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How do different surfaces affect the preservation of finger prints?
Question Date: 2011-10-21
Answer 1:

Fingerprints left on a surface are known as latent fingerprints. These fingerprints contain mostly water, fatty acids, and triglycerides, with small amounts of amino acids and chlorides. In order to detect the fingerprint, we have to be able to detect one of these groups of chemicals. The general idea is to make the fingerprint easily visible and then photograph it as a means of preservation. It is usually easier to rely on a photograph than actually trying to preserve the fingerprint itself.

One way to make the fingerprint visible is to dust with a powder that will stick to the fingerprint and create contrast between the fingerprint and its background. The fingerprint can be photographed in place or lifted from the surface with lifting tape and placed on a contrasting surface. This technique works well on smooth surfaces.

Another way to make the fingerprints visible is to make them react with chemicals. For instance the amino acids in the fingerprint will react with ninhydrin or diazafluorenone. Ninhydrin turns blue or purple after the reaction so the fingerprints can easily be seen. When fingerprints treated with diazafluorenone are illuminated with blue light, they will glow yellow. There are many other methods for dying and staining fingerprints. Chemical developing methods like these are important because it allows for fingerprint detection on porous surfaces. It is usually difficult to detect fingerprints on these kinds of surfaces because the latent print can actually be absorbed into the material so dusting for prints does not work.

Fingerprints fresh enough to still contain water (< 24 hours old) can also be preserved with superglue (ethyl cyanoacrylate). Super glue liquid is made up of many molecules of ethyl cyanoacrylate. When it is exposed to air, the moisture in the air will cause it to polymerize, meaning that all of the molecules react together to form chains. If fingerprints are exposed to ethyl cyanoacrylate vapor, the ethyl cyanoacrylate will react with the moisture in the fingerprint and polymerize on the ridges of the fingerprint, forming a hard surface and preserving it.

While I am doing research on your questions, here is something interesting that I came across that I think you will find helpful. It is the procedure for collecting fingerprint evidence from the Missouri State Highway patrol. Hopefully it will help you get started and let you know that we haven't forgotten you while I find out more about some of the detection techniques. The handbook does talk quite a bit about different surfaces which was one of your questions.

Missouri State Highway Patrol Forensic Laboratory Forensic Evidence Handbook, SHP-145D 06/2007

2. Fingerprints
Generally, latent fingerprints on nonporous materials deteriorate rapidly upon prolonged exposure to high temperature and humidity.Consequently, items should be processed and/or forwarded to a lab as soon as possible. With the assistance of the Automated Fingerprint Identification System (AFIS), we are able to search the entire fingerprint file without a suspect. A thorough file search can be made only if correct processing procedures are followed to obtain the best latent print evidence. Also, homicide and sexual assault cases will be searched, without a request, through the FBI's Integrated Automated Fingerprint Identification System (IAFIS), if no candidate is generated through AFIS. All other types of crimes will be searched through IAFIS only upon request.

Comparison Standard/Inked Prints
Collect fingerprints and palm prints of suspect, other persons under investigation, and any other person who may have touched the object bearing the latent print. Designate either on the print card or in a cover letter whether the inked prints are from the suspect or are for elimination. Place fingerprint cards in a stiff envelope to protect them from being bent. Seal the envelope and label it with your initials, the date, and an exhibit number. (See page 11.)

Photographing Latent Prints
A ruled scale should be used when photographing latent prints.Photographs to be used for comparison purposes should be 1:1 or natural size prior to submission to the laboratory.

Latent Prints on Absorbent Materials
Place the paper, cardboard, or other absorbent material in a plastic bag or cellophane protector. Do not handle the material with your fingers wear gloves or use forceps to maneuver the materials. Do not attempt to develop latent fingerprints on absorbent surfaces yourself.Questioned document evidence can be destroyed if processed for fingerprints; therefore, questioned document examinations must be completed prior to evidence being processed for fingerprints. Label the container with your initials, the date, and an exhibit number. Collect all necessary comparison standards.

Latent Prints on Hard Surfaces
Use fingerprint powder to dust plastic cards, metal plates, glass bottles, or other hard surfaced objects for latent

Answer 2:

Here it might be helpful to discuss the difference between "patent," "latent," and "plastic" fingerprints.With patent you would have a visible impression on a surface left with some other material, such as flour or oil. Latent isn't necessarily visible by the human eye. Lastly, plastic fingerprints are 3-D, "molded" prints left in a material, such as putty. So, the preservation of these different types of fingerprints are affected by exactly what material they were left behind with.

Some additional websites that may be helpful to look over:

w iki
a little history
f actors affecting latent prints

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