DETECTION OF LATENT FINGERPRINTS BY LASER-EXCITED

A Novel Approach to Fourier Spectral Enhancement of Laser-Luminescent Fingerprint Images. Emre Kaymaz , Sunanda Mitra. Journal of Forensic Sciences ...
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ANALYTICAL APPROACH

DETECTION OF LATENT FINGERPRINTS BY LASER-EXCITED LUMINESCENCE Ε. Roland Menzel Center for Forensic Studies and Department 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 arti­ cles of physical evidence plays a key role. The detection of latent finger­ prints 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 laborato­ ries. 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 hand for fingerprint evidence to be of any use. Since the introduction of Automated Fingerprint Identification Systems in the mid-1970s, single fingerprints of known origin can be stored in digital form. An unknown fingerprint is en­ tered into the computer and compared with prints on file. The computer deliv­ ers ranked matches, and the respective file prints are then compared with the unknown print by an examiner of la­ tent prints. Usually, the unknown print corresponds to one of the top two com­ puter-delivered matches. Latent fin­ gerprint development has become more valuable than ever because cold searching (searching in the absence of a suspect) is now possible. 0003-2700/89/0361-557A/$01.50/0 © 1989 American Chemical Society

Traditional methods When a finger is pressed against a sur­ face, ~ 0.1 mg of material is transferred to the surface, forming a latent finger­ print. Of this, 98-99% is water that soon evaporates to leave ~ 1 μ.% of resid­ ual material. Approximately one-half of this is inorganic material (e.g., NaCl). The rest is a complex organic mixture (e.g., amino acids, lipids, and vitamins). The two most widely used tradition­ al methods of latent fingerprint devel­ opment are dusting, usually with black powder, for relatively fresh finger­ prints (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 fin­ gerprints on porous surfaces (primarily paper, but also cardboard, wood, leath­ er, or wallboard). Ninhydrin (I) reacts with the amino acids in fingerprint res­ idue to form a purplish-blue product known as Ruhemann's Purple (II). Spray cans of solutions of ninhydrin are commercially available and often are used by investigators, who spray

surfaces under examination a t t h e scene of a crime. Occasionally, silver nitrate or iodine vapor is used for fingerprint develop­ ment, and prints on adhesive tapes are often developed by using crystal violet. The visualization of latent fingerprints by all of the traditional methods in­ volves observing t h e difference be­ tween the ambient light reflected from the areas between the ridges and that surrounding the prints. For weakly de­ veloped prints where a small amount of powder adheres to the fingerprint resi­ due or only a little Ruhemann's Purple is formed, fingerprint visualization in­ volves the detection of a small differ­ ence between two relatively large light signals, and this is an inherently insen­ sitive detection mode. Fingerprint detection by lasers In 1976 researchers began to investi­ gate the use of fluorescence for the de­ tection of latent prints because of the high sensitivity of this technique. Al­ though fluorescent dusting powders had been used for some time in special situations, the use of fluorescence de-

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ANALYTICAL APPROACH tection was not widespread. The advantage of detecting a small signal (fluorescence) versus an equally small difference between two large signals (absorption/reflectance) is clearly demonstrated by the eye: Stars are readily observed at night but not in daylight. For practical reasons, the fluorescence of a latent fingerprint, either from material inherent to the fingerprint residue or resulting from a fingerprint treatment, should be visible to the naked eye. Obtaining such intense fluorescence is difficult because the detection of fluorescence from constituents of the fingerprint residue itself (e.g., riboflavin) involves nanograms or less of material. The use of a reagent that reacts with components of the fingerprint residue (e.g., amino acids) to form a fluorescent product involves small amounts of material in the fingerprint residue as well. The excitation of visible fingerprint fluorescence requires an intense light source (of appropriate color); this requirement led quite naturally to the use of lasers. From the perspective of power, color, and ease of use, the argon ion laser was, in 1976, the most useful type of laser for fingerprint work. It was used in the fluorescent detection of untreated fingerprints or those dusted with fluorescent powders, stained with fluorescent dyes, or treated with reagents that form fluorescent products. Some law enforcement agencies now also use copper vapor lasers and frequency-doubled Nd:YAG lasers; however, the most widely used laser is the Ar laser. For fingerprint detection in a

laboratory setting, Ar lasers of 5-20 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, filtered lamp systems have been produced as well, but they do not provide the sensitivity of the large Ar or Cu vapor lasers. The fingerprint fluorescence detection procedure is simple. The laser light passing through an optical fiber 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 fingerprint 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 fingerprint 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 (7), and numerous law enforcement agencies had begun to use lasers routinely for fingerprint work. At that time, fingerprint treatments

