Latent Print and Impression Evidence
Latent Print and Impression Evidence
This domain tests the detection, enhancement, and recovery of latent prints and impressions, and the decision sequence that preserves evidence for comparison.
Core rules
- Match the process to the surface. Porous surfaces such as paper, cardboard, and raw wood take amino acid reagents such as ninhydrin, DFO, or physical developer. Non-porous surfaces such as glass, metal, and plastic take powders, cyanoacrylate fuming, vacuum metal deposition, or small particle reagent.
- Process sequentially from least destructive to most destructive; photograph before and after each step.
- Blood-contaminated prints need protein stains such as amido black or leucocrystal violet after appropriate fixation, not powder first.
- Footwear and tire impressions: photograph with scale, then cast with dental stone or lift dust with gelatin lifters.
- Document location, orientation, and substrate for every impression.
Worked example
A questioned document on porous paper. Do not powder it. First examine under oblique light and photograph; then use DFO or ninhydrin in a humidity-controlled chamber, followed by heat. The reasoning: powders sit on the surface and can clog pores, blocking the amino acid reaction; a porous substrate requires a chemical that reacts with residue within the paper. If the print is in blood, use a protein stain after fixing the blood.
Traps
- Powdering porous surfaces first.
- Applying cyanoacrylate to a surface already coated with powder.
- Lifting before photographing.
- Casting a wet impression without a release agent.
- Failing to record the enhancement sequence.