Establishing Identities: Fingerprinting
If you look closely at the tips of your fingers you will notice a series of tiny raised lines called friction ridges. These ridges help you grip and handle objects easily. Whenever we touch something with our fingers, we can leave impressions of these ridges behind. These are called fingerprints.
One of the challenges in ongoing investigations is identifying the people involved, such as the victim(s), suspect(s), or even the father in a paternity lawsuit. Numerous techniques are used to determine the identification of people.
If you look closely at the tips of your fingers you will notice a series of tiny raised lines called friction ridges.
If you look closely at the tips of your fingers you will notice a series of tiny raised lines called friction ridges.
|
Definitions:
Friction Ridges
|
Classes of Fingerprints
Loops
|
Whorls
- Whorls have a circular pattern with no entrance or exit. Whorls can be classified as plain whorls, central pocket loop whorls, double loop whorls, and accidental whorls.
- 30 - 35% of all people have whorl fingerprints.
- Some of your fingerprints might be arches, some whorls, and some loops.
- But even if they're all the same class, they're slightly different from each other.
- The distance between friction ridges, the length of each ridge, the space and depth between ridges - all of these things and more make each fingerprint unique.
- That is why the police fingerprint all ten fingers when they arrest a suspect.
- Not only are all of your fingerprints different from each other, they're also different from everyone else's fingerprints. When we say every fingerprint is different we really mean every fingerprint!
Something to Try
- Rub a pencil on a piece of paper. Then rub the tip of your index finger over the paper until it's covered in lead.
- Put the sticky side of a piece of Scotch tape over your fingertip. Carefully peel the tape off to reveal your fingerprint. Stick the tape on a sheet of paper.
- Look closely at your fingerprint. What class is it - a loop, arch, or a whorl? How can you tell? Try your other fingers. Are they all the same class?
Latent Fingerprint Detection Techniques
Crime scenes are usually packed with fingerprints.
- Some of these are easy to see (like a bloody fingerprint on a doorknob), but others are not.
- Fingerprints that are completely invisible to the naked eye are called latent fingerprints.
- Finding these obviously takes more time than ones in plain view.
- There are many techniques for finding latent fingerprints.
- If the surface is non-porous, the identification officer will typically use a dusting powder.
- This powder collects on the moist areas of fingerprints, making them visible.
- Expose the object to cyanocrylate adhesive, commonly known as superglue.
- Cyanocrylate gives off fumes that stick to sweat in fingerprints.
- These fumes harden, leaving a solid white impression of the fingerprint.
- A luminescent stain or powder is then applied to the fingerprint to bring out the details.
- A new technique called vacuum metal deposition (VMD).
- VMD is especially effective for detecting old, degraded fingerprints and ones on tricky surfaces like plastic bags.
- In this process, the item is placed in a vacuum then coated with very thin layers of gold and zinc.
- The gold deposits evenly over the object, including the friction ridges in a fingerprint; the zinc deposits on everything except the friction ridges.
- The result is a gold fingerprint outlined against a zinc background. Very cool!
- Different techniques are required for finding latent fingerprints on porous surfaces like paper, drywall, or wood.
- One technique is to coat the surface with a chemical solution known as ninhydrin.
- Ninhydrin reacts with the sweat in fingerprints to create a purple image.
- A second technique is to coat the area with a chemical called DFO.
- DFO acts in much the same way as ninhydrin but results in a light pink image of the fingerprint.
- DFO works much more quickly than ninhydrin and reveals far more fingerprints.
- A third technique is to coat the substance with physical developer.
- This solution reacts with the fats, oils, and waxes in fingerprints to create a silver-grey coloured deposit.
- Developed prints appear as dark grey images against a light grey background.
Matching Fingerprints to a Known Sample
- After a fingerprint is revealed at a crime scene, it is lifted (collected) using one of several different techniques.
- It is then entered into a computer database containing the fingerprints of convicted criminals
- The computer compares the unknown fingerprint against all of its records and creates a "shortlist" of possible matches.
- The identification officer then compares the prints by hand and confirm which one matches the print found at the crime scene.
- Vóila - the suspect has now been identified.
Important Terminology
|
|
|
Terminology Defined
- Friction Ridge: Tiny ridges on the fingertips and toes that are used for gripping.Latent Prints: Fingerprints that are invisible to the naked eye.
- Non-porous: A surface that is impenetrable to water or air (e.g., glass, metal, or plastic).
- Dusting Powder: A fine powder (often aluminum based) used to reveal fingerprints left on non-porous surfaces like glass or doors.
- Cyanocrylate: An adhesive commonly known as superglue. The fumes from this substance stick to the moist surfaces on a latent fingerprint, making a visible white layer.
- Luminescent: Something that emits light.
- Porous: A surface that can be penetrated by water or air (e.g., paper or cardboard)
- Ninhydrin: A chemical solution used to reveal latent fingerprints on a porous surface like wood or paper. When ninhydrin is applied to the surface the fingerprint is revealed as a purple image. Very similar to but slower and less effective than DFO application.
- DFO: A chemical solution used to reveal latent fingerprints on a porous surface like wood or paper. When DFO is applied to the surface the fingerprint is revealed as a light pink image.
- Physical Developer: A chemical solution similar to one used to develop photographs. Sometimes used to reveal latent fingerprints left on porous surfaces.