Contamination in Forensic Science

In 1991, hikers discovered a body in a mountain pass as a glacier thawed. Initially, it was thought to be modern. On further study, scientists learned the corpse was over 5000 years old. The man was nick-named ‘Ötzi’. Of course, the genome was mapped. It strangely suggested that Ötzi had Steppe ancestry. However, the Steppe people entered Europe more recently. A new study of the genome has removed that Steppe ancestry. Modern DNA contaminated the first sample. Even this important find tracing human ancestry,  had impurities inadvertently added to the sample (Marshall, 2023). If a study this important was defective, how do we avoid contamination in Forensic Science?

What is Contamination?

When a case is presented before a court of law, everyone must be absolutely certain that the results are correct. Due to the sensitivity of modern tests, contamination is becoming more of an issue. But what is contamination? The two types are contamination and cross-contamination.

Contamination

Contamination is defined as additional material added to the evidence from a different source.

Cross-Contamination

Cross-contamination is when material from one piece of evidence becomes mixed with material from a second piece of evidence (GFJC, 2013).

The Forensic Science Regulator in the UK makes the further distinction for DNA analysis. The additional material must be added ‘at or after the point when a controlled forensic process starts’. In many crime scenes, background DNA is present. This must be compensated for so that it does not interfere with the investigation (Rustidge, 2021). But Contamination in Forensic Science does not just involve DNA, all tests and processes might be subject to impurities.

Where does contamination take place?

If contamination happens after the start of the forensic process, how can it occur at all? The guidelines on evidence collection were put in place to prevent just that. To start with, forensic scientists are human and errors take place. Problems take place at any point in the process, whether it be incorrect collection, put in the wrong packaging, transported in a way that damages the packaging, stored incorrectly, or even during analysis. This is the reason for all the Best Practice Guidelines, to minimize rather than prevent potential contamination (Baldwin, 2017).

How does evidence become contaminated?

When a large number of people trample over a crime scene, it is more likely to yield defective evidence.

Image 1: A crime scene from before the guidelines came into practice.

More people on a scene can increase doubt as to whether or not the footwear marks are valid evidence. If we apply Locard’s Exchange Principle, that ‘Every Contact Leaves a Trace’ (Fuller, 2023), then each person who enters the scene can potentially leave a small amount of incorrect evidence. As I mentioned earlier, the increasing sensitivity of forensic tests can lead to false leads. Or, worse still, the additional material could cover up the evidence that would lead to potential suspects (Baldwin, 2017).

Wind and rain can destroy the evidence at a crime scene. On a hot day, bacteria will build up in a blood sample causing deterioration. Rain could dilute a sample below the limits of the test boundaries (Baldwin, 2017).

Crimes which involve firearms are particularly subject to cross-contamination. If a person is arrested by police who use firearms, then there is a high possibility to transfer Gun Shot Residue (GSR) to the arrestee. Even other police officers who work the same shift as firearms officers can be contaminated and then pass on that contamination to anyone they arrest (Cook, 2016). Chemical tests for GSR are very sensitive and regularly provide false positives.

Contamination Case Study: Shirley McKie

In 1997, Shirley McKie was an officer in Police Scotland. Her role was to stand outside a crime scene and preserve evidence. However, fingerprint evidence suggested she had entered the crime scene. She swore she had never entered the crime scene. However, she was suspended from the police force.

Image 2: Fingerprints

It was later proven that the finger mark found at the scene was not hers, but not before the muddle of evidence collection had led to her arrest and trial. She was acquitted but was never reinstated to the job she loved. Because of the faulty evidence, the suspect’s conviction was declared unsafe and he was released from prison (McDermid, 2014).

What can be done to reduce potential contamination?

Properly controlling a crime scene is the most successful way of reducing potential contamination.

Controlling the crime scene

Surrounding the scene in barrier tape and recording all those who enter is just the first step. A Crime Scene Manager should assess the site for a Common Approach Path. And all those entering should wear the correct Personal Protective Equipment (PPE)  to reduce the excess fingerprints and trace evidence of hair and fibers. This is discussed in more detail in a previous article.

Transferring evidence to the laboratory

Evidence should always be packaged in new, not previously used containers. They should be sealed and labeled immediately at the crime scene. Even the packaging should be handled with disposable gloves. This prevents another source of contamination when the packaging is re-opened for analysis. All items should be correctly stored appropriately for the evidence. Some items will need drying, others freezing to retain the evidence potential. Some evidence will be in paper packets, while others will be in plastic bags. Each type of container is chosen specifically to preserve the evidence and prevent contamination.

Image 3: The correct packaging for powdered drugs.

Even after analysis, the evidence should be resealed, in the same packaging if possible, as the sample may require re-analysis (Baldwin, 2017).

Contamination of DNA Evidence

DNA evidence is considered a gold standard for conviction, but this can only be true if the courts are convinced no contamination or cross-contamination has occurred. To be sure of the effectiveness of anti-contamination procedures in a laboratory, databases of the analyst’s DNA can be utilized.

Case Study: DNA contamination

A man was charged with a sexual assault but denied it. His excuse was that he was arrested for a different alleged assault and could not have been at the first one. Re-evaluation of the evidence placed the suspect’s DNA at only one of the crime scenes. During the re-analysis of both cases, it proved that the same instruments were used for both cases and could have caused cross-contamination (Avenell, 2022). Thankfully, the initial evidence was stored correctly, preventing further contamination.

If the courts are to believe the evidence, then best practice must reduce the potential for contamination in Forensic Science analysis.

Finally

In the first case study, that of Shirley McKie, the suspect was released from prison because the information on fingerprints was faulty. The second case study allowed the police to focus their efforts away from an incorrect suspect. Contamination in forensic science leads to false convictions and suspects going free. Evidence must be as pure as possible, to retain the confidence of the courts of law, and also for archaeological data.

And, hopefully, the next time an ancient human is discovered, contamination will not attempt to rewrite history.

References

Avenell, P. (2022) The Risk of Contamination in Forensic DNA Profiling. Forensic Access. Jan 06. https://www.forensic-access.co.uk/news/insight/the-risk-of-contamination-in-forensic-dna-profiling/#:~:text=Contamination%20could%20occur%20at%20the,transfer%20via%20an%20intermediate%20surface

Baldwin, H.B. & May, C.P. (2017) Crime Scene Contamination Issues. Crime Scene Investigator Network. Aug 18. https://www.crime-scene-investigator.net/crime-scene-contamination-issues.html

Cook, M. (2016) Gunshot Residue Contamination of the hands of Police Officers following start-of-shift handling of their firearm. Forensic Science International. Vol 269. pp 56-62. Dec. https://www.sciencedirect.com/science/article/abs/pii/S0379073816304789?via%3Dihub

Fuller, J. (2023) How Locard’s Exchange Principle Works. How stuff works. https://science.howstuffworks.com/locards-exchange-principle2.htm

GFJC. (2013) A Simplified Guide to Crime Scene Investigations. Global Forensic and Justice Center. https://www.forensicsciencesimplified.org/csi/glossary.html

McDermid, V. (2014). Forensics: The Anatomy of Crime. Profile Books LTD.

Marshall, M. (2023) Ötzi the iceman may have been bald. New Scientist. Aug 26.

Rustidge, M. (2021) Reducing Contamination in Forensic Investigations. AZO Life Sciences. Aug 11. https://www.azolifesciences.com/article/Reducing-Contamination-in-Forensic-Investigations.aspx