Universal IR Fluorescent Latent-Print Detection Method

A universal latent print detection method and associated tools supporting field use have been identified.  Visible fluorescent dye candidates were screened for potential use in latent print detection.   Dye-candidate screening factors included: (1) reactivity or attraction of the dye to constituents in latent print residue that produce a fluorescent complex, (2) reactivity/association with constituents contained in both eccrine and sebaceous residues, (3) low fluorescent background when treated (high ridge detail contrast), and (4) applicability to latent print detection on most all surfaces, both porous and nonporous.

Initially, a thorough literature review was conducted to produce a candidate list of fluorescent dyes with structural potential for association with print residue.  Specifically, the potential for dye reactivity with amino and fatty acids was desired in order to support the detection of both eccrine and sebaceous prints.  Next a spectral assessment of each candidate was conducted to determine optimum absorption and anticipated emission wavelengths in appropriate solvents.  Illuminators and filtered glasses were selected based upon the spectral excitation/emission parameters for each dye candidate in order to assess fingerprint development potential on various surfaces.  Finally, image capture and documentation was achieved for each dye candidate screened, using either a band-pass or long-pass filter fitted to a point and shoot camera.   This configuration allowed the fluorescent print image to be captured with high contrast in a manner that filtered out the illumination source as much as possible, thus optimizing the signal to noise ratio. 

Using the spectral information from the literature review, each dye candidate was evaluated for actual development performance on paper, soft plastic, and glass. Promising candidates were further investigated to identify optimum development conditions with respect to deposition surface. The selected dyes were then exposed to the test prints using a variety of portable medical nebulizers designed to produce a fine mist for asthma treatment.  Each candidate dye was applied to the prints via these nebulizers and tested on various print/surface combinations. Each candidate was then assessed for print/dye complex emission intensity, print integrity, and background/contrast.  When studying the dye development of latent prints, it was found that the dyes interact differently with latent print residue in a manner that is dependent upon the deposition surface; additionally certain dyes perform better to detect prints on some surfaces over others.  Overall the best fluorescent dye for use as a latent print developer on a large variety of surfaces was found to be 3,3’-Diethyloxacarbocyanine iodide (CAS 905-96-4, Sigma-Aldrich, Co.).  Optimum performance was obtained when the dye was initially dissolved in ethanol (1 mg/mL), and subsequently diluted with an equal volume of water.  Additionally, low cost commercially available tools to support a fluorescent dye based latent print development process and a detection kit has been identified and the associated performance detailed.  An overview of research findings on dye candidate performance, imaging specifications on a range of porous and nonporous surfaces, and an overview of the tools supporting field kit utility are documented within this final report.