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Improved Fingerprint Acquisition

Fingerprint analysis has been a staple of forensic investigation since the inception of crime investigation. In many cases, it has been the one of the definitive piece of evidence used by a jury to reach a conviction. Recently, DNA analysis largely has overshadowed the importance of fingerprint evidence. However, finding a perpetrator’s fingerprints at a crime scene still places the perp at the scene at some point in time. More important, while the DNA of identical twins will be identical in base sequences, they will have slight, yet detectable, differences in their fingerprint patterns.

Fingerprints arise because oils secreted by the skin collect on the ridges of the patterns existing on the tips of the fingers. When an object is picked up or touched, a small amount of this oil is transferred from the finger to the surface of the object. The oil leaves an exact replica of the ridge pattern of the finger on the object. Because the oil does not readily evaporate, the impression remains for a long period of time or until the surface is cleaned. Fingerprints are unique to each individual and permanent; you are born with and die with the same patterns.

The original method for detecting fingerprints was dusting the surface with finely ground carbon powder applied from a fine camel-hair brush lightly brushed over the print in a series of short strokes. The image left behind was then be photographed and kept for comparison to a potential perpetrator. As the method advanced, white powders (alumina) and other materials were developed for visualizing prints on objects with dark or colored surfaces. Dusting is still the most widely used method for latent prints on items that cannot be removed from a crime scene.

In the movies, fingerprints were always found on the clear wine glass or ash tray. Unfortunately, in the real world latent prints most often occur on colored objects like writing pads, vinyl gloves, soda or beer cans, fabrics, money or the side of a trash Dumpster. Methods have been developed to visualize these fingerprints, and this technology has greatly improved the evidentiary value of latent prints.

Ninhydrin reagent, which reacts with amino acids in the oil and produces a blue-purple stain pattern, was an early addition to print identification. Other chemical methods have followed, including fuming an object with iodine vapor, treating with silver nitrate or forming metal complexes.

In 1990, a new chemical reagent more sensitive for visualizing prints on paper objects was introduced. After processing, this reagent, generally called DFO (1,8-diazafluoreneone), produces a pale print as compared to ninhydrin, but one that is highly luminescent (visible) using specialized light sources and can even be visible under room light. DFO prints are also developed in a period of about 30 minutes, whereas ninhydrin prints may take longer than 24 hours to develop.

A number on ninhydrin derivatives have been developed in an attempt to increase their ability to enhance latent prints. One of these, 1,2-IND (1,2-Indanedione), is now widely used by many departments. It combines the advantages of both DFO and ninhydrin and is a less expensive reagent.

A number of fluorescent powders have been developed for print dusting. In order to visualize these prints they must be viewed under an alternative light source, or ALS device, or a laser. The fluorescence produced depends on the color of the dye being used and the wavelength of light produced by the filter on the light source.

The advantages to this approach are that even weak latent prints can be visualized at the crime scene, and this can lead to acquisition of prints that might otherwise be overlooked. The vivid fluorescent color also provides greater contrast to background colors and makes for better more well-defined photographic records. These fingerprints can be visualized in room light and even in outdoor situations.

ALS devices come with a variety of light sources and wavelength filters, which can produce light in the UV, visible and infrared (IR) ranges. Newer lights that use small, light emitting diode (LED) sources are becoming popular because they can more accurately produce a specific wavelength of light. All of these devices work by shining the light of a particular wavelength on the print (treated or untreated) in order to induce the print to emit light at a different wavelength, a phenomenon called photoluminescence.

More recently, lasers have been introduced for print visualization. Typically, an Argon ion laser is used to induce a photoluminescent response from a fingerprint. The major drawback to ALS and laser detection system is the cost of the equipment, specialized cameras and lighting equipment needed to record the results.

For large objects (rifles, baseball bat, etc.) latent prints can be developed by fuming them with cyanoacrylate or “superglue.” Objects are placed in a closed cabinet and exposed to cyanoacrylate that is vaporized by heating. The cyanoacrylate bonds with and fixes the biochemical materials in the print to produce a solid cast which can then be viewed directly under an ALS or treated with various stains or reagents and subsequently viewed.

A variety of other methods are under study by the government and independent research facilities. These include the use of micro-X-ray fluorescence (MXRF), infrared spectromicroscopy (FTIR-SM), and others. These methods may prove very sensitive, but their equipment costs will limit them to only the very largest CSI departments. They may prove valuable in developing prints on objects of great value (antiques, paintings, etc.) or where a vital trace print is needed to ensure a conviction.

Doug Hanson, Ph.D., is a biochemist who has operated toxicology and analytical chemistry laboratories for more than 25 years. He is also a freelance writer who has written extensively for law enforcement, EMS and first responder magazines. He published a book, “The Eider Files,” which is a novel about bioterrorism. He can be reached at dougmh@comcast.net.

Published in Law and Order, Jul 2006

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