Forensic Fingerprint Analysis

Fingerprint Collection for Forensic Use

Recovering fingerprints, even partial fingerprints, from a crime scene is essential to a criminal investigation. Forensic fingerprint analysis is used to identify suspects and solve crimes. As such, proper fingerprint collection is considered one of the most critical parts of forensic analysis and an extremely valuable tool used by law enforcement agencies around the world.

This document provides an overview of the types of fingerprints collected, the methods of lift fingerprints, and fingerprints development or visualization methods.

Types of Fingerprints

There three major Categories of Fingerprints: Loop Fingerprints, Whorl Fingerprints, and Arch Fingerprints. The total number of different types of fingerprints that make up the three (3) categories is eight (8). Each of the eight (8) types of fingerprints is unique. Furthermore, no two people have the exact same fingerprint. The three (3) categories and eight (8) types of fingerprints are as follows:

Loop Fingerprints

Loop fingerprints are the most common type of fingerprint. Sixty to seventy percent (60% – 70%) of all prints are a type of loop fingerprint. Loop fingerprints are called loop fingerprints because the prints recurve to form a loop shape. They are characterized by a circular or oval-shaped pattern.

There are three (3) subcategories of loop fingerprints:

Ulnar Loops
Double Loops
Radial Loops

An Ulnar Loop has a circular pattern that runs from the thumb toward the pinky finger. The name Unlar Loop is a reference to the ulna bone.

A Double Loop has two separate loop formations and at least one recurving ride within the inner pattern.

A Radial Loop is one that joins the hand on the same side as the thumb and flows in a downward slope from the pinky finger toward the thumb. The name Radial Loop is a reference to the radium bone.

Whorl Fingerprints

Next after Loop Fingerprints, Whorl Fingerprints are the second most common type of fingerprint. They make up about thirty percent (30%) of all fingerprints. Whorl Fingerprints are characterized by its spiral-like pattern. A whorl is a pattern of spirals or concentric circles.

There are three (3) subgroups of Whorl Fingerprints:

Plain Whorl
Accidental Whorl
Central Pocket Loop Whorl

Plain Whorl Fingerprints include concentric circles, with one complete circle and two deltas.

Accidental Whorl Fingerprints are irregularly shaped. That’s why they’re called “Accidental”.

Central Pocket Loop Whorl Fingerprints are a loop with a whorl at the end of it. The whorl can be oval, circular, or spiral.

Arch Fingerprints

The least common type of fingerprint is Arch Fingerprints. They make up about ten percent (10%) of all fingerprints. Arch Fingerprints are characterized by its wave-like pattern.

There are two (2) different types of Arch Fingerprints:

Plain Arch
Tented Arch

In a Plan Arch Fingerprint, the pattern begins on one side and slightly cascades in an upward direction.

In a Tented Arch Fingerprint, the arch is in the center ridges and does not show a continuous arch.

Now that we know the different types of fingerprints, let’s take a look at the different methods to lift fingerprints for forensic analysis.

Methods to Lift Fingerprints

There was a time when the most common method used to “lift” fingerprints from a crime scene was to use powder and a brush to reveal the fingerprint on a surface. This method is still used today. However, fingerprint forensics have advanced. While there are several new methods to accurately collect fingerprints at a crime scene the most common are:

Latent Prints
Patent Prints
Cyanoacrylate
Chemical Developers

Each method works in different ways and have different levels of usefulness.

Latent Prints

Latent Prints are not visible to the naked eye. That’s why they are the most common type of print collected. However, the process is very messy. If you’ve watched a crime show on television chancer are you’ve seen Latent Print fingerprint collection. The forensic investigator will lightly dust the area where a fingerprint is suspected to be. Powders used to dust the area include:

Magnetic Fingerprint Powder
Black Granular Fingerprint Powder
Aluminum Fingerprint Powder
Fluorescent Fingerprint Powder

The collection of a Latent Print is an effective method of fingerprint collection. However, the oils, grease, and biomatter that are released by human skin pores bind with the powder to create a big mess. It takes a lot of work to clean up after a Latent Print collection. We don’t leave a mess behind. We clean up after ourselves as part of our service to commercial and residential properties, no matter what it takes.

Patent Prints

The Patent Print method involves the simple taking a photograph of what’s visible to the naked eye. For example, a law enforcement officer can take a photograph as evidence if he finds a bloody fingerprint on the wall at the scene of a crime. The issue is whether the photograph is sufficient to accurately identify an individual. The value of a patent print is in its use to document evidence and keep track of where the print was located at the scene of the crime.

Cyanoacrylate

When you think of Cyanoacrylate think of superglue. It’s the same stuff and can be used on most anything. Cyanoacrylate reacts to dyes and fingerprint powders. It works especially well on non-porous surfaces. It’s applied before powders and dyes through fumes. Once applied, the fingerprint can be viewed by using a white light source or oblique ambient light.

Chemical Developers

While some fingerprints can be seen without using any special techniques, others require a chemical reagent to lift it. An example of when a chemical reagent is used is a fingerprint located on a paper surface. Chemicals such as ninhydrin and chemical developers are used to help lift such prints. The chemicals adhere to inorganic salts and amino acids left behind.

As forensic technology evolves, new and better fingerprint collection methods will emerge. In the meantime, Latent Prints, Patent Prints, Cyanoacrylate, and Chemical Developers are the most common and efficient methods used today.

