Forensic scientists collect or process trace evidence such as hair, skin, blood or semen samples that is found at crime scenes.

Crime scene evidence can include a wide variety of substances such as hair, bodily fluids and fibres.

Collecting evidence

When a crime is discovered, the scene is examined in order to look for clues that will identify suspects and provide evidence for the courts. In New Zealand, evidence is collected from crime scenes by Police officers or scientists from Environmental Science and Research (ESR). They must follow strict guidelines while gathering evidence so that samples are not contaminated or degraded and their analysis is admissible in court.

Detecting evidence

Crime scene evidence can include a wide variety of substances such as hair, bodily fluids, fibres, paint chips, soils or gunshot residue. For substances to be useful as evidence they are usually compared to similar items from suspects. Because of this, particular care is taken to ensure all substance are collected carefully and kept free of contamination.

In some cases, forensic scientists use biotechnology techniques to help detect important evidence. For example, a chemical called luminol, which glows brightly in the presence of blood, is used to detect small amounts of blood that are not visible to the naked eye.

Analysing evidence

Evidence at a crime scene may only be found in small, trace amounts so forensic scientists use a variety of techniques including microscopic analysis, mass spectrometry, chromatography and DNA analysis.

Once samples have been collected from a crime scene, ESR technicians carries out forensic analysis. They might analyse skin, blood or urine looking for the presence of drugs or process DNA evidence in the hope of identifying someone. Find out more in the articles Forensics and DNA and DNA profiling.

Interpreting evidence

The results of these analyses need to be interpreted. This could mean comparing them to a standard reference, such as legal blood-alcohol limits for driving a car, or comparing DNA profiles of victims or suspects. New Zealand has a national DNA databank where DNA profiles are stored.

Presenting the findings

The findings may be presented in court either as a written report or by ESR scientists acting as expert witnesses.

Activity idea

Use the activity DNA detective and Mobile forensic kit – unit plan to explore the use of forensics in crime scene investigations.

Published 16 November 2007, Updated 29 November 2016