Fingerprinting
Fingerprinting
Fingerprinting analysis has been used for more than a century, yet it is still widely used in law enforcement agencies. A fingerprint is an individual characteristic. It is yet to be found that prints taken from different individuals possess identical ridge characteristics. Furthermore, a fingerprint will remain unchanged during an individual’s lifetime. Finally, fingerprints have general characteristic ridge patterns that permit them to be systematically classified. Because of its unique characteristic, a print is conclusive evidence and a valuable tool among advanced technology even today. In a judicial proceeding, a point-by-point comparison must be graphically demonstrated for at least 12 different, but corresponding, points in order to prove the identity of a specific person.
The method used for obtaining latent prints depends on the type of surface to be examined, the manner in which the prints were left, and the quantity of material left behind. After the prints have been photographed, lifted and taken into the crime lab, they are then compared to the prints of all persons known to be at the scene of the crime or who had access to the crime scene. This procedure eliminates all but the criminal’s prints.
The individuality of any fingerprint is based not upon the general shape or pattern that it forms, but instead upon its ridge structure and specific characteristics (also known as minutiae). The recognition of these ridges, their relative number, and the approximate location of them, on the observed print, are the special characteristics that make the fingerprint a specific identifying characteristic of each individual. There are at least 150 individual ridge characteristics on the average fingerprint. If between 10 and 16 specific points of reference for any two corresponding fingerprints identically compare, a match is assumed.
As mentioned previously, the individuality of any fingerprint is based not upon the general shape or pattern that it forms, but instead upon its ridge structure and specific characteristics (also known as minutiae). The recognition of these ridges, their relative number, and the approximate location of them, on the observed print, are the special characteristics that make the fingerprint a specific identifying characteristic of each individual. There are at least 150 individual ridge characteristics on the average fingerprint. If between 10 and 16 specific points of reference for any two corresponding fingerprints identically compare, a match is assumed.
There are three types of fingerprints that exist at crime scenes.
1) Visible prints: Made from a finger which has been stained with colored materials such as ink, blood, and grease.
2) Plastic prints: Formed by pressing onto a soft surface such as clay, soap, and wax.
3) Latent prints: Invisible print left on an object by the body’s natural greases and oils. Because it cannot be seen by naked eyes, fingerprint powders, chemicals, and even lasers are used to make it visible on the crime scene evidence.
The structural elements of fingerprint patterns can be categorized into three basic formations:
1) Loops: Lines that enter and exit on the same side of the print.
2) Arches: Lines that start on one side of the print, rise into hills, and exit on the other side of the print.
3) Whorls: Circular patterns that do not exit on either side of the print.
* Note: Approximately 60% of the total population has loops, 35% have whorls, and 5% have arches.
Type Lines: The two diverging ridges coming into and splitting around a loop or obstruction.
Delta: Ridge point nearest the type line divergence.
Core: Center of the pattern.
The three major groups are also subcategorized based upon smaller differences existing between the patterns within the specific group. These three subcategories are as follows:
Arch: Plain Arch, Tented Arch
Loop: Radial Loop, Ulnar Loop
Whorl: Plain Whorl, Central Pocket Whorl, Double Loop, Accidental
Plain Arch: Simplest of all fingerprint patterns, formed by ridges entering from one side of the print and existing on the opposite side. These ridges tend to rise at the center of the pattern, forming a wavelike structure.
Tented Arch: Similar, but instead of rising smoothly at the center, there is either a sharp up thrust or spike, or the ridges meet at an angle that is less than 90 degrees. (Arches do not have type lines, deltas, or cores).
Loops: A loop must have one or more ridges entering from one side of the print, re-curving, and exiting from the same side. If a loop opens toward the little finger, it is called an Ulnar Loop. If it opens toward the thumb, it is a Radial Loop. The patterned area of any loop is surrounded by two Type Lines. All loops must have one Delta.
