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Applications of laser-excited fluorescence can be found in numerous areas of criminalistics, including document examination, fiber analysis, serology and latent fingerprint development. This paper focuses on the latter application. The present mature procedures for laser fingerprint detection are described to set the stage for discussion of current research, which primarily deals with time-resolved imaging to take advantage of the difference between the lifetimes of substrate and fingerprint fluorescences.
The principal procedure for laser detection of latent prints on smooth surfaces involves first the fuming with cyanoacrylate ester, which results in the formation of a polymer on the fingerprint residue. Thus stabilized latent prints can then be solution stained with fluorescent dyes. Porous items are treated with ninhydrin or its analogs, which react with amino acids of the fingerprint residue. Subsequent reaction with group II B metal salts yields intensely fluorescent products.
Time-resolved imaging involves modulated laser excitation and gated imaging with a microchannel plate image intensifier. The critical requirement for such imaging is the identification of reagents or stains that yield lifetimes which are longer than those of the substrates (typically 1 ns or less) and also at least comparable to the laser cutoff and image intensifier turn-on time (about 5 ns). Two general approaches are presented. One involves compounds that yield long-lived d-π* intramolecular charge transfer states. The other involves chemical reactions with fingerprint residue that lead to formation of organo-rare earth complexes which display long-lived rare earth emission. The structural and photophysical features which underlie the luminescence properties of fingerprints treated with ninhydrin or its analogs and subsequently with metal salts are discussed with the aim of providing guidelines for the design of optimized fingerprint detection reagents.
lasers, fluorescence, criminalistics, fingerprints, ninhydrin, benzo(, f, )ninhydrin, 5-methoxyninhydrin, zinc chloride, europium chloride hexahydrate, tris(2,2′-bipyr-idyl)ruthenium(II) chloride hexahydrate, time-resolved imaging, microchannel plate image intensifier, luminescence spectroscopy
Professor, Texas Tech University, Lubbock, TX