The gross patterns of fingerprints, namely arches, loops, and whorls, sometimes referred to as first-level detail, are primarily employed for classification or elimination purposes. The identification of individuals by fingerprints involves instead the minutiae of fingerprint patterns, such as ridge endings, bifurcations, and so forth, called second-level detail. The locations of pores on fingerprint ridges and the very structures of ridges, referred to as third-level detail, can, if the fingerprint is of good quality, be employed for identification as well.
A fingerprint obtained from an article of evidence is compared with an inked fingerprint if a suspect is on hand. Otherwise, it is entered into the Automated Fingerprint Identification System (AFIS) to be searched against the fingerprint database. This is referred to as cold searching. Similarly, a live fingerprint may be scanned (Livescan), mostly for security purposes, and compared with the fingerprint database. We do not in this entry belabor the AFIS and Livescan computer technology but focus instead on the methods for detecting fingerprints on articles of evidence to begin with, emphasizing milestones of fingerprint detection methodology.
Current State of the Art
The detection of fingerprints for criminal identification dates back to the late 1800s. At that time, it involved mainly dusting, iodine fuming, and silver nitrate processing. A number of additional techniques were devised over the years for specialized situations, but no truly major advance occurred until 1954, when Oden and von Hofsten reported the use of ninhydrin. This compound was discovered in 1910 by Ruhemann, who recognized it as an amino acid reagent. Soon thereafter, ninhydrin became a universal reagent for amino acid assay in the bioscience community.
It is unfortunate that it took so long for ninhydrin, which since the mid-1960s has become the workhorse of chemical fingerprint detection, to enter the fingerprint arena. From the mid-1960s to the early 1980s, a number of techniques were explored, mostly in England, including among others metal deposition in vacuo and fuming with radioactive sulfur dioxide. These techniques did not reach wide use, being applicable to special instances only. Physical developer, devised in England as well in the early 1980s, targets lipids in fingerprints. It is a photographic process reminiscent of the silver nitrate treatment. It is based on the formation of silver on fingerprints from a ferrous/ferric redox couple and metal salt mixture.
Physical developer reached wide use by the late 1980s.
Of these techniques, only dusting, nin-hydrin, and physical developer remain in wide use. Fingerprint visualization with these techniques involves the basic phenomena of light absorption and reflection by substances, namely the principles of ordinary everyday color or black-and-white vision. This visualization is often referred to as ”colorimetric.”
The year 1976 marks the advent of fingerprint detection with lasers. The basic phenomenon involved is fluorescence, also referred to as luminescence or photolumi-nescence. Fluorescence detection differed markedly from the absorption/reflection-based, namely colorimetric, techniques then in use. Operationally, the article of evidence is illuminated with a high-intensity light source of the appropriate color and the article is visually inspected through a filter that blocks the illumination reflected from the article but transmits the fingerprint fluorescence produced via the illumination. The examination is conducted in a darkened room to eliminate interfering ambient light. The fingerprint is literally seen to glow in the dark, much like a firefly.
The rationale for attempting to detect fingerprints by photoluminescence techniques is that such techniques, regardless of the field of science, are quite generally characterized by very high sensitivity. Initially, the fluorescence detection of fingerprints by laser involved the inherent fluorescence of fingerprint material, dusting with fluorescent powder, and staining with fluorescent dye. When a finger touches an article, only very little material is deposited on the article in the form of a latent fingerprint. Nonetheless, this fingerprint must be made to fluoresce sufficiently intensely to be visible to the naked eye, hence the high-intensity requirement for the illumination source.
Initially, high-power argon-ion lasers (blue-green) were employed. Some agencies, notably in Israel, followed with adoption of copper vapor lasers, which have a green output and a yellow output. The latter is only of limited utility for fingerprint work. By the mid-1980s, frequency doubled Nd:YAG lasers, which operate in the green at 532 nm, began to see use as well. Their powers then were low, however. The technology has since matured, such that high-power 532-nm lasers that are easy to use are on the market today. A number of agencies have adopted them for fingerprint work.
