20 f latent prints (student version)
- 1. Latent Fingerprinting Methods and Techniques This presentation includes mildly graphic pictures
- 2. Behind the Prints Skin has two layers: • epidermis (upper/outer) • dermis (lower/inner) Feet and hands have specialized skin called friction ridge skin. These ridges originate in the dermis. Skin contains three types of glands: • eccrine • sebaceous • appocrine
- 3. Latent Print Identification • there are three types of prints potentially found at a crime scene visible plastic latent • from fingers, palm, or feet (thumb print is best for identification) • fingerprints are unique to an individual • fingerprints fall into 3 major categories: arches (5%), loops (60- 65%), and whorls (30-35%)
- 4. Fingerprints Methods of fingerprint identification rely on materialMethods of fingerprint identification rely on material present in prints:present in prints: a. Fatty acids “oils” C-C-C-C-C-C-C… b. Amino acids -- from protein c. Urea -- found in urine and perspiration d. Inorganic salts -- sodium chloride (NaCl), perspiration Each print contains about a microgramEach print contains about a microgram ((µµg) of material depending on pressure andg) of material depending on pressure and glandular activityglandular activity
- 5. Fingerprints There are three common techniques used toThere are three common techniques used to visualize latent fingerprints:visualize latent fingerprints: a. Powders b. Fuming c. Dips or Sprays
- 6. Powders Most common identification method partly because they are cheap and fast Powders stick to the oily components of the print Black and silver most common depending on background although fluorescent and magnetic powders are fast becoming preferred Must lift print with special tape or photograph
- 7. Fuming Iodine • iodine undergoes sublimation (solid gas) • iodine is non-polar – dissolves in the oil of fingerprints Superglue/Cyanoacrylate • cyanoacrylate dissolves in the oils of the print to form very visible white ridges • very sensitive method
- 8. Fuming Iodine • iodine undergoes sublimation (solid gas) • iodine is non-polar – dissolves in the oil of fingerprints Superglue/Cyanoacrylate • cyanoacrylate dissolves in the oils of the print to form very visible white ridges • very sensitive method
- 9. Dips or Sprays Ninhydrin • intense purple good for light colored backgrounds • sensitive to the amino acid or urea components of prints (eccrine secretions) • most widely used method for latent print detection from paper
- 10. Dips or Sprays Silver Nitrate • the silver reacts with chloride (in sweat to form silver chloride which then decays into silver producing a permanent black stain in UV (ultraviolet) light AgNO3 + Cl - AgCl Ag (in UV) • good for wood products, paper products, leather, and many metal surfaces (including ammunition and casings) • silver nitrate is generally used as a last resort technique
- 11. Dips or Sprays Crystal Violet • also called gentian violet • stains fatty acids and protein components of fingerprints • purple solution is best for detecting latent prints on all types of tape (duct, Scotch, masking, electrical, etc) • permanent stain is dissolved in alcohol before use Amido Black • black protein dye best for blood-contaminated latent prints • good for non-porous surfaces (metal, plastic, glass) • corrosive – it will damage surfaces, collect samples of blood for typing or DNA fingerprinting first
- 12. Dips or Sprays Fluorescent / Luminescent Chemicals • fluorescent – reradiation of light; luminescent – produces light by chemical means • dyes adhere to fatty acids; most common are Rhodamine and Ardrox (both fluorescent) • extreme sensitivity • must be viewed under alternate light source (black light, UV, laser) before after
- 13. Superglue fuming, fluorescent powder, UV light, orange filter These techniques can be used in combination…