Whorl Patterns: All whorl patterns must have type lines and a minimum of two deltas. A Plain Whorl and Central Pocket Loop have at least one ridge that makes a complete circuit. This ridge may be in the form of a spiral, an oval, or any variant of a circular form. The main difference between these two patterns can be shown if an imaginary line is drawn between the two deltas contained within the two patterns. If the line touches any one of the spiral ridges, the pattern is determined to be a plain whorl. Alternatively, if no ridge is touched, then the pattern is a central pocket loop.
The Double Loop is made up of any two loops combined into one fingerprint. Any print classified as Accidental either contains two or more patterns (not including the plain arch) or the pattern is not covered by other categories i.e., a combination loop and a plain whorl or a loop and tented arch.
The image below contains graphic examples of all the structural elements listed below.
Core: 6
Bifurcations: 1 , 4 , 5
Ridge Ends: 2 , 3 , 7 - 11
Analytical Methods
The currently used methods of fingerprint detection may be classified into two categories. Both classes, in fact all conventional fingerprint detection methods, require a chemical or physical treatment of the exhibit under examination.
1) Powder methods. Those which are dependent on the adherence of inert materials to fingerprint residues.
2) Those which rely on chemical interaction of a detection reagent with specific components of the latent print.
Fingerprints are often present, but not visible in standard conditions of white light illumination. Patterns of perspiration in such latent prints can be made visible by brushing certain types of chemical powders over them. The choice of powder depends partly on the type of surface on which the print is found, and partly on how it is to be preserved. For example:
1) Carbon powder is typically used on white or lightly colored surfaces
2) Lanconide Powder is typically used on black surfaces
3) Aluminum powder is typically used on hard or dark colored surfaces as well as smooth surfaces (e.g. mirrors and metal surfaces)
Fluorescence is the emission of
radiation (esp. visible light) by a substance, typically during exposure to
external radiation (lasers, X-rays, UV light). The substance absorbs light
and re-emits the light in wavelengths longer (lower
energy) than that of the illuminating source. The
mechanisms of absorption and emission are based on transitions of quantum
electron energy levels. The energy difference between the absorbed and emitted
photons ends up as molecular vibrations or heat. Various chemicals present in
the natural oils or perspiration in fingerprints may fluoresce under appropriate
(typically UV) lighting conditions. Fluorescent body secretions include
semen, saliva, and
sweat. Elements of particular interest here include various
fluorescent enzymes and proteins contained in body fluids.
In 1976, a method for the detection of latent fingerprints
by their inherent luminescence using continuous-wave (CW) argon-ion laser
excitation was discovered by E.R. Menzel at Xerox Research Centre of Canada,
where the first detection by this method of an identified print from an actual
criminal exhibit (a fingerprint on the sticky side of a piece of black
electrical tape) was also achieved. (1, 2)
The procedure involved illumination of the exhibit under scrutiny with
blue-green light from an argon-ion laser and photography of the resulting
yellow-green luminescence of the fingerprint. The fingerprint will fluoresce
with a brightness (or intensity) proportional to its absorbance. This absorbance
is fairly flat from the long wave UV to the green with a small peak at 510 nm
in the blue-green. The absorbance at 510 nm is believed due to
riboflavin (Vitamin
B). Thin layer chromatography (TLC) has also indicated that the
fluorescent components of fingerprint residue are
lipid in nature, and may include
pyridoxine (Vitamin
B6).
In a similar manner to the analysis of semen on fabric (or skin), the key element in a fingerprint identification and recording system is contrast enhancement. The fingerprint ridges must be made to stand out with respect to the background.
Fingerprints can be located from their intrinsic absorbance. The absorbance of skin oils and amino acids found in a fingerprint is ~ 20 times greater in the short wave UV (260-300 nm) than the long wave UV (320-400 nm). Under these conditions, fingerprint residues have been found to fluoresce between 310 - 380 nm.
So in the short wave UV, fingerprints often show up in reflectance. To get contrast requires a UV sensitive camera and a difference in both absorption and surface texture between the fingerprint and the underlying surface. This usually requires smooth surfaces.
