Plastic is constantly finding its way into the ocean – it comes from ships, unsecured landfill sites and the sewage system. Over the years, it has repeatedly been claimed that a large proportion of marine plastic is made up of tiny man-made fibres, with polyester and viscose being given as examples. These particles are even said to have been detected at great depths. When conducting this kind of research, it is absolutely crucial that appropriate detection methods are carefully selected, but, as a study by TU Wien has now shown, this point has often been ignored in the past. It has been discovered that some methods of measurement do not make any distinction between natural and synthetic microparticles. This means that what is believed to be plastic in environmental samples may often merely be contamination in the form of natural fibres from lab coats.
Measurements prone to errors
“When you are looking for plastics in water samples, there is always a risk of the materials detected actually originating from the laboratory setting rather than the sample itself,” says Prof. Bernhard Lendl from the Institute of Chemical Technologies and Analytics at TU Wien. As there was already a level of awareness of this problem, some research groups went to great lengths to ensure no synthetic fibres were present in the lab when they were trying to detect plastic in environmental samples. This involved these experiments being performed in special clean rooms, where no clothing made of synthetic fibres was permitted. Nevertheless, tiny fibres from clothes would have invariably made their way into the samples and distorted the results.
All this time, one thing was being overlooked, however: viscose is a wood-based cellulose fibre that cannot be equated with plastic. The fact that viscose consists of natural cellulose means it is biodegradable, which is not true of synthetic plastic. It is difficult to tell synthetic fibres and natural cellulose fibres (such as viscose and cotton) apart. And unless the right analytical methods are used, it is entirely possible for contamination with fibres from a cotton lab coat to produce results that could be incorrectly interpreted as proof of plastic having been detected.
In the past, we’ve seen this sort of laboratory contamination with beer and honey samples. In those cases too, microplastics were initially detected, before someone realised at a later stage that the results could actually most likely be traced back to impure laboratory conditions.
Infrared spectroscopy is the method traditionally used to detect traces of plastic in water samples. When samples are exposed to infrared radiation, some of it is absorbed and, as different chemical substances absorb different ranges of the infrared spectrum to different extents, it is possible to assign specific infrared fingerprints to individual chemicals.
“We took a range of samples and made sure we knew exactly what they contained. We then analysed them with several different infrared spectroscopy methods,” says Bernhard Lendl. This demonstrated just how easy it is to produce erroneous results in tests of this kind. “If you choose the right method and take extreme care when setting the measurement parameters, you can expect to get completely reliable results. However, it’s simply not possible to distinguish between synthetic fibres and natural substances with the technology that has been used in this context until now,” says Lendl. “According to the results of our research, it may be that the synthetic fibres supposedly found at the depths of the ocean are simply the result of a measurement error.”
Of course, this doesn’t mean that we don’t have to worry about the world’s seas being contaminated by plastic. The reality is that there are huge amounts of plastic floating around in our oceans – from plastic bottles to lost fishing nets. There is no question about that. “But when it comes to detecting traces of microplastics, it is essential that appropriate scientific methods are used,” reiterates Bernhard Lendl. “Any other approach is not to be trusted and will not do any good for the world’s oceans or for science.”
Photo: © Wikimedia Commons, M.Danny25, Lizenz: CC Share Alike 4.0
Prof. Bernhard Lendl
Institut für Chemische Technologien und Analytik
Getreidemarkt 9, 1060 Vienna