Fingerprints are permanent, unique, and universal markers of human identity. They are the distinctive ridge outlines that appear on the inside of the end joints of fingers and thumb and form an important piece of evidence found at every crime scene. They are called the visiting card of a visitor, which proves the presence of a person at a particular crime scene.
Fingerprints based on their encounter at the crime scene can be categorized into-visible, plastic, and latent fingerprints.
Visible or patent fingerprints are prints that can be easily identified by the naked eye, such as fingerprints stained with blood or paint or mud or any other such matrix. Plastic prints are three-dimensional fingerprints produced on soft surfaces like butter, mud, etc.
Both these types of fingerprints are easy to identify and locate at the crime scene. The real problem is with the latent fingerprints which are invisible to the naked eye and are present in abundance at the crime scene.
What are Latent Prints?
The word latent means ‘hidden’ or ‘unseen’, and latent fingerprints can be defined as the invisible markings or prints of oily matter or perspiration from the skin glands, left upon any surface with which the hands or fingers have been touched.
Latent fingerprint residue is composed of several chemicals exuded from the pores in the fingertips and consists of sweat and will fairly dry after a short period, and the other components of latent fingerprints are solid hence they remain on the surface for a longer period.
How to Develop Latent Prints?
Though the latent prints are invisible, they form a crucial piece of evidence. Therefore it is necessary to develop these prints. The presence of residual material on the prints forms the basis for making the latent prints visible.
There are two categories of methods for developing latent fingerprints, which are categorized as follows-
1. Physical Method for Developing Latent Prints
Physical methods applied to develop latent fingerprints do not use any chemical reaction for visualization and are based on the fact that perspiration and greasy matter retain certain substances without fusion. They include powder methods, iodine fuming, and manga brush methods.
In this method, a variety of fine powders are used, which stick to the fingerprints making them visible and easy to collect. The different types of powders used are as follows-
- Black Powder
The black powder is prepared by mixing charcoal, graphite, and lampblack. It is usually applied to develop the latent fingerprints present on light-colored surfaces such as paper, chinaware, etc.
In contrast to light black powder, which has the propensity to paint or smear the surface, black powder that contains a mixture of heavy metal salts like manganese dioxide and graphite powder is favored when prints are to be produced on absorbent surfaces like paper.
This powder is commonly used for developing latent fingerprints because of its’ low cost, extensive coverage, and reduced substrate painting. However, it cannot be used on permeable surfaces.
- White/Grey Powder
The main constituents of grey/white powder are mercury and chalk which produces excellent results on glassware, silver wares, and plated wares.
However, due to the toxicity of mercury, its use has been reduced and instead, white powders containing titanium oxide, zinc oxide, and gum acacia are used to develop latent fingerprints.
- Red Powder
It is also known as dragon’s blood because it is prepared by finely powdering the red resin from the fruit of a palm that is used in the manufacture of zinc engravings.
The latent fingerprint is developed when the red powder is applied to the suspected surface and then heating it. The powder produces a fine print on the surface where the prints are fresh. It is usually applied to papers with fresh prints.
- Silver Powder
Silver powder is prepared from fine aluminum dust that produces excellent prints on hard surfaces which are polished or varnished. It is also used for developing prints on objects like cellophane, feather, etc.
- Fluorescent Powder
It is not always possible that the latent fingerprint will be produced only on dark or light surfaces. There are numerous surfaces that are multicolored and an analyst cannot use any of the above-mentioned powders to developing latent fingerprints on these surfaces.
Therefore for such surfaces, fluorescent powders are used which work on every multicolored surface, such as magazine covers, calendars, multicolored tins, calendars, etc. The common fluorescence powders used are rhodamine-B and anthracene.
These powders are first on the suspected areas of the surface and moved to a dark room where they are exposed to UV light which produces fluorescence prints. These prints can be then photographed without any interference from the colorful background.
