UNIT-2
Development of Fingerprints
1. Latent Prints
Definition: Latent prints are invisible fingerprint impressions left by friction ridge skin when it comes into
contact with a surface. These prints are called "latent" because they require specialized techniques to be
visualized.
Characteristics
- Found on various surfaces like glass, paper, plastic, and metal.
- Their visibility depends on the material of the surface and the quality of the residue left by the finger.
Importance in Forensics
- Unique to every individual, including identical twins.
- Used for personal identification and crime-solving.
Constituents of Sweat Residue
Eccrine Sweat
- Secreted from eccrine glands distributed across the body.
- Primary components: Water (~98%), salts (sodium and potassium chloride), urea, lactic acid, and amino
acids.
Sebaceous Sweat
- Secreted from sebaceous glands found near hair follicles.
- Primary components: Lipids like triglycerides, fatty acids, and wax esters.
Apocrine Sweat
- Produced in specific areas (armpits, groin).
- Contains proteins, carbohydrates, and pheromones.
Sweat residue plays a crucial role in forming latent fingerprints. When a person touches a surface, the
friction ridges on their fingers leave behind tiny amounts of sweat and oils. This residue contains water,
salts, amino acids, lipids, and other substances that can form a fingerprint pattern. Although the print itself is
invisible (latent), the sweat residue is key to revealing the print through various detection methods, such as
powder dusting, chemical reactions (e.g., ninhydrin), or cyanoacrylate fuming. These residues allow
investigators to visualize, lift, and preserve the fingerprint for identification.
Role in Fingerprint Detection
- The chemical composition of sweat residue interacts with detection reagents (e.g., amino acids react with
ninhydrin, salts with silver nitrate).
Latent Fingerprints Detection by Physical and Chemical Techniques
Physical Techniques
1. Powder Dusting
- Common powders include black powder (charcoal-based), white powder (titanium dioxide), and
fluorescent powders.
- Powders adhere to sweat or oil residues, revealing the print.
- Best suited for smooth, non-porous surfaces like glass, plastic, or metals.
, 2. Magnetic Powders
- Require no brush, reducing surface damage.
- Adhere to prints on non-porous surfaces without smudging.
Chemical Techniques
1. Ninhydrin
- Reacts with amino acids in sweat to produce a purple print (Ruhmann’s purple).
- Commonly used on porous surfaces such as paper or cardboard.
Here are the short steps involved in using ninhydrin to detect latent fingerprints:
1. Prepare the Surface: Ensure the item with potential latent prints is clean and dry. Ninhydrin works best
on porous surfaces like paper, cardboard, or wood.
2. Apply Ninhydrin: Lightly spray or immerse the surface with ninhydrin solution. Ensure even coverage.
3. Allow Development: Let the surface air dry. Ninhydrin reacts with amino acids in sweat from the
fingerprint, turning the print purple or blue over time.
4. Heat or Wait: To accelerate development, gently heat the surface (using a heat gun or drying oven) if
needed. Otherwise, wait for the reaction to fully occur.
5. Photograph the Print: Once the print appears, photograph it for documentation before it fades.
6. Preserve: If necessary, lift the print using tape or other appropriate methods, or simply preserve the item
for further analysis.
2. Silver Nitrate
- Reacts with sodium chloride in sweat to form silver chloride, which darkens under UV light.
- Suitable for porous surfaces.
3. Cyanoacrylate Fuming
- Superglue vapors polymerize on sweat residues, creating a hard, white deposit.
- Effective for non-porous materials like glass, metal, and plastic.
4. Iodine Fuming
- Sublimed iodine interacts with lipid residues, producing a yellowish-brown temporary print.
- Stabilized using starch to preserve visibility.
5.Small Particle Reagent (SPR)
- A liquid suspension containing molybdenum disulfide or other particles.
- Effective for detecting prints on wet or greasy surfaces.
Mechanism of Detection Using Different Developing Reagents
- Powders Fine particles adhere to moisture, grease, or oils in fingerprints via physical interactions (adhesion
or van der Waals forces).
- Ninhydrin: A chemical reaction occurs between the free amino acids in sweat and ninhydrin, producing a
visible purple compound.
- Cyanoacrylate Fuming: Heat accelerates the release of cyanoacrylate vapors, which polymerize in the
presence of moisture, coating fingerprint ridges.
- Silver Nitrate: Forms silver chloride upon reacting with salts in sweat. Exposure to light reduces silver
chloride to metallic silver, making the print visible.
