Forensic Science: Evidence handling & chain of custody

Moist biological material sealed in plastic promotes bacterial and fungal growth that degrades DNA. Air-drying away from heat and sunlight, then packaging in breathable paper, preserves the stain.

Chain of custody records every person who handled an item, when and why, proving the exhibit presented in court is the same, unaltered item that was collected.

A label must uniquely link the item to its case and record who collected it, what it is and when, so the item can be tracked and identified reliably.

EDTA chelates magnesium ions needed by nucleases and prevents clotting, protecting DNA. Formalin damages DNA and clot activator tubes complicate extraction.

Separate packaging keeps trace material, stains and cells from moving between exhibits, which could otherwise create false associations.

Photographs record the location, orientation and state of evidence in situ, supporting scene reconstruction and later testimony.

Keeping the original seal intact shows it was not broken. The new opening is resealed with tamper-evident tape, initialled and dated, extending the custody record.

Gloves can transfer cells and body fluids from one item to another. Changing them between items prevents cross-contamination.

Forceps avoid adding the collector's DNA. A folded paper packet (druggist fold) securely contains small trace items without moisture build-up.

Known samples come from an identified source and are compared with questioned samples of unknown origin recovered from the scene or victim.

Edmond Locard proposed that every contact leaves a trace. This underpins the search for transferred fibres, hairs, cells and other trace evidence.

Volatile hydrocarbons escape from porous containers and diffuse through polyethylene. Clean, unused metal cans, partly filled and sealed airtight, retain vapours for headspace analysis in the laboratory.

Tamper-evident tape, signed or initialled and dated across the seal edge, makes any unauthorised opening visible and identifies the person who sealed it.

Sharps go into rigid, puncture-resistant containers, sealed and marked with biohazard and sharps warnings, to protect handlers and preserve residues. Needles are not recapped by hand because of needlestick risk.

Amplicons are present in huge copy numbers. Separating pre- and post-amplification work, with dedicated equipment and unidirectional movement, stops them contaminating new samples.

A reagent blank contains all reagents but no sample. Any DNA detected in it indicates contamination from reagents, consumables or handling during that batch.

A lightly moistened sterile swab lifts dried cells. The swab is air-dried before packaging in a breathable container to prevent microbial degradation.

A break in custody raises doubt that the item is authentic or unaltered. The defence can challenge admission or argue the evidence deserves less weight.

Freezing minimises nuclease activity and hydrolysis. Extracts are stored frozen in closed tubes, avoiding repeated freeze-thaw cycles.

A substrate control shows whether the material itself gives a reaction or carries background DNA, helping interpret results from the stained area.