Identification of Microplastics in Environmental Samples
Combination of Particle Analysis with FTIR and Raman Microscopy
- Particles from a water sample (117 liters, river Warnow), distributed over 4 filters (Explanation and results in the text)
- Fig. 1: Output of the GEPARD program (spectrum, size and polymer type of each single particle, size distribution of all MP particles in the sample, distribution of all MP particles of the sample according to polymer type).
- Fig. 2: Identification of MP particles using FTIR and Raman spectroscopy.
- Fig. 3: Size and polymer type of the MP particles in a water sample from the river Warnow (Explanation in text).
Determining microplastics (MP) in environmental samples quickly and reliably is a challenging task. With a largely automated combination of optical particle analysis with FTIR and Raman microscopy and spectral database search, particle sizes, particle size distributions and the type of polymer including color particles can be determined.
Macroplastics and microplastics in the environment and in food is one of the major environmental topics of our time. The numbers: The ~8.3 billion tons of plastic produced worldwide by 2015 are offset by ~6.3 billion tons of plastic waste produced in the same period. Plastics are estimated to remain in the environment for 100 - 500 years, depending on the type of plastic. By 2015, only 9% of the plastic waste produced worldwide was recycled, 12% of the plastic waste was incinerated and recycled for energy, and 79% of the plastic waste was landfilled or landed in the environment. According to estimates, approximately eight million tons of plastic waste find their way into the sea every year via rivers, wind, waste water, etc. The waste is disposed of in the environment, mostly in the sea and in the seabed . Of this, 80% comes from the mainland. Poor and missing waste management systems in many countries, especially in Asia and South America, are jointly responsible for a large part of the macroplastic in the sea, which decomposes into MP by wind, waves and solar radiation. In principle, a distinction is made between macroplastic waste and primary and secondary microplastic waste. Microplastics are solid and insoluble plastic particles smaller than 5 millimeters and larger than 1 µm (below it is nanoplastics), all larger plastic parts and products are referred to as macroplastics. Primary microplastics is divided into type A and type B. Type A is formed during production (e.g. friction bodies in cosmetics or plastic pellets), while type B is formed during the utilization phase, e.g. by tire abrasion, paint abrasion, the construction industry (insulation) or by washing synthetic clothing . Secondary microplastics are weathered and fragmented macroplastic waste in the environment, especially in rivers, lakes and oceans.
Additives, such as flame retardants, stabilizers, fillers, color pigments or plasticizers, which are added to plastics during production to improve their properties, can be released by degradation of the MP.
In addition, organic pollutants such as pesticides and pathogenic germs can “dock” to the surface of MP. All these additives and pollutants can then be ingested by animals, especially in the marine environment, which mistake MP with food, and reach humans via the food chain. From a toxicological perspective, these additives and pollutants probably pose a greater risk to humans and the environment than the plastics themselves .
It is necessary to note that plastics themselves are not the problem. On the contrary, plastics are valuable substances from which excellent and indispensable products are made, but which must not remain in the environment after use. Therefore, the problem is what happens to plastic products at the end of their life cycle. In order to achieve the fastest possible reduction of the MP load in the environment, there is a considerable need for research to the main sources, transport routes and whereabouts, as well as their distribution in the environment. Decisive for evaluations is the knowledge of how much MP, which MP and which size is present in different environmental areas (oceans, sediments, rivers, soils, air). Reliable and comparable sampling, measurement and analysis methods are necessary to determine reliable data. Particularly desirable are analytical methods that can quickly and reliably determine particle size, particle size distribution and polymer type. Such methods are described, for example, in .
Identification and Quantification of MP with a Combination of Optical Particle Analysis with FTIR and Raman Microscopy
Since MP can already be contained in the laboratory air and in tape water, the analysis must take place in a microplastic-free “clean room” (particle filter air purifier, flow box, plastic-free laboratory equipment and special work clothing) so that the MP samples are not contaminated. In this flow box all pre-cleaned water, sediment, soil and sewage sludge samples are filtered onto special silicon filters with a pore size of 50 and 10 or 5 µm. Particles larger than 500 µm are sorted out beforehand and measured individually. These filters were developed as measuring substrates for FTIR and Raman microscopy . Depending on the particle load on the sample, this can be up to 10 filters per sample. These filters usually contain 500 - 40,000 particles. Of these, however, “only” about 1% or less is MP. In addition, a blank sample with MilliQ water is taken to clarify that the samples are not self-contaminated.
