E. Roland Menzel
Center for Forensic SMies and Dapartment of physics Texas Tech University Lubbock, TX 79409 Maintaining a tolerable level of public safety means relying heavily on crime solving, where the examination of articles of physical evidence plays a key role. The detection of latent fingerprints is especially important, because fingerprint evidence historically has been the strongest physical evidence that can be introduced in court. DNA fingerprinting is a new technique of equal probative value (when biological evidence is on hand). However, such procedures are complex and currently are performed by only a few laboratories. In fact, protocols for routine use of DNA analysis for law enforcement have yet to be developed. In the past, fingerprint files were composed of inked impressions on 10finger cards. Because manual searching of card files usually was not feasible, a suspect generally had to be on band for fingerprint evidence to he of any use. Since the introduction of Automated Fingerprint Identification Systems in the mid-l970a, single fingerprints of !mown origin can be stored in digital form. An unknown fingerprint is entered into the computer and compared with prints on file. The computer delivers ranked matches, and the respectivt file prints are then compared with t h i unknown print by an examiner of latent prints. Usually, the unknown print correspondsto one of the top two computer-delivered matches. Latent fingerprint development has become more valuable than ever because cold searching (searching in the absence of a suspect) is now possible. 000527001891096 1-557AI$01.5010 @ 1989I American Chemical Society
TladRiaral methods
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When a finger is pressed against a surface, 0.1 mg of material is transferred to the surface, forming a latent fingerprint. Of this, 9&9% is water that soon evaporates to leave -1 fig of residual material. Approximately one-half of this is inorganic material (e.g., NaCI). The rest is a complex organic mixture (e.& amino acids, lipids, and vitamins). The two most widely used traditional methods of latent fiigerprint development are dusting, usually with black powder, for relatively fresh fingerprints (i.e., no older than a day or two) on smooth surfaces such as metals, some plastics, or glass; and ninhydrin development for both fresh and old fingerprints on porous surfaces (primarily paper, but also cardboard, wood, leather, or wallboard). Ninhydrin (I) reacts with the amino acids in fingerprint residue to form a purplish-blue product known as Ruhemann’s Purple (11). Spray cans of solutions of ninhydrin are commercially available and often are used by investigators, who spray
surfaces under examination at the scene of a crime. Occasionally, silver nitrate or iodine vapor is used for fimgerprint development, and prints on adhesive tapes are often develoDed bv usine mrstal violet. The visualition 6f la& fingerprints by all of the traditional methods involves observing the difference between the ambient light refleeted from the areas between the ridges and that surrounding the prints. For weakly developed prints where a small amount of powder adheres to the fingerprint residue or only a little Ruhemann’s Purple is formed, fingerprint visualization involves the detection of a small difference between two relatively large light signals, and this is an inherently insensitive detection mode. --byIn 1976 researchers began to investigate the use of fluorescence for the detection of latent prints because of the high sensitivity of this technique. Although fluorescent dusting powders had been used for some time in special situations, the use of fluorescence de-
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A NALYrICA L APPROACH teetion was not widespread. The advantage of detecting a small signal (fluorescence) versus an equally small difference between two large signals (ahsorption/reflectance) is clearly demonstrated by the eye: Stars are readilv observed at nieht but not in dayliiht. For nractical reasons. the fluorescence k a latent fingerprint, either from material inherent to the fmgerprint residue or resulting from a fmgerprint treatment, should be visible to the naked eye. Obtaining such intense fluorescence is difficult because the detection of fluorescence from constituents of the fmgerprint residue itself (e.g., riboflavin) involves nanograms or less of material. The use of a reagent that rea& with components of the fmgerprint residue (e.g., amino acids) to form a fluorescent product involves small amounts of material in the fmgerprint residue as well. The excitation of visible fmgerprint fluorescence requires an intense light source (of appropriate color); this requirement led quite naturally to the use of lasers. From the perspective of power, wlor, and ease of use, the argon ion laser was, in 1976, the most useful type of laser for fmgerprint work. It was used in the fluorescent detection of untreated fwerprints or those dusted with fluorescent powders, stained with fluorescent dyes, or treated with reagents that form fluorescent producta. Some law enforcement agencies now also use copper vapor lasers and frequency-doubled NdYAG lasers; however, the most widely used laser is the Ar laser. For fwerprint detection in a I
laboratory setting, Ar lasers of EL20 W (all lines blue-green) generally are used, whereas for crime scene work, portable Ar lasers are available. However, portability is achieved at substantial sacrifice in sensitivity (laser powers are -200 mW). In the last few years, fdtered lamp systems have been produced as well, but they do not provide the sensitivity of the large Ar or Cu vapor lasers. The fmgerprint fluorescence detection procedure is simple. The laser light p w i n g through an o p t i d fiher conveniently illuminates the article under scrutiny. The examination is conducted in a darkened room. Illuminated areas, typically -10 cm in diameter, are visually inspected through goggles equipped with filters that block the laser light reflected from the article but that transmit the fmgerprint fluorescence. Once a fluorescent print is observed, it is photographed through the same filter. Bandpass filters are sometimes employed for suppression of background fluorescence. Detection by inherent fingerprint fluorescence is possible only on surfaces (smooth or porous) that display little or no background fluorescence, because inherent fmgerprint fluorescence generally is weak. Because many surfaces show substantial background fluorescence, fingerprint treatment prior to the measurement of fluorescence is critical in most cases. By 1980 several treatments had been developed (I), and numerous law enforcement agencies had begun to use lasers routinely for fmgerprint work. At that time, fmgerprint treatments
Fkure 1. Detection sensitivity achieved with ninhvdrin/ZnCIJiaser examination.