Once the print has been located by intrinsic reflectance or fluorescence, the ridge contrast can be further enhanced using dyes preferentially bound to the ridges of the print. This is done on smooth surfaces with cyanoacrylate fuming (“SuperGlue Method”) which polymerizes preferentially on the ridges. This protects the ridges during dusting or staining with a fluorescent dye. The cyanoacrylate provides a surface on which the dye (often tied to a binder) can preferentially adsorb.
Fluorescent dusting powders (which are often combined with fluorescent stains and dyes) include: ANS Zinc Chloride Rhodamine 6G Safranin O as well as fluorescent reagents in vapor phase such as 9-methylanthracene . All of these dyes can be visualized under UV light, Argon laser, or alternative light sources (e.g. high intensity quartz halogen, xenon arc or indium arc). The fluorescence of the dye bound to the fingerprint ridges can then be photographed under ALS illumination. On rough surfaces a different approach is often taken. Some of the fluorescent reagents utilized under such conditions include:
Ninhydrin (Triketohydrindane hydrate) is a chemical used to detect ammonia or primary and secondary amines. Thus, compounds such as benzo(f)ninhydrin and 5-methoxyninhydrin and 5-methylthio ninhydrin react with the amino acids left over from peptides and proteins (terminal amines or lysine residues) sloughed off in fingerprints. When reacting with these free amines, a deep blue or purple color is evolved. The blue colored substance (Ruhemann’s Purple) is formed by the reaction of some of the ninhydrin with its reduction product (hydrindantin) and ammonia. Several amino ninhydrins have also proved to be effective in this context. (3)
DFO (1,8-diazafluoren-9-one) makes fingerprints glow when they are lit by laser or blue-green light (4). This organic reagent reacts with amino acids present in the fingerprint perspiration to form highly fluorescent derivatives. For example, excitation with light at ~ 470 nm results in red emissions at ~ 570 nm.
1,2-indanedione as a latent fingerprint reagent on some types of paper was found in many cases to exceed DFO in performance and quality (5). It even exceeds the performance of the sequence, DFO, followed by ninhydrin. For example, no new prints could be observed when ninhydrin was applied after indanedione.
Alternatively, fluorescent powders can be used to treat surfaces (or substrates) before the crime has been committed. Available in powder and aerosol forms, these powders may be used to dust paper currency and documents and sprayed in areas where recurring theft takes place. In this context, they are not classified as finger print powders because they are used a priori, or prior to the deposition of the prints. For example, DFO, fluorescamine and 0-Phthalaldehyde react almost instantaneously with primary amino groups from body secretions, yielding highly fluorescent images and patterns. These reagents are used on multi-colored surfaces.
After the suspect has handled the money or touched the dusted area, the area is examined in UV light. The latent print is easily visualized, and may be photographed using proper photographic techniques. These and other procedures are outlined in the FBI Processing Guide for Developing Latent Prints (6).
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References & Reading
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(1) Menzel, E.R., Duff, J.M.
"Laser Detection of Latent Fingerprints"
Journal of Forensic Sciences, Vol. 24 [p. 96 and p.582] (1979)
(2) Menzel, E.R.
Fingerprint Detection With Lasers p.48
Marcel Dekker Inc. (1980)
(3) Menzel, E.R., Bartsch, RA, Hallman, JL
"Fluorescent Metal -Ruhemann's Purple Coordination Compounds: Applications
to Latent Fingerprint Detection"
J. Forensic Sci. Vol. 35 (January, 1990)
(4) Pounds, C.A., Phil, M., Grigg, R., Mongklolaussavartana, T.
"Use of DFO for the Fluorescent Detection of Latent Fingerprints on Paper"
J. Forensic Sci., Vol. 35, p. 169 (1990)
(5) Wiesner, S., Springer, E., Sasson, Y., Almog, J.
"Latent fingerprints: 1,2-indanedione has come of age"
J. Forensic Sci., Vol. 46 (September, 2001)
(6) FBI Processing Guide for Developing Latent Prints
Forensic Science Communications (January, 2001)
(7) Saferstein, R., Criminalistics p. 336
Pearson Prentice Hall, Upper Saddle River, NJ