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 6G, 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 be 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

Figure 1. Detection sensitivity achieved with ninhydrin/ZnCI 2 /laser examination. (a) Room light photograph of strong fingerprint on white paper developed by ninhydrin, (b) room light photograph of weak fingerprint developed by ninhydrin, and (c) fingerprint b developed by laser-excited fluorescence after zinc chloride treatment.

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examination. Because Ruhemann's Purple does not fluoresce, efforts were begun in the early 1980s to convert it to a fluorescent product by a second, postninhydrin chemical reaction. The procedure developed (4) is simple and involves spraying the ninhydrin-treated article with zinc chloride dissolved in a volatile solvent system to prevent "bleeding" of fingerprint detail. Usually, a 1:5 mixture of methanols,1,2trichlorotrifluorethane is used. Zn 2+ forms an intensely fluorescent coordination compound (III) with Ruhemann's Purple. Ambient humidity is necessary for the reaction to occur.

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C Figure 1 shows an example of the detection sensitivity achieved with ninhydrin/ZnCl2/laser examination. Figure la 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 Figures l b and lc. Figure l b is a photograph taken in room light of a weak print treated with ninhydrin, with no visible development; Figure lc shows this print after ZnCl2 treatment and laser examination. The fused-ring benzo analogue of ninhydrin (IV) and the 5-methoxy analogue (V) can be used in place of ninhydrin and yield intensely fluorescent zinc complexes. The 5-methoxy dériva-

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ANALYTICAL CHEMISTRY, VOL. 61, NO. 8, APRIL 15, 1989 · 559 A

ANALYTICAL APPROACH tive of ninhydrin is particularly effec­ tive in this regard and may become the reagent of choice since it has recently become commercially available (2,2-dihydroxy-5-methoxy-l,3-indanedione; Aldrich). The structural and photophysical features that lead to fluores­ cence of the zinc complexes are de­ scribed elsewhere (5). Time-resolved imaging Laser fingerprint development has be­ come a widely applicable, highly sensi­ tive method. More than 100 law en­ forcement agencies in the United States now use it, as do agencies in oth­ er countries (including Canada, Great Britain, Israel, and the People's Re­ public of China). Until very recently, a number of surfaces ubiquitous at crime scenes (cardboard, wood, leather, vari­ ous plastics, surfaces painted with strongly fluorescent paint, and some adhesive tapes) remained intractable because of their excessively intense background fluorescence. To permit background fluorescence (short lifetime) suppression by timeresolved imaging, we are now using staining dyes and reagents that yield long luminescence lifetimes. Instead of continuous Ar laser illumination, the Ar 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 ex­ amination. The fingerprint lumines­ cence passes through a filter as before, but in this case is incident on a gatable

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 fre­ quency is adjusted so that the laser-off period is comparable 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 be photo­ graphed or video-recorded. The details of our time-resolved imaging system are described elsewhere (6). Although fingerprint work on stains and reagents t h a t produce intense, long-lived emission is still in its infan­ cy, two potentially effective fingerprint treatments have already emerged. The first treatment involves staining fin­ gerprints with tris(2,2'-bipyridyl)ruthenium(II) chloride hexahydrate(VI) (7). This compound displays an intense

