Automated analysis of microplastics

Tracing invisible particles

“Our approach is fundamentally different,” says Dr. Ivleva: “It is particle-based. That means that instead of destroying the particles, we analyze them directly.” To do this, the researchers use a method known as Raman microspectroscopy. It works by shining a monochromatic laser source onto a sample and detecting the light scattered by the molecules. Comparing the scattered light against the laser source provides information on the substance under investigation. To analyze plastic particles with a diameter greater than 1 µm (micrometer), they must first be filtered out of the aqueous solution, detected under the microscope and then illuminated with laser light. Because plastics such as polyethylene, polystyrene and polyvinyl chloride scatter the photons in characteristic ways, they each generate signals as unique as a fingerprint.

It took years to develop the tracing process: “When we started, we still had to make manual measurements,” recalls the chemist. “It took us months to investigate a few thousand particles.” In the meantime the team has succeeded in automating the detection of microplastics. A single analysis no longer takes weeks, but only a matter of hours. Although the tiny particles still have to be filtered out of the aqueous solution, followed by placement of the filter under the Raman microspectroscope, all remaining steps are carried out by the software developed by the team. The plastic particles are first localized with a light microscope, photographed and measured, and the particles are distinguished from fibers. The software uses these data to compute the number of particles and fibers and to select the image sections needed for a statistically significant result in the subsequent Raman spectroscopy.

In the next step, the laser is directed onto the sample and the scattering is detected and analyzed. This allows quick and reliable analysis of the number, size, shape and composition of the microplastics. The open-source TUM-Particle Typer 2 software is now available to researchers around the world.

To investigate nanoparticles with diameters of less than 1 µm, however, Dr. Ivleva’s team is already working on a modified process. “Nanoparticles like these are difficult or even impossible to discern under a light microscope. To detect them, we first have to carry out size fractionation and then identify them,” explains the researcher.

For this purpose, a field flow fractionation (FFF) system is used. This creates a water flow that captures the particles – depending on their size – and separates them by transporting them at varying speeds. A specially developed device, combined with Raman spectroscopy, permits the chemical characterization of different types of nanoplastics.

“The new analytical processes permit rapid and precise investigation of concentration, size and composition of micro- and nanoplastics,” sums up Dr. Ivleva. “This will now make it possible to study the influence of these particles on the environment and human health.”