Microplastic
Taking aim at tiny threats
AWI experts offer recommendations on detecting microplastic
[21. September 2020]
Many experts are concerned by the increasing pollution of the environment with tiny plastic particles. But we still don’t know enough about the scope of the problem: though there are various methods for detecting microplastic in water and in other samples, it’s often difficult to compare the results. An international team led by Dr Sebastian Primpke and Dr Michaela Meyns from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) has now presented possible solutions in two new studies released in the journal “Applied Spectroscopy”.
The world is full of plastic. In the form of minuscule, barely visible particles it slumbers beneath the soil, floats through our rivers and lakes, and drifts across the ocean. It can be tiny fragments of larger pieces of plastic, stem from microfiber clothing, or be granulates from cosmetic products. These by-products of civilisation can even be found in the most remote reaches of the Earth’s poles. Their impacts remain unclear, but experts fear that microplastic could pose a serious threat to the environment and human health alike.
Accordingly, scientists around the globe are working on strategies to reduce this pollution. But to do so, they need to first gauge the scale of the problem. “For instance, from July 2021, the drinking water in California will have to be routinely checked for these particles,” says Sebastian Primpke. And microplastic will likely also be covered in the EU’s planned new drinking water guidelines.
But the real question is how these tiny threats can best be detected in water or other samples. “We already have a range of methods for doing so,” Primpke explains. “The problem is that it’s often difficult to compare the results of the respective methods.” For routine testing, there would have to be standard protocols that delivered accurate and intercomparable results at an acceptable cost. And they have yet to be developed.
As a first step, Primpke and his colleagues within a team of international experts prepared an overview of the currently available methods, together with their strengths and weaknesses. “In addition to the ability to reliably detect the particles, we were interested in the cost factor,” he points out.
How high are the costs?
In this regard, optical methods, which involve manually sorting the particles into certain predefined size categories under the microscope, are considered particularly affordable. The required microscopes cost a few thousand euros to a few tens of thousands – making them far less expensive than any other analytical instruments. “But you have to bear in mind that, for this type of analysis, the sample usually has to be constantly monitored, and the interesting particles have to be painstakingly sorted out,” says Primpke. Accordingly, anyone who wants to analyse a typical water sample using this method will likely need a whole day to do so.
In other methods, technological systems take more of the work off of users’ hands. For example, either an image analysis technique can be used to automatically select the relevant particles or the whole filter membrane surface measured. Subsequently, they are then analysed using a Fourier-transform infrared (FT-IR) microscope. Not only can an FT-IR mircoscopemeasure numbers and sizes of particles each sample contains, it can also determine which ones are plastic, and what kind of plastic. To do so, it scans the sample with infrared radiation. When the rays collide with particles, a portion of their wavelength is absorbed – and that portion varies from material to material. The remaining spectrum can be used to identify the particles, just like a fingerprint.
As such, this approach yields more information than analysis under the optical microscope – and means far less work for laboratory staff. “Identifying the particles usually only takes one or two hours personal work time,” Primpke says. “The machine does the rest on its own.” That being said, the price for this type of microscopes is usually in the six-figure range. “So, depending on your situation, you have to consider whether the working time you save can justify the larger investment.”
In Primpke’s view, given the high price tag, it’s hardly feasible for every site to install a FTIR microscope, which is why he and his colleagues recommend using a two-step approach for future routine tests. In the first step, the particles could be inexpensively counted under the microscope. Only if the presumed particle concentration exceeded a certain threshold, above which there was a realistic health risk, could the sample in question be
Special software helps with large amounts of data
But even having an expensive FT-IR machine doesn’t mean all your problems are solved. Microplastics identification with infrared microscopy produces massive quantities of data, and it takes special-purpose computer programs to analyse them. Spectrometers from different manufacturers use proprietary software, databases and analysis mechanisms – virtually none of which are compatible with one another. As a result, anyone who uses FT-IR can generate detailed lists of the particles, particle sizes, and types of plastic contained in their samples; but they can’t compare the lists with the data of colleagues who use other brands.
In response, the AWI experts, working together with colleagues from Aalborg University, Denmark, have developed a software package called siMPle (‘Systematic Identification of MicroPLastics in the Environment’), which can solve these compatibility problems. And it’s already passed the first field test: researchers from the Netherlands, Great Britain, Denmark and Germany used a variety of FT-IR instruments to test treated wastewater samples from sewage treatment facilities. Not only did they arrive at similar findings, regardless of the brand used; to the team’s surprise, the results were even more accurate than those from the commercial software they had previously applied.
“Meanwhile, several colleagues at other institutions in Germany and abroad have begun using it,” Sebastian Primpke reports. As he explains, the more who do so, the more intercomparable the data will become; in addition, the software and connected free database will make it much easier to simultaneously assess studies conducted by different research groups, or to introduce an international monitoring programme for microplastic pollution. “With our nano-FTIR data from a new microscopic technique we are implementing to identify even smaller nanoplastics we work with siMPle right away, so we promote comparability from the beginning”, Michaela Meyns points out. Furthermore, siMPle can also be used in the classroom or for other educational purposes; together with examples and corresponding databases. It is available free of charge online (www.simple-plastics.eu). Accordingly, everything’s ready for the next stage in the hunt for tiny plastic threats.
Original publications:
Sebastian Primpke, Silke H. Christiansen, Win Cowger, Hannah De Frond, Ashok Deshpande, Marten Fischer, Erika B. Holland, Michaela Meyns, Bridget A. O’Donnell, Barbara E. Ossmann, Marco Pittroff, George Sarau, Barbara M. Scholz-Böttcher und Kara J. Wiggin: Critical Assessment of Analytical Methods for the Harmonized and Cost-Efficient Analysis of Microplastics. Applied Spectroscopy 0003702820921465, DOI: 10.1177/0003702820921465
Sebastian Primpke, Richard K. Cross, Svenja M. Mintenig, Marta Simon, Alvise Vianello, Gunnar Gerdts und Jes Vollertsen: Toward the Systematic Identification of Microplastics in the Environment: Evaluation of a New Independent Software Tool (siMPle) for Spectroscopic Analysis. Applied Spectroscopy 0003702820917760, DOI: 10.1177/0003702820917760
Additional information:
Meyns, M., Primpke, S. & Gerdts, G. Library based identification and characterisation of polymers with nano-FTIR and IR-sSNOM imaging. Anal. Methods 11, 5195-5202, doi:10.1039/C9AY01193E (2019).