d -* 7Γ* charge-transfer phosphores­ cence with a lifetime of ~ 1 μβ, which is sufficiently long compared with back­ ground fluorescence lifetimes (typical­ ly 0.1-1 ns). The ruthenium compound lends itself to staining of smooth sur­ faces and certain special surfaces such as adhesive tapes. It can also be incor­ porated into dusting powders. The second treatment is used for de­ velopment of fingerprints on porous surfaces. Ninhydrin, its 5-methoxy an­ alogue, or its benzo derivative reacts with fingerprint residue. EUCI3 • 6H2O is used instead of ZnCl2, and the result­ ing Eu complexes display a ligand-toEu intramolecular energy transfer that results in long-lived Eu 3 + lumines­ cence. Spectroscopic details are report­ ed elsewhere (8). Fingerprint t r e a t m e n t with the benzo analogue of ninhydrin and then EUCI3 · 6H2O is particularly effective. Figure 2 depicts a realistic situation of­ ten 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 benzo analogue of ninhydrin/ EuCl 3 · 6H 2 0. A stain of the laser dye, 3,3'-diethyloxadicarbocyanine iodide (DODC), was placed next to the print. Figure 2a 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 de­ rivative of ninhydrin, and this mark would change color to violet after EuCl 3 · 6 H 2 0 application. The fact

Figure 2. Detection sensitivity achieved with the benzo analogue of ninhydrin/EuCI3 · 6H20/laser examination. (a) Room light photograph of fingerprint on white paper developed by benzo(/)ninhydrin/EuCl3 · 6H20. Next to the fingerprint is a DODC stain, (b) Fluorescence photograph of same under CW UV Ar laser excitation, (c) Photograph of gated intensifier phosphor screen image of same.

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that nothing is seen in the photograph taken in room light attests to the weak­ ness of the print. Near-UV Ar laser ex­ citation was used next to produce the orange-red Eu 3 + luminescence of the fingerprint of Figure 2a. Under this ex­ citation, DODC fluoresces intensely in the same spectral range. Figure 2b shows the luminescence obtained under the customary CW la­ ser illumination. The noteworthy fea­ tures of Figure 2b are the intense DODC fluorescence and the absence of any fingerprint detail because of exces­ sive background fluorescence from the paper. Figure 2c shows the results ob­ tained with time-resolved imaging. Note the complete suppression of the DODC fluorescence, and the suppres­ sion of the paper background as well, so that clear fingerprint detail emerges. Although the benzo analogue of ninhydrin/EuCl 3 · 6H 2 0 procedure yields a luminescence that is weak compared with t h a t obtained by ninhydrin/ ZnCl2, the sensitivity of gatable image intensifiers permits detection of latent prints on strongly fluorescent sub­ strates. If fingerprints detected by time-resolved imaging are recorded by video cameras interfaced with comput­

ers, computer image processing for im­ proved sensitivity is possible. We are currently studying a range of rare earths (e.g., Tb 3 + ) and ninhydrin ana­ logues to improve luminescence inten­ sities. We are also investigating a range of transition metal complexes t h a t yield long-lived emissions. This article summarizes the results of a research program that has been supported by Xerox Corpo­ ration (1976-79), the National Science Founda­ tion (1980-86), the U.S. Department of Justice National Institute of Justice (1987-89), and sever­ al industrial companies (Spectra-Physics, ALM, Laser Photonics, Coherent). References (1) Menzel, E. R. Fingerprint Detection with Lasers; Marcel Dekker: New York, 1980. (2) Kendall, F. G.; Rehn, B. W. J. Forensic Sci. 1983,28,777-80. (3) Menzel, E. R.; Burt, J. Α.; Sinor, T. W.; Tubach-Ley, W. B.; Jordan, K. J. J. Fo­ rensic Sci. 1983,28, 307-17. (4) Herod, D. W.; Menzel, E. R. J. Forensic Sci. 1982,27, 513-18. (5) Menzel, E. R.; Bartsch, R. Α.; Hallman, J. L. J. Forensic Sci., in press. (6) Mitchell, K. E.; Menzel, E. R. Proc. SPIE, in press. (7) Menzel, E. R. Proc. SPIE 1988,910,4551. (8) Menzel, E. R.; Mitchell, K. E. J. Foren­ sic Sci., in press.

E. Roland Menzel is professor of phys­ ics and director of the Center for Fo­ rensic Studies at Texas Tech Univer­ sity. He received his B.S. degree (1967) and his Ph.D. (1970) in physics from Washington State University. After completing postdoctoral fellowships at Simon Fraser University (British Columbia, Canada), Purdue Universi­ ty, 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 insula­ tor damage by fluorescence probes.

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