A reliable determination of MP and the respective polymer type is only possible by infrared (FTIR) and Raman microscopy, which is described in detail in . We use both spectroscopic methods in combination with optical particle recognition. The GEPARD program package  was developed for this combination of methods in order to be able to carry out the measurements largely automatically. The program first determines all particles on a filter optically and segments not isolated particles with different algorithms. The coordinates (x,y,z), the optical image and the size of each particle are stored. The coordinates are automatically transferred to the FTIR and/or Raman microscope, where all particles are automatically measured and identified using spectra databases. In order to obtain high-quality spectra for the most important polymers and additives, proprietary databases for polymers and copolymers, commercial pigments, lacquers and dyes as well as nonplastic materials (laboratory materials, inorganics, biota, etc.) were created in addition to commercial databases already in use. In addition to MP, all color particles in the samples can be identified. All results can be transferred to national and international databases (e.g. the “Marine Plastic Data Base”) and comfortably output by the GEPARD program (fig. 1).
The analyzed samples come from all environmental areas. Water samples (tape water, surface water from rivers and the Baltic Sea, deep water), samples from the seabed, sediments from rivers, samples from soils, beaches, sewage treatment plants, rainwater overflows and from the air are investigated. The samples will be analyzed within 4 large collaborative projects (EU-BONUS, BMBF-FONA “Plastics in the environment”), which take the samples from the environment. Information on the projects and project partners can be found in .
Determination of MP in Environmental Samples
The example of MP particles from the sediment of the river Warnow is used to illustrate the procedure. The localized flakes in figure 2 (center) were measured with FTIR and Raman. The database search revealed a polyester resin for the FTIR spectrum, in the Raman spectrum the filler titanium dioxide and a blue color pigment (phthalocyanine) were identified. This shows that often only both measurements provide a complete result, especially for color particles.
Another example is the identification of MP particles < 500 µm in a water sample (117 l) from the river Warnow. This sample was prepared and purified at the Leibniz Institute for Baltic Sea Research in Warnemünde (IOW). All particles were filtered in a 2-stage filtration process to 4 filters, 2 filters with 50 µm and 2 filters with 10 µm pore size (see cover picture). A total of 68000 particles and fibers were measured on all 4 filters. A total of 407 MP particles were found, of which 305 are definitely MP from the Warnow. 102 particles were excluded because they could have got into the sample during sampling and sample preparation (PTFE and silicone are built into the sampling device; the dyes PV23 and PB 15 are used for handling and the transport bottles were sealed with parafilm). The distribution by polymer type and particle size is shown in figure. 3. The bulk plastics PP, PE, PET, PMMA and PS make up by far the largest part.
A risk assessment of the ecotoxicological effects of MP requires reliable data on its sources, sinks and transport routes in the environment. Suitable and reliable analytical methods for identification and quantification are FTIR and Raman microscopy. In combination with optical particle recognition and automatic measurement and evaluation, these can also be fast enough for higher sample throughput.
Dieter Fischer1, Andrea Käppler1, Franziska Fischer1, Josef Brandt1, Lars Bittrich1, Klaus-Jochen Eichhorn1
1Leibniz-Institute of Polymer Research Dresden, Germany
 World Economic Forum, Ellen MacArthur Foundation & McKinsey & Company. The new plastics economy. (2016)
 Bertling, Bertling, & Hamann. Kunststoffe in der Umwelt: Mikro- und Makroplastik. Fraunhofer Umsicht (2018)
 Galloway. Micro and Nanoplastics and Human Health. In: Marine Anthropogenic Litter. Springer. (2015)
 Wendt-Potthoff, Imhof, Wagner, Primpke, Fischer, Scholz-Böttcher, Laforsch. Mikroplastik in Binnengewässern (Kapitel V-6). In: Handbuch der Limnologie, Wiley-VCH (2017)
 Käppler, Windrich, Löder, Malanin, Fischer, Eichhorn, Voit. Identification of microplastics by FTIR and raman microscopy: a novel silicon filter substrate opens the important spectral range below 1300 cm-1 for FTIR transmission measurements. Analytical and Bioanalytical Chemistry 407 (2015) 6791-6801
 Käppler, Fischer, Oberbeckmann, Schernewski, Labrenz, Eichhorn, Voit. Analysis of environmental microplastics by vibrational microspectroscopy: FTIR, Raman or both? Analytical and Bioanalytical Chemistry 408 (2016) 8377-8391
 GEPARD (Gepard Enabled PARticle Detection), OPEN SOURCE Software, https://gitlab.ipfdd.de/GEPARD/gepard
 https://www.ipfdd.de/de/organisation/abteilungen-und-gruppen/institut-makromolekulare-chemie/analytics/microplastics-group/projects/ und die Links dort zu den einzelnen Projekten