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were not yet effective. The superglue fingerprint treatment was first used in the late 1970s to early 1980s (2).Articles were exposed to cyanoacrylate ester fumes (cyanoacrylate is the main ingredient in superglue). The cyanoacrylate ester polymerizes on fingerprint ridges to form a white product. Before this treatment was developed, staining of fingerprints on smooth surfaces with solutions of fluorescent dye was difficult because the solvent tended to wash away latent prints. The polymerization resulting from the superglue treatment, however, not only stabilizes latent prints but also provides preferential adherence of highly fluorescent dyes such as rhodamine 6G. It is not necessary for the polymerization to proceed to such an extent that a visible white print is obtained. The combination of superglue, rhodamine 6'2, and laser examination (3) has become an effective procedure for fingerprint detection on smooth surfaces. In most instances this staining procedure provides better results than dusting with fluorescent powders. Thus dusting is usually performed at crime scenes only on objects (such as walls) that cannot be transported to the laboratory. If a portable laser is on hand, dusting and staining can he performed at the crime scene. However, movable items are best examined in a laboratory setting. For example, we have examined items as large as refrigerators, car doors, water heater shells, and windows in the laboratory. Frequently, porous items treated with ninhydrin subsequently are sent to a laser-equipped facility for further
examination. Because Ruhemann'r Purple does not fluoresce, efforts were begun in the early 1980sto convert it t c a fluorescent product by a second, postninhydrin chemical reaction. The procedure developed (4) is simple and involves spraying the ninhydrin-treat. ed article with zinc chloride dissolved in a volatile solvent system to preveni "bleeding" of fingerprint detail. Usually, a 1 5 mixture of methanol:1,1,2trichlorotrifluorethane is used. Zn2+ forms an intensely fluorescent coordination compound (111) with Ruhemann's Purple. Ambient humidity is necessary for the reaction to occur.
Activhion of
Carbon Dioxide
Figure 1shows an example of the detection sensitivity achieved with ninhydrin/ZnCIdaser examination. Fig. ure l a is a photograph taken in room light of a strong print developed by ninhydrin. Zinc chloride treatment and laser examination often are necessary in criminal casework, as illustrated by Figurea Ib and IC.Figure Ib is a photograph taken in room light of a weak print treated with ninhydrin, with no visible development; Figure l e shows this print after ZnClz treatment and laser examination. The fused-ring benzo analogue of ninhydrin (IV) and the 5-methoxy analogue (V) can he used in place of ninbydrin and yield intensely fluorescent zinc complexes. The 5-methoxy deriva-
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ANALYTICAL APPROACH tive of ninhydrin is particularly effective in this regard and may become the reagent of choice since it has recently become commercially available (2,Z-dihydroxy-5-methoxy-l.3-indanedione; Aldrich). The structural and photophysical features that lead to fluorescence of the zinc complexes are deacrihed elsewhere (5). lime-resolved imaging Laser fingerprint development has become a widely applicable, highly sensitive method. More than 100 law enforcement agencies in the United States now use it, as do agencies in other countries (including Canada, Great Britain, Israel, and the People’s Republic of China). Until very recently, a number of surfaces ubiquitous at crime scenes (cardboard, wood, leather, various plastics, surfaces painted with strongly fluorescent paint, and some adhesive tapes) remained intractable because of their excesaively intense background fluorescence. To permit background fluorescenc (short lifetime) suppression hy time resolved imaging, we are now using staining dyes and reagents that yield long luminescence lifetimes. Instead of continuous Ar laser illumination, the AI laser beam is chopped, either by a mechanical light chopper or an electrooptic modulator. The laser light then passes through an optical fiber and illuminates the article under examination. The fingerprint luminescence passes through a filter as before, but in this case is incident on a gatahle
image intensifier. The intensifier is gated to turn on during the “laser-off” portion of the chopping period (with a delay with respect to the laser cutoff) such that the background fluorescence has already decayed when the image intensifier turns on. The chopping frequency is adjusted so that the laser-off period is comparahle to the fingerprint luminescence lifetime. The image of the fingerprint is visible to the naked eye at the output phosphor screen of the image intensifier and can he photographed or video-recorded. The details of our time-resolved imaging system are described elsewhere (6). Although fingerprint work on stains and reagents that produce intense, long-lived emisaion is still in ita infancy, two potentially effective fingerprint treatments have already emerged. The first treatment involves staining fingerprints with tris(Z,Z’-hipyridyl)ruthenium(I1) chloride hexahydrate(V1) (7). This compound displays an intense
