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Plastic can be evidence in murder case, UvA PhD student proves

Sija van den Beukel,
15 april 2024 - 14:18

Plastic has a unique, chemical fingerprint that could link a piece of tape at a crime scene to the roll of tape in a suspect’s house, showed UvA PhD student Mirjam de Bruin-Hoegée in a publication in Forensic Chemistry. “A plastic jerrycan sometimes contains very small amounts of gold and arsenic.”

Is a lot of plastic used in crimes?

“Yes, especially plastic tapes. In violent crimes, tapes are used to tie people up or put together an improvised explosive device. This again involves using electrical wires with a plastic sheath. Plastics are also used to carry drugs, toxins, or chemical weapons, such as plastic baggies or jerrycans and sprayers that you can use to disperse toxins.”

“If you find a piece of tape at the crime scene and later come across a piece of tape during a search of the suspect’s home, you can determine if the piece of tape came from that roll”

Is every plastic unique?

“That varies by type of plastic and by application. In fact, we can detect a unique chemical profile reflected in the contaminants in the plastic. These are already in the oil and catalysts from which the plastic is made, but also in added dyes such as white titanium oxide. In plastic tapes or jerrycans, you find many of these elements: iron and copper, but also small amounts of gold and arsenic. Those impurities are so characteristic that you can trace the plastic back to where it was produced. The guidelines for food packaging are stricter, so there are fewer impurities in those plastics, making them harder to trace.”


How do you measure the chemical fingerprint of a plastic?

“We measure that with an expensive device called the LA-ICP-MS that determines such a fingerprint within a minute. It works very simply. You put the piece of plastic under the laser, which scratches the plastic and causes it to evaporate and be absorbed into the device. There the substance is heated strongly so that the substance disintegrates. The mass of the individual atoms can then be measured because each element such as zinc or copper has its unique weight. Together, those substances determine the chemical profile.”


How can you use that to solve a crime?

“If you find a piece of tape at the crime scene and later come across a roll of tape during a search of the suspect’s home, you can use the chemical profile to determine whether the piece of tape came from that roll.”


With what probability can you say that?

“That depends on the size of your database, that is, how many other pieces of tape were examined. For this study, I had around 200 objects and the evidence value went up to a factor of a thousand. That is, the evidence is a thousand times more probable that the piece of tape came from that particular roll than if it didn’t. In principle, a factor of two already makes sense in court, because multiple pieces of evidence are always being collected. But it would be nice if we could increase the probability to a million times.”


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Foto: Mirjam de Bruin-Hoegée
The impurities found in plastics that produce a unique, chemical fingerprint.
“It might make sense to be able to trace microplastics in the environment back to a product or source”

Would criminals change their behavior if they knew plastic could give them away?

“Good question. I don’t think so. Alternatives like metals are usually too heavy and might be traceable in other ways. I would have some ideas. A clear sandwich bag is much harder to trace than a colored one. But, of course, I don’t want to give too many hints.”


What else can you use a chemical fingerprint of plastic for?

“Since there is a lot of plastic pollution and microplastics are floating around in the environment everywhere, it might make sense to be able to trace them back to a product or a source. Are the bathers on the beach causing the plastic pollution here, or the cars or the agriculture?”


Before doing your PhD, you used chemical profiles in many ways to detect drugs in blood and to demonstrate the use of chemical weapons in blood and plants. Are those methods already being used?

“Partially. There was a recent report on the use of chemical weapons by the IS in Syria. The question was whether the chemical weapons were produced by the Syrian regime or whether the IS had set up its own production line. Research into the impurities in the chemical weapons found that the latter was the case. Yet chemical profiles are not otherwise being widely used in forensic investigations. Only in drug and explosives research are they beginning to appear. The field is still in its infancy since it didn’t used to be possible to make such sensitive measurements.”