Magnetic Powder and Magna Brush Method
This is a special powder method in which a fine magnetic powder and a bristle-less brush are used.
The magna brush is first brought near the magnetic powder, where the powder is deposited on the tip of the brush due to magnetism. Then the brush is brought near the latent fingerprint where the magnetic powder loses its magnetism and gets deposited on the fingerprints.
The magnetic powder selectively adheres to the latent fingerprints due to its affinity for oils and moisture, leaving the substrate “pain-free.”
It is an easy-to-use method that can be applied on both porous and non-porous surfaces as well as colored surfaces. However, it is less effective on the metallic surface.
Iodine Fuming Method
In this method fumes of iodine are used to develop the latent fingerprints. Though it uses a chemical substance it is included in the physical method as it is not causing any chemical reaction with the constituents of the prints. This is useful on porous surfaces like paper, cardboard, and plaster wall.
In this method, the vapors of iodine fumes are forced on the suspected areas of the surface. The iodine fumes are physically absorbed by the fatty or oily deposits that develop a yellow-brown colored print.
It is advised that the developed prints should be photographed immediately because, with the passage of time, the prints fade due to the release of iodine from the prints.
2. Chemical Method
Chemical methods use the chemical reactions between the different chemical compounds and the constituents of the perspiration altering them directly, which gives certain coloration to the latent fingerprints.
Some of the chemical methods used for the development of latent fingerprints are discussed below:
Silver Nitrate Development
This method is based on the reaction between Silver nitrate (AgNO3) and chloride salts in the skin.
When silver nitrate solution, 3% (0.75 g AgNO3 in 25 mL water) is applied to the latent prints, it reacts with the chlorides to form silver chloride, which turns grey when exposed to UV light.
The developed prints must be photographed immediately as the prints may fade with the passage of time. This method gives excellent results on paper, cardboard, plastics, unvarnished and light-colored woods, etc.
It has been found that prints, as old as two years old can be developed by this method.
Ninhydrin is the most widely used chemical reagent for the detection of those latent fingermarks, which are not visualized by the iodine fuming or silver nitrate methods.
Ninhydrin reacts with the amino acid present in the perspiration followed by heating and produces a pink/purple print. After the ninhydrin is sprayed on the suspected surface, the sample is directed to a heating chamber where it is heated at a temperature up to 80o-110o Celsius for 10 minutes.
This method is applicable on porous surfaces such as paper, cardboard, etc.
DFO is another method based on the reaction with amino acids of perspiration. The reagent DFO (1,8-diazafluoren-9-one) in reaction with the amino acids produces pale pink or purple color.
The surface containing the latent fingerprint is immersed into the DFO solution for 5 seconds, dried, and again reimmersed followed by heating in a heating chamber at 110 degrees Celsius for 10 minutes.
Under white light pale pink-purple prints are produced and when the prints are illuminated to the light of wavelength 470 nm the prints show fluorescence.
Similar to ninhydrin this method is also applicable to porous surfaces like paper, wood, cardboard, etc.
Cyanoacrylate Fuming Development
Cyanoacrylate is also known as the superglue method, where on the heating of the superglue, fumes of cyanoacrylate are produced. When these fumes come in contact with the latent fingerprints, a reaction between cyanoacrylate and fatty acids in perspiration.
The reaction produces a white residue on the surface when heated for 10-15 minutes. This method is applicable on non-porous hard surfaces.
Latent fingerprints are the chance prints that are left by a person at a particular place unknowingly. These invisible prints are of crucial importance in forensic science because these prints are not visible to the naked eye and there is very less possibility that these prints will be washed off by the criminal at the crime scene.
Considering the significance of latent prints, it becomes an important step to develop these invisible fingerprints. Forensic analysts utilize a lot of methods to develop the latent fingerprints present on various surfaces.
The basic principle behind the development of latent prints is based on the interaction (physical and chemical) of the developing agents with the various constituents of perspiration (salts, oils, and amino acids).
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