The argon-ion laser and copper vapor laser are expensive and somewhat cumbersome to use. The 532-nm lasers are expensive as well. Ordinary lamps, such as xenon arc lamps, equipped with band-pass optical filters to extract the blue-green light used mostly for fingerprint fluorescence production, were examined from the very outset as well. Such lamps began to be commercialized by the mid-1980s and are often referred to as alternate light sources or forensic light sources. They do not provide nearly the sensitivity the large lasers are capable of, but they are cheaper and easy to use. One can equip them with a range of band-pass filters for a variety of forensic examinations that do not involve fingerprint detection.
In the early days of fingerprint detection with lasers, there was a need for a chemical fingerprint detection procedure that would lend itself to fluorescence visualization in order for the fluorescence approach to be a universal one, capable of detecting fingerprints, fresh or old, on porous items such as paper. Ruhemann’s purple (RP), namely the product of the reaction of ninhydrin with amino acid, is not fluorescent, which is unfortunate given the wide use of ninhydrin for fingerprint work. The remedy came in 1982 with the discovery that a simple follow-on treatment with zinc chloride after ninhydrin would convert the nonfluorescent RP to a highly fluorescent complex, thus greatly increasing the sensitivity of ninhydrin.
In the early 1980s, efforts began, most notably in Israel, to synthesize ninhydrin analogs that would have the same chemical reactivity as ninhydrin but that would have superior colorimetric or fluorescence (in concert with zinc chloride) properties. Benzo(f)ninhydrin in particular, first synthesized by Almog and coworkers in Israel, was found, in concert with zinc chloride, to be very nicely tailored to the 532-nm laser, whereas ninhydrin/zinc chloride is not very effective with this illumination, being tailored, instead, to the argon-ion laser. In 1988, Pounds reported the use of 1,8-diazafluoren-9-one (DFO) for the fluorescence detection of fingerprints on paper. This reagent has since been adopted by many fingerprint examiners as an alternative to ninhydrin/zinc chloride. In 1998, 1,2-indanediones were reported for fluorescence detection of fingerprints. These compounds resemble ninhydrin but their reaction chemistry with amino acid differs from that of ninhydrin. There are claims that 1,2-indanedione is the most sensitive amino acid reagent yet devised for the fluorescence detection of fingerprints.
In the early days of laser fingerprint detection, old fingerprints on smooth surfaces, such as plastics, that might not be amenable to dusting, posed considerable problems. Development of such prints by staining with fluorescent dye proved tricky in that the dye solution tended to wash fingerprints away, as would solutions of chemical reagents. The situation changed for the better with the advent of cyano-acrylate ester fuming (Super Glue fuming). This material polymerizes on fingerprints to stabilize them. A white polymer is formed by which the fingerprint becomes visible. The technique was first devised in 1978 by the Japanese National Police Agency and was subsequently introduced to the United States by latent print examiners of the U.S. Army Criminal Investigation Laboratory in Japan and of the Bureau of Alcohol, Tobacco and Firearms.
From the fluorescence perspective, cyanoacrylate ester fuming is marvelously compatible with subsequent fluorescent dye staining and laser examination. The dye intercalates within voids of the polymer. Very high sensitivity gain over the Super Glue fuming alone is realized. A number of fuming procedures have been developed, including heat acceleration, chemical acceleration, and, most recently, fuming in a vacuum chamber. With the latter, the formed polymer tends to be colorless rather than white. Thus, the vacuum fuming is invariably followed by fluorescent dye staining.
In the late 1980s, fingerprint detection by reflective ultraviolet imaging systems (RUVIS) was developed, first in Japan. The light source typically is deep ultraviolet, namely a mercury lamp operating at 254 nm. When a fingerprint is located on a smooth surface, the ultraviolet light is specularly reflected from it but diffusely from the fingerprint. The article is inspected through a camera or night vision goggle that converts the UV light—invisible to the eye—to visible light via an image in-tensifier. The illumination is at a suitable angle such that the image intensifier only sees the diffuse reflectance (from the fingerprint). RUVIS is a variant of oblique lighting, long used for shoeprint examination, for instance. RUVIS is expensive, cumbersome, and useful only in special instances and thus is not widely employed.
