-development when immersing large quantities of ammunition simultaneously, thus affording a successful development of 92 % of a total of 320 FMs from 20 donors. Due to its high sensitivity, robustness, and suitability for large quantities, this simple-to-use protocol has shown a high potential for the future implementation in operational use by latent fingermarks laboratories and CSI field units.