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Fingerprints (This article is reprinted from Scientific Sleuthing, issue unknown. Thanks to Bill Corson, N.C.I.S. for the contribution.) Two continually nagging questions about latent fingerprint evidence concern their presence/absence and their permanence. Not infrequently, although an individual is known to have touched or handled an object, e.g. a firearm, a bank robbery note, etc., processing of the item yields either no usable prints or no prints associated with a suspect. Another issue highlighted numerous times in the pages of Scientific Sleuthing Review involves determination of the time of placement of a latent print subsequently developed at a crime scene. Despite a grab--bag of claims to the contrary, no reliable method exists to demonstrate that a print was recently deposited. The ability of the examiner to develop a latent print is related to both the development method used and the extent to which the component(s) relied upon for development was deposited on the surface. Another factor in the ability to develop a latent print on a piece of physical evidence is the nature of the surface itself. Similarly, studies conducted to determine the persistence of a latent impression exposed to conditions expected to rapidly degrade the print components strongly suggest that the major factor in persistence is the initial quality of the latent print itself. To a lesser, but significant, extent, the surface onto which the impression is placed affects persistence as also will environmental factors. Considerations affecting transfer of papillary secretions to a surface may include; adhesive forces, (inversely related to the temperature of the surface), the degree of roughness of the touched surface and electrostatic effects exerted on the perspiration drops by the surface itself. A variety of factors may affect retention of print components on non--porous surfaces ranging from glass, ceramics and metal to plastics of different types. Low density, porous materials including: paper, wood and raw ceramics have a micro capillary structure. Electrolytic dissociation affects the ability of components in an aqueous medium, which fresh print impressions are, to migrate horizontally along the surface and to greater depths in the material. Salts dissociate extensively and thus migrate rapidly whilst amino acids undergo little dissociation at near neutral pH and so tend to remain near the original point of deposition. Although little of the information presented should cause an experienced examiner to rocket out of his (or her) chair, a review of the mechanism of deposition and retention can be valuable in planning an approach to development of latent prints on a particular material or explaining why none were found. Bobey, “Fingerprints and Factors Affecting Their Condition,” 45 J.F.I.. 176 (1995).
This article was printed in “THE PRINT” |