Chemistry Everyday for Everyone edited by
Applications and Analogies
Ronald DeLorenzo Middle Georgia College Cochran, GA 31014
Solving the Mystery of Fading Fingerprints with London Dispersion Forces Doris R. Kimbrough Chemistry Department, Box 194, University of Colorado at Denver, P.O. Box 173364, Denver, CO 80217-3364 Ronald DeLorenzo Department of Chemistry, Middle Georgia College, 1100 Second St. SE, Cochran, GA 31014-1599
The use of fingerprints for identification purposes is one of the most widespread and best-understood tools of forensic science. Fingerprinting is such a common occurrence in detective novels, cinema, and television that we all know it is best to wear gloves when committing a crime. However, the fingerprints of the perpetrator are often not the only ones of importance in the collection of evidence. The fingerprints of the victim can be extremely important as well, and obtaining them when the victim is a child can be a tricky and frustrating endeavor. In 1993, an 8-year-old in Knoxville, Tennessee, was kidnapped and then luckily escaped. She was able to describe and later identify the car in which she was held captive. When the car was recovered four days later, police were unable to find any of her fingerprints anywhere in the car; it was as if she had never been there (1, 2). This puzzling fact significantly weakened the prosecution’s case against the accused kidnapper. How her fingerprints vanished in such a short time was a mystery that was solved by exploring the chemistry associated with fingerprints. Background The presence of distinct and recognizable patterns of ridges on the hands and feet is common to all primates. The evolutionary purpose of these ridges is thought to include the exudation of perspiration, the improvement of tactile facility, and the provision of a gripping surface (3). That these patterns are unique to each individual has been known in different cultures for centuries. In 2000 B.C.E., the Babylonians used fingerprints made in clay to accompany documents to prevent forgery (4). Handprints as signatures are mentioned in the Book of Job (3); they were used in ancient India to prevent duplication of the payment of wages (3). The Native Americans of North America recognized the individuality of fingerprints long before the arrival of Europeans (4 ). The utility of fingerprints as a forensic tool first became important in the late 1800s (3). A classification system was in place in Scotland Yard by 1901 (3), and fingerprinting was accepted as a definitive identification by court systems in most countries shortly thereafter (4 ). Today, fingerprints are used for routine identification purposes as well as in the collection of evidence against persons accused of criminal behavior.
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Composition of Latent Fingerprints Fingerprints can be visible (e.g., if left by a hand with a surface layer of ink, paint, or blood) or latent (i.e, invisible to the naked eye). A latent fingerprint must be detected and made visible using some chemical or physical means (1, 5). Whenever one touches a surface a thin residue of perspiration from the finger is left behind. This latent fingerprint consists primarily of water, but also contains oils, fatty acids, esters, salts, urea, and amino acids (1). Luckily, for the purposes of gathering evidence, one typically experiences increased perspiration when engaged in an act for which one fears detection (6 ). There are numerous methods used to detect fingerprints, most of which involve reagents that are sensitive to the amino acids that are present (5). Latent fingerprints do not last forever and will smear, fade, or wash away from nonporous surfaces over time. The fading is caused by simple evaporation of the substances composing the fingerprint and, not surprisingly, is accelerated by heat. It has recently been discovered that children’s fingerprints fade much more quickly than those of adults (1, 2). Children’s Fingerprints What happened to the fingerprints of the 8-year-old in Tennessee? This case initiated research into the behavior of children’s versus adults’ fingerprints, and simple forensic experimentation revealed that children’s fingerprints on glass and plastic disappear faster than the fingerprints of adults, particularly in a warm environment (2). Although the kidnapper was subsequently convicted of the crime, the unexpected behavior of children’s fingerprints prompted further research by the criminologist involved in the case (1, 2 ). Gas chromatograph–mass spectrometric (GC–MS) analysis of extractions of the perspiration from the fingers of adults and children yielded some fascinating differences. The oils in the finger extractions of adults contained much larger concentrations of long-chain alkyl esters than found in extractions from children (2 ); these esters have on the order of 32 carbons (e.g., C15H31CO2C16H33). Conversely, children’s finger extractions contained much higher amounts of smaller, more volatile free fatty acids that contain on the order of 13 carbons (e.g., C12H25CO2H). The primary intermolecular forces that govern the volatility of both classes of
Journal of Chemical Education • Vol. 75 No. 10 October 1998 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone
compounds are London dispersion forces, also called induced dipole–induced dipole interactions. These forces result from weak instantaneous dipole moments caused by temporary uneven distributions in electron density. Dispersion forces increase in strength with molar mass and with increased surface area. Since the molar masses of the fatty acids in a child’s fingerprints are much smaller than those of the fatty acid esters found in adult fingerprints, the children’s fingerprints are more volatile and can vanish in a matter of hours in a warm environment (1, 2). It is likely that the Knoxville 8-year-old’s fingerprints simply evaporated from the warm car. The oil composition in fingerprints is thought to change during puberty. The esters found in adult fingerprints are probably not actually secreted from the finger, but from oil glands in the face. The material is transferred to the finger whenever one touches one’s face. The changes in the production of facial oil glands during and after puberty is fairly well understood and does indeed account for the differences in the behavior between children’s and adults’ fingerprints. It has also been determined that children’s fingerprints contain a larger quantity of cholesterol, but there are currently no
detection methods that are sensitive to cholesterol (2). Current research is focused on developing a new test for children’s fingerprints to enable police to detect them after the substances that respond to conventional methods have evaporated. This study also revealed differences between male and female fingerprints, but those differences are more subtle and as yet not well characterized (2). There also exist potential clinical applications of this research, where skin excretions can be used for disease and metabolic assays as well as tests for drugs and their metabolites (2). Literature Cited 1. Noble, D. ChemMatters 1997, 15(February), 9. 2. Noble, D. Anal. Chem. 1995, 67(13), 435A. 3. Berry, J. In Advances in Fingerprint Technology; Lee, H. C.; Gaensslen, R. E. Eds.; Elsevier: New York, 1991. 4. Block, E. Fingerprinting: Magic Weapon Against Crime; McKay: New York, 1969. 5. Clark, S.; Quigley, M. N.; Tezak, J. J. Chem. Educ. 1993, 70, 593. 6. Moenssens, A. A. Fingerprint Techniques; Chilton: Philadelphia, 1971.
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