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Law Enforcement and Latent Fingerprints—How Do They Find Them at Crime Scenes

By: Administrator
The wildly popular TV show, “CSI-Crime Scene Investigation” has changed the public’s view on how crimes are investigated and solved. In a sense this show has accomplished two points:

1.  It has given the public first hand information about the tools and procedures that crime scene investigators use to solve crimes.

2. It has given the public unrealistic expectations in believing all law enforcement agencies should be able to catch the criminal in just 42-44 minutes.

Budget constraints often hamper investigative agencies in procuring the latest tools needed for effective crime scene investigation. Budgets also restrict training opportunities for officers too. Yet the locating, collecting, preserving and evaluating physical evidence from crime scenes is what helps to get convictions and /or exonerate a falsely accused suspect..

Latent fingerprints remain one of the most valuable kinds of physical evidence, and although DNA evidence is becoming increasingly valuable, latent prints have an excellent track record.

Latent fingerprints are the result of physical contact with a surface by the fingers or palms of the hands. Both the palms and soles of human feet are covered with “friction ridges,” and these ridges are dotted with sweat pores.  These pores are part the body’s waste disposal system and they secrete mostly water that is populated with a variety of waste products like amino acids, urea, sugar, creatinine and choline.

But the fingertips are often contaminated by oily secretions from contact with other parts of the body that harbor sebaceous glands. The face, ears and upper body are covered with pores that secrete oily sebaceous content, which includes fatty acids, glycerides and hydrocarbons.
Several methods are available to CSIs for use at the crime scene. Below are tools and materials used to locate and develop latent fingerprints:

1. Latent print development powders:  Latent powders include a variety of formulations. The first consideration facing the evidence collection team is the type of surface to be processed. Nonporous surfaces like metal, painted wood, glass and most plastics generally require the use latent print powders. But specialized powders have been developed that improve chances of getting useable latent prints on a variety of surface textures. Background color is also a consideration. A light colored powder is used on dark surfaces and dark powders are used on light surfaces to ensure good photographic contrast.  Latent powders are applied to surfaces using soft bristle brushes.

Highly polished surfaces like chrome-plating and silver objects require the use of powders formulated from metal—like aluminum, copper and brass particles. Other non-porous surfaces are processed with “Oxide” formulated powders like black, white, gray, and red.

 

2. Porous surfaces such as paper, cardboard and raw wood are best treated with chemicals like DFO, Ninhydrin, Silver Nitrate or Physical Developer.  These chemicals are used in liquid form and are either sprayed onto the evidence or development in a tray is used.

3. Chemical fuming is the third choice for latent print development.  There are two popular fuming processes: Iodine fuming and Cyanoacrylate (Superglue) fuming. Iodine fuming works best on porous surfaces (paper, cardboard and raw wood).  Iodine crystals undergo a transformation when low heat is applied to them. This causes the Iodine to sublimate, which is when a solid like the crystals transforms in to a gas (fumes). Iodine fuming is practiced in an enclosed area of some sort. At the crime scene a zip-top plastic bag may be used as a “fuming chamber.” Iodine fumes react with the oily contents of latent print residue and an orange-brown color appears that conforms to the ridge structure of the print.

Cyanoacrylate or superglue fuming is used on nonporous surfaces like metal, painted wood, glass and plastic. Again a sealed chamber is needed for development. Superglue forms a very persistent bond with many surfaces, but it does not work unless moisture is present on the surfaces to be bonded.

Normal humidity can fulfill the moisture requirement. A small amount of superglue is paced inside the chamber with the evidence undergoing evaluation. It could take typically two to three hours for the fumes to “polymerize,” or to cover and harden on the moisture content of a latent print.

Development may be accelerated by the addition of heat (like a small coffee warmer) or by applying the glue to a cotton pad. The cellulose content of cotton causes considerable heat to develop.

In review, latent powders, chemicals and fuming techniques fill the crime-solving toolkit of the CSI.

How Police Find latent prints Part 1

See Some of the Basic Tools and Equipment used at Crime Scenes

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