Biological and Trace Evidence Handling
Biological and Trace Evidence Handling
This domain tests contamination avoidance and recovery of DNA, blood, semen, saliva, hair, fibers, paint, glass, and gunshot residue.
Core rules
- Wear gloves, mask, and hairnet; change gloves between items. DNA contamination is cumulative and invisible.
- Air-dry biological stains and package them in paper. Swab dried stains with a sterile, slightly moistened swab; prepare a control swab using the same water.
- Collect reference samples, such as known buccal or blood, separately and label them.
- Trace evidence: use clean forceps, tape lifts, or vacuum filters; package in paper, druggist folds, or rigid containers. Clean or change tools between items.
- Collect gunshot residue with stubs before packaging or moving the subject; avoid cross-contamination.
Worked example
A dried bloodstain on a windowsill. Photograph with scale, then swab with a sterile swab moistened with distilled water using a circular motion; air-dry the swab, place it in a paper envelope, seal, and label. Prepare a substrate control swab from an adjacent unstained area. The reasoning: the control shows whether the substrate or the water contributed DNA; the moist swab improves transfer from a dry surface, but excess water dilutes the sample.
Traps
- Talking or coughing over evidence.
- Reusing swabs or forceps.
- Missing substrate controls.
- Packaging wet swabs in plastic.
- Collecting gunshot residue after the person has changed clothes.