Fluorescence represents the most sensitive approach to fingerprint detection currently available, and it is the approach of choice, especially in serious cases. Inherent fingerprint fluorescence, dusting with fluorescent powder, staining with fluorescent dye after Super Glue, and chemical development with ninhydrin/zinc chloride, DFO, or 1,2-indanedione are the bread-and-butter routine procedures. They have amply proved their mettle in worldwide use.
Meanwhile, fluorescence approaches have spilled over into other areas of forensic analysis, such as the detection of elusive fibers, body fluids (mostly semen), or bone and tooth fragments and the examination of documents for erasures, alterations, obliterations, or the like. Indeed, the labeling of DNA, which early on involved radioactive tags, now makes use of fluorescent tags as well. As a result of concern with terrorism, fluorescence is beginning to find its way into field methods for the detection of traces of explosives and nerve agents. It is safe to say that fluorescence methodology has become a new paradigm in forensic analysis.
Future Developments
Fluorescence techniques, though generally very sensitive, suffer from a major difficulty, namely that of background fluorescence. In the fingerprint context, that background would come from the article on which the fingerprint is located. Background fluorescences are ubiquitous and often so intense that they completely mask the fingerprint luminescence. Moreover, they frequently are spectrally very broad, such that there is a background component of the same luminescence color as that of the fingerprint. Thus, optical filtering is ineffective.
To make fingerprints detectable by fluorescence under these conditions, time-resolved techniques can be employed. Their details are left to the references. The feasibility of time-resolved fingerprint imaging was demonstrated by Menzel as early as 1979, but the technology to produce a practical instrument was not available then. Worse still, a range of fingerprint treatments that would produce luminescences with long lifetimes, as required in time-resolved imaging, did not exist either. With the advent of microchannel plate image intensifiers in the mid-1980s, it became possible to construct instruments that had practicality potential. By the early 1990s, the instrumentation issue was largely under control. The accompanying fingerprint treatment issue took longer to come to grips with. By the late 1980s, researchers began to gravitate toward europium-based fingerprint treatments because europium luminesces with an exceptionally long lifetime. By the mid-1990s, luminescent europium powders and staining dyes had materialized.
The chemical development of fingerprints, akin to ninhydrin, especially for old fingerprints, was more recalcitrant. That chemistry is now in hand as well, using SYPRO Rose Plus Protein Blot Stain, although, no doubt, improvements in europium-based chemical fingerprint processing remain desirable.
Although much progress has been made in the time-resolved methodology, several factors have so far mitigated against its adoption by law enforcement. The main one is the complexity of the instrumentation, which calls for a highly trained operator. Instrument cost is an issue as well. Furthermore, specialized fingerprint treatments pertain that otherwise would not be used.
An instrumentation design change reported in 2004 by Menzel and Menzel makes for a system that is much simpler and cheaper than the earlier ones, and even shows portability potential. The expensive computer controlled intensified and gateable CCD camera of earlier instruments is eliminated altogether and is replaced by the human eye for the observation of the time-resolved fingerprint. Most any garden variety photographic camera can then be employed for the recording of the fingerprint. The system still uses a laser, but one can foresee a further simplification and cost saving by replacing the laser with an electronically controlled array of light-emitting diodes (LEDs). The LED technology has matured greatly in recent years, such that such controlled arrays are now available. One can thus envision miniaturizing the instrument to where it may be worn at the crime scene, much like night vision goggles. It should be emphasized, though, that the instrumentation is still wedded to the europium-based fingerprint treatment strategy.
Even though the time-resolved fluorescence detection of fingerprints is not entirely mature yet, there is little doubt that in time it will become part of the fingerprint examiner’s arsenal.
