Metal deposition techniques for the detection and enhancement of latent fingerprints on semi-porous surfaces

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Fingerprints can provide critical evidence in the investigation of crime. Most fingerprints are latent, or invisible, and hence need detection and enhancement before they can be used in the investigation of crime. While techniques exist for fingerprint development on a range of surfaces, the detection of prints on semi-porous substrates is particularly difficult. Vacuum metal deposition (VMO) and multi-metal deposition (MMO) were investigated for their ability to detect latent prints on such surfaces. VMD is recognised as a very sensitive method of latent print visualisation but inconsistent resu1ts are commonly encountered. This research shows that reproducible VMD development is only obtainable when the amount of gold deposited is carefully controlled. The type and quality of VMD development obtained is influenced by a number of critical factors. These factors include the amount of gold deposited, the polymer type of the surface being treated, the nature of the latent print itself, and the presence of any other contaminants on the surface (whether due to the history of the exhibit or treatment with other fingerprint reagents). MMD has not found routine application due to its complexity and mediocre results. Recent research that sought to overcome these problems resulted in the development of a new formulation known as MMDII. Several MMD formulations were trialled in this study and MMDII proved to be the superior formulation, giving better overall print detail. On non-porous surfaces, MMDII may offer further print development than that achieved with cyanoacrylate fuming (CAF) and luminescent staining, but VMD always gave superior results to MMD. Polymer banknotes are a difficult semi-porous surface, causing particular problems for fingerprint laboratories. A procedure utilising CAF followed by VMD and finally luminescent staining enables successful print development on this substrate. Success on new notes was achieved with prints up to 18 months old; unfortunately, the possibility of successful print development decreases on used banknotes. VMD has two important advantages for print visualisation on polymer notes: i) it is sensitive enough to detect the small amounts of residue remaining; and, ii) the zinc layer removes a significant amount of interference caused by the banknote design. MMD and VMD were compared to standard techniques on other semi-porous surfaces. MMD proved to be the technique of choice on these surfaces. The ability of MMD to react with print residue within and on the surface is believed to be important to its success.
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