Visualization Methods or Fingerprints Development

Typically, people are nervous when conducting a criminal act. They sweat. Perspiration helps to produce Latent Prints. The visibility of a latent fingerprint depends on:

The physical condition of the bad actor’s skin
The substrate with which contact is made
Temperature and climatic factors
Amount of time passed

Fingerprints are developed using a wide array of physical, chemical, and optimal methods. The method used to develop a fingerprint to be collected depends on:

Surface type. The surface can be porous or non-porous. Porous surfaces are absorbent. Examples of porous surfaces include wood, paper, cardboard, and other forms of cellulose. Non-porous surfaces do not absorb. Examples of non-porous surfaces are painted or lacquered wood, metal, glass, rubber, and plastics.
Condition of the substrate. Is it clean, dirty, greasy, tacky, sticky?
Environmental conditions.
Subsequent forensic examination to be undertaken and the consequences of the development method used.
Amount of time elapsed.

Powder Method for Fingerprint Development

The powder method for fingerprint development is commonly used for the development of a latent fingerprint. This method is one of the oldest and simplest methods available. Latent prints can be visualized using different kinds of powers. The powder method is most often used on non-porous surfaces. A powder that has find particles will physically adhere to the latent print residue on non-porous surfaces. Powder is used because it has an affinity for moisture and adheres to the residue deposited by friction ridges.

Since latent fingerprints are fragile as evidence, the brushes used for dusting powder need to have soft bristles to prevent any damage to the print during development. Typically brushes used for fingerprint development are made with animal hair, feathers, or fiberglass filaments. For obvious reasons, brushes must be kept clean, tangle free, and dry.

Applying Powder to Fingerprints

There are five (5) basic steps to properly apply powder to fingerprints:

“Load” the brush. Apply fingerprint powder on the brush by lightly dipping the tip of the brush into a sterile, wide-mouth container that holds a small amount of powder.
Remove excess powder by shaking the brush.
Prior to applying the powder to all areas, the powder is applied to a small portion, or to a substrate similar in nature, for testing. Testing is done to determine the optimum amount of powder needed for that substrate and to prevent background painting.
Once testing is completed, the suspected area is brushed with light and even strokes.
When the print starts to appear, the brush is moved in the direction of the ridge flow to prevent damage to the print.

Fluorescent Powder

Fluorescent powder is based on the principle of luminescence. It is created by adding laser dye to the binders. The mixture is then allowed to evaporate. Finally, the dried mass is grounded into the latent fingerprint powder. Fluorescent powder is most often used on multicolored backgrounds.

Magnetic, or Magna Powder

This method uses the ferromagnetic properties of iron powder mixed with pigment powders. The powder is applied to the fingerprint area with the help of a magnetized rod that has no bristles. When the magnetic applicated is dipped into the powder a magnetic powder ball is formed. The magnetic powder ball acts as a brush which is much softer than conventional filament brushes and causes less damage to fragile latent prints. The magnetic powder ball serves as an effective carrier of pigment particles and is passed back and forth over the surface to develop latent prints. The disadvantage of magnetic, or magna powder, is it is less effective on ferromagnetic substrates such as steel because of the magnetic attraction which can cause contact between the applicator and substrate damaging the latent print in the process.

Preservation of a Powdered Print

The most common method for preservation of powdered prints is tape lifting. Tape lifting is easy to implement and economical. In the tape lifting technique, a sufficient amount of tape is pulled from the roll. A tab is then made by folding a small portion from the corner to conveniently hold the tape. Then the tape is applied to the print. It is important to make sure no air bubble is formed when the tape is applied. The forensic investigator will rub the applied tape gently to make sure there is good adhesion to the print. Once this is done, the tape is pulled off and transferred to a contrasting background, such as black fingerprint powder placed on a white backing card. Hinge lifter is another form of adhesive lifting materials. As a precaution, is always recommended that a dust mask is worn, or work done on a downdraft table.

Chemical Method of Development

Ninhydrin Method

The Ninhydrin Method is used to develop latent prints on paper or other porous substances. Sweat has small but detectable amounts of amino acid and amino acids have high affinity for cellulose. When our fingers come into contact with paper, a small amount of sweat (containing amino acid) gets impregnated on the paper. The amount of amino acid present in the print reduces over time. For this reason, its best to develop the print as soon as possible.

Ninhydrin is a non-specific amino acid reagent. It is highly suitable for the development of latent prints. When ninhydrin reacts with the amino acids that are present in sweat it forms ruhemann’s purple along with aldehyde derivative of the amino acid and carbon dioxide as byproducts. Prints are developed in purple reddish-brown color. Ruhemann’s purple degrade in the presence of oxygen and light. Therefore, it is recommended to store the developed print under cool and dark conditions. Developed prints should be photographed before storing. A green filter is used for better contrast on colored backgrounds.

Cyanoacrylate Fuming

Cyanoacrylate Fuming is used for developing prints on non-porous surfaces. When the surface that is suspected to have a latent print is exposed to vapor of ethyl or methyl cyanoacrylate it produces a white deposit of cyanoacrylate polymer. The white deposit can be enhanced using fluorescent dyes.

Silver Nitrate Method

The Silver Nitrate Method is used for developing prints on porous surfaces such as documents, currency, and unpainted wood. Silver nitrate reacts with sodium chloride present in sweat and forms an unstable white silver chloride. When exposed to sunlight, silver chloride breaks into components (silver and chlorine) giving reddish brown prints.

Iodine Fuming Method

Prints containing fats or oils are developed using iodine fuming method. When iodine crystals are heated, they turn into vapors without turning into liquid phase (sublimation) and the fumes generated reacts with the fatty acids or oils present, providing yellowish brown prints. Once treating the surface with iodine fumes is stopped, the developed print starts to fade away. Therefore, prints should be photographed as soon as they are developed.