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d T * charge-transfer phosphorescence with a lifetime of -1 ps, which is sufficiently long compared with background fluorescence lifetimes (typically 0.1-1 ns). The ruthenium compound lends itself to staining of smooth surfaces and certain special surfaces such as adhesive tapes. It can also he incorporated into dusting powders. The second treatment is used for development of fingerprints on porous surfaces. Ninhydrin, its 5-methoxy analogue, or its heuzo derivative reacts with fingerprint residue. EuCl3. 6Hz0 is used instead of ZnClz, and the resulting Eu complexes display a ligand-toEu intramolecular energy transfer that results in long-lived Eu3+ luminescence. Spectroscopicdetails are reported elsewhere (8). Fingerprint treatment with the benzo analogue of ninhydrin and then EuC13. 6Hz0 is particularly effective. Figure 2 depicts a realistic situation often encountered in physical evidence examination. It gives an example of the background suppression that can be achieved and shows a weak latent print on white note paper developed with the henzo analogue of ninhydrin/ EuC13. 6Hz0. A stain of the laser dye, 3,3’-diethyloxadicarhocyanineiodide (DODC), was placed next to the print. Figure 2s shows a photograph of the print and stain in room light. A strong fingerprint would develop as a green mark on treatment with the benzo derivative of ninhydrin, and this mark would change color to violet after EuCls. 6Hz0 application. The fact
Figure 2. Detection sensitivity achieved with the benzo analogue of ninhydrin/EuCl3-6H2011aserexamination. (a) Ronm l i w photographof fingerprint On white paper developed by benzo(rlninhydrin/EM&. &I&. N e d 10 Um fingerprint is a W D C slain. (b) Fluaescenc? phologaph d san-m under CW UV Ar laser exckation. (4Photcgraphof gated intensilier plwsphw screen image of S a m .
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that nothing is seen in the photograph taken in room light attests to the weakness of the print. Near-UV AI laser excitation was used next to produce the orange-red Eu3+ luminescence of the fingerprint of Figure 2a. Under this excitation, DODC fluoresces intensely in the same spectral range. Figure 2h shows the luminescence obtained under the customary CW laser illumination. The noteworthy features of Figure 2b are the intense DODC fluorescence and the absence of any fingerprint detail because of excessive background fluorescence from the paper. Figure 2c shows the results obtained with time-resolved imaging. Note the complete suppression of the DODC fluorescence, and the suppression of the paper background as well, so that clear fingerprint detail emerges. Although the henzo analogue of ninhydrin/EuC13. 6H20 procedure yields a luminescence that is weak compared with that obtained by ninhydrin/ ZnClz, the sensitivity of gatable image intensifiers permits detection of latent prints on strongly fluorescent suhstrates. If fingerprints detected by time-resolved imaging are recorded by video cameras interfaced with comput-
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P
ers, computer image processing for improved sensitivity is possible. We are currently studying a range of rare earths (e&, Th3+) and ninhydrin analogues to improve luminescence intensities. We are also investigating a range of transition metal complexes that yield long-lived emissions. This article summarizes the resulb of B research program that has been supported by XeroxCorporation (197679). the National Science Foundation (1980-86). the US. Department of Justice National Institute of Justice (1987491, and several industrial companies (Spectra-Physics. ALM, Laser Photonics. Coherent).
Refer(1) Menzel, E. R. Fingerprint Detection with Lasers; Marcel Dekker: New Yolk, iwn
(2) Kendall, F. G.; Rehn.
B. W. J . Forensic Sei. 1983.28,777-80. (3) Menzel, E. R.; Burt, J. A,; Sinor, T. W.; Tubach-Ley, W. B.; Jordan, K. J. J . Forensic Sci. 1983.28.307-17, (4) Herod, D. W.; Menzel, E. R. J . Forensic Sei. 1982,27,513-18. ( 5 ) Menzel, E. R.;Bartach, R. A,; Hallman, J. L. J. Forensic Sci., in press. (6)Mitchell, K. E.; Menzel, E. R. Proc. SPIE, in press. (7) Menzel, E. R.Proe. SPIE 1988.910,4561
(Ei;Menze!, E. R.; Mitchell. K.E. J . ForenS I C Scr., in press.
E. Rolana Menzei is professor of physics and director of the Center for Forensic Studies at Texas Tech Uniuersity. He received his B.S. degree (1967J and his Ph.D. (1970) in physics from Washington State University. After completing postdoctoral fellowships at Simon Fraser University (British Columbia, Canada),Purdue Uniuersit y , and the University of Kentucky, and an industrial research position at the Xerox Research Centre of Canada, he joined Texas Tech University in 1979. His research interests include photoluminescence applications in criminalistics and the study of insulator damage by fluorescenceprobes.
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