Shining green

Media, environmental protection organizations and authorities report on microplastics. These particles pose danger to humans and animals and are already found in their excretions. But research on the direct impact of microparticles in the body is in its infancy.

Since many publications keep developing theoretical models and work with “freshly generated” microplastics, the idea here is to recycle used material, shred it and then use it for testing purposes. Thus, instead of putting new and more polymer into the environment, existing polymer is used; it is about micro quantities here, not kilograms or tonnes. And the materials are real-life samples with true properties of the microparticles that the organisms must deal with by necessity.

Microplastics are dangerous to the environment mainly because of their small size, which means that a large number of organisms ingest them, because they mistake them for food.

A next step in the development of innovations in environmental research could be fluorescent microplastics. They may make it easier to determine whether and where these materials accumulate in organisms. This would then enable better risk assessment.

According to an application, it should be possible to produce fluorescent microplastics using the lipophilic dye Nile Red. But here the question arises, why produce them when there are enough polymers with fluorophores (part 1 + part 2) (figure 1)? So technical polymers and a very striking greenish-yellow PET beverage bottle (figure 2) were ground to produce microparticles. [1, 2]

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Figure 2: PET bottle bottom (caffeinated lemonade) and the fluorescence under a xenon light source. The bottle was mounted in the sample chamber of the RF-6000.

The Carat company from Bocholt, Germany has developed the idea further. It specializes in the analysis of thermoplastics. For the application at hand, Carat has found a way to break down, specify and certify the polymers. With the RF-6000 spectrofluorometer and a solid sample holder, Shimadzu has supplied a solution that can measure these small particles.

Measurement challenge microplastics

Microplastics lead to special requirements in analytical technology. Fluorescence is known for its high detection limits in liquids. Can this property also be transferred to particle size? In the two previous issues of the Shimadzu customer magazine (see 1 and 2/2020), polymer pieces (2-3 cm) and granulates (5 mm) were examined.

Microscopy is useful for samples smaller than 100 µm, for instance in the identification of particles with infrared spectroscopy. This third part of the article series looks at the identification or simply recovery of particles smaller than 500 µm with fluorescence spectroscopy. Common polymer granules in the size of 5 mm show more or less strong fluorescence, which arises from polymers with π-electron molecular structure (double bonds) or from fluorescent additives. Now PC (polycarbonate) and PET (polyethylene terephthalate) are being examined more closely.

In order to simulate microparticles, polymers were ground to a size smaller than 500 µm using the cryo-mill (Carat’s method). The polymer particles (part 2) were sieved and divided into fractions after being ground to ensure fluorescence, because homogeneity of additives in polymers can also be a factor. This resulted in particles identified as 3,000 (containing up to 1,500 µm), 1,500 (containing up to 500 µm) and 500 µm (and smaller), with each sieve fraction containing a particle distribution; see figure 3. [3]

Sample preparation and analyses

Particles from each fraction were scattered into the center of a bed of barium sulphate (BaSO₄), acting as a fixative, and pressed down with a quartz plate (figure 4). These measurements consistently showed homogeneity down to below 500 µm and presence of fluorescence (figure 5). Depending on the polymer and/or additive, the fluorescence intensity is attenuated with the microparticles compared to higher concentrations (larger surface areas and layer thickness of the polymers).

In an experiment with PET microparticles, grinding of the bright bottle would be suitable. With a very small amount of microparticles, the fluorescence can still be intense, so a standard fluorescence instrument can detect the presence of the particle (figure 6). Measurement parameters and sample preparation were not changed for comparison reasons.