Definition of Contamination of Forensic Sample

Forensic sample contamination is a serious consideration when collecting evidence.

Juries expect to see forensic evidence before they make a decision in a case which relies on that evidence. Because of this, attorneys on both sides try to discredit forensic evidence that does not support their clients' interests. This requires crime scene investigators be especially careful to guard against contamination in the forensic samples taken from a crime scene.

Definition

A forensic sample is contaminated when any trace materials are added to a crime scene after the crime is committed. This can happen before, during and after authorities take samples of the evidence from a scene. Many people can contaminate the evidence at a crime scene, including witnesses, suspects, victims, emergency response personnel, fire fighters, police officers and crime scene investigators.

Consequences

There are many serious consequences for forensic sample contamination. First, the evidence could be thrown out because of contamination, and the case could be lost because of a lack of evidence. Perhaps a more serious consequence is the possibility that the wrong person could be convicted of a crime. The Innocence Project, which freed 258 people who had been wrongfully incarcerated through DNA testing, found that at least 50 percent of the cases convicted the wrong person because of sloppy, forensic evidence.

Scan the Scene

Many things can be done to make sure evidence in a crime scene is not contaminated. Most important, the scene should be carefully evaluated before people enter the scene. After a crime scene investigator has evaluated the scene, he can decide what samples need to be taken and coordinate other people entering the scene.

Avoid Contamination

Tools used to collect evidence should be carefully cleaned between uses. Items like a fingerprint brush and even the powder used to dust for fingerprints, can hold DNA from a previous crime scene. In fact, crime scene consultant Dick Warrington suggests a new fingerprint brush should be used for each case and the brush itself should become evidence after it is used. Warrington also says it is important for investigators to change their gloves every time they think the pair they are wearing has been compromised. Investigators can compromise their gloves in many ways, from touching blood to scratching their face or covering their mouth when they cough.

References

• USA Today: 'CSI effect' has juries wanting more evidence
• Forensic Magazine: Crime Scene Contamination Part 1
• Forensic Magazine: Crime Scene Contamination Part 2
• The Innocence Project: Unvalidated or Improper Forensic Science