Development of Fingerprints
1. Latent Prints
Definition: Latent prints are invisible fingerprint impressions left by friction ridge skin when it comes into
contact with a surface. These prints are called "latent" because they require specialized techniques to be
visualized.
Characteristics
- Found on various surfaces like glass, paper, plastic, and metal.
- Their visibility depends on the material of the surface and the quality of the residue left by the finger.
Importance in Forensics
- Unique to every individual, including identical twins.
- Used for personal identification and crime-solving.
Constituents of Sweat Residue
Eccrine Sweat
- Secreted from eccrine glands distributed across the body.
- Primary components: Water (~98%), salts (sodium and potassium chloride), urea, lactic acid, and amino
acids.
Sebaceous Sweat
- Secreted from sebaceous glands found near hair follicles.
- Primary components: Lipids like triglycerides, fatty acids, and wax esters.
Apocrine Sweat
- Produced in specific areas (armpits, groin).
- Contains proteins, carbohydrates, and pheromones.
Sweat residue plays a crucial role in forming latent fingerprints. When a person touches a surface, the
friction ridges on their fingers leave behind tiny amounts of sweat and oils. This residue contains water,
salts, amino acids, lipids, and other substances that can form a fingerprint pattern. Although the print itself is
invisible (latent), the sweat residue is key to revealing the print through various detection methods, such as
powder dusting, chemical reactions (e.g., ninhydrin), or cyanoacrylate fuming. These residues allow
investigators to visualize, lift, and preserve the fingerprint for identification.
Role in Fingerprint Detection
- The chemical composition of sweat residue interacts with detection reagents (e.g., amino acids react with
ninhydrin, salts with silver nitrate).
Latent Fingerprints Detection by Physical and Chemical Techniques
Physical Techniques
1. Powder Dusting
- Common powders include black powder (charcoal-based), white powder (titanium dioxide), and
fluorescent powders.
- Powders adhere to sweat or oil residues, revealing the print.
- Best suited for smooth, non-porous surfaces like glass, plastic, or metals.
, 2. Magnetic Powders
- Require no brush, reducing surface damage.
- Adhere to prints on non-porous surfaces without smudging.
Chemical Techniques
1. Ninhydrin
- Reacts with amino acids in sweat to produce a purple print (Ruhmann’s purple).
- Commonly used on porous surfaces such as paper or cardboard.
Here are the short steps involved in using ninhydrin to detect latent fingerprints:
1. Prepare the Surface: Ensure the item with potential latent prints is clean and dry. Ninhydrin works best
on porous surfaces like paper, cardboard, or wood.
2. Apply Ninhydrin: Lightly spray or immerse the surface with ninhydrin solution. Ensure even coverage.
3. Allow Development: Let the surface air dry. Ninhydrin reacts with amino acids in sweat from the
fingerprint, turning the print purple or blue over time.
4. Heat or Wait: To accelerate development, gently heat the surface (using a heat gun or drying oven) if
needed. Otherwise, wait for the reaction to fully occur.
5. Photograph the Print: Once the print appears, photograph it for documentation before it fades.
6. Preserve: If necessary, lift the print using tape or other appropriate methods, or simply preserve the item
for further analysis.
2. Silver Nitrate
- Reacts with sodium chloride in sweat to form silver chloride, which darkens under UV light.
- Suitable for porous surfaces.
3. Cyanoacrylate Fuming
- Superglue vapors polymerize on sweat residues, creating a hard, white deposit.
- Effective for non-porous materials like glass, metal, and plastic.
4. Iodine Fuming
- Sublimed iodine interacts with lipid residues, producing a yellowish-brown temporary print.
- Stabilized using starch to preserve visibility.
5.Small Particle Reagent (SPR)
- A liquid suspension containing molybdenum disulfide or other particles.
- Effective for detecting prints on wet or greasy surfaces.
Mechanism of Detection Using Different Developing Reagents
- Powders Fine particles adhere to moisture, grease, or oils in fingerprints via physical interactions (adhesion
or van der Waals forces).
- Ninhydrin: A chemical reaction occurs between the free amino acids in sweat and ninhydrin, producing a
visible purple compound.
- Cyanoacrylate Fuming: Heat accelerates the release of cyanoacrylate vapors, which polymerize in the
presence of moisture, coating fingerprint ridges.
- Silver Nitrate: Forms silver chloride upon reacting with salts in sweat. Exposure to light reduces silver
chloride to metallic silver, making the print visible.
