Identification of Microplastics in the Marine Environment by Raman Microspectroscopy and Imaging

Each year, 5–13 million tons of plastic debris enter the oceans.1 The main reason for this is the inappropriate disposal of waste from industrialized nations. This plastic debris floats on the water and is found in seabed, beach and deep sea sediments.2

Because of their long-term stability and resistance to degradation, plastics persist in the environment. Macro-sized plastic parts in the ocean are reduced to small pieces, named microplastics, in the 500-nm to 5-mm range, as a result of mechanical forces or weathering from sunlight. This article focuses on 1) primary microplastics, that is, microbeads in cosmetics (e.g., skin care products, toothpaste, etc.), cleaning agents and industrial raw materials that are incorrectly disposed of, and 2) secondary microplastics, which are the fragments from macrodebris.

Microplastics cause numerous problems for marine animals, birds and humans. When mistaken for food, their ingestion is harmful.3,4 Microplastics contain additives such as plasticizers and flame retardants and can absorb toxic hydrophobic contaminants.5 Potential pathogenic microorganisms like viruses, bacteria and microbes6 can be transported into the human food chain via microplastics and hence endanger lives.

Knowledge of the occurrence, composition, size and distribution of microplastics is paramount to understanding their risk. A very important step in microplastics investigations is sampling in the sea and in seabed sediments and subsequent sample preparation using extraction steps such as enzymatic treatment to remove organic layers from the particles to correctly identify the type of plastic. This must be done in a clean laboratory with extreme care taken to avoid contamination. Blind sampling must also be used. At the end of the procedure, the particles are placed onto a filter with an area of 11 × 11 mm.

Raman microspectroscopy and imaging

An effective method for identifying the type of polymer and determining particle size and distribution of microplastics particles is Raman microspectroscopy and imaging. In comparison to FTIR, a significant advantage of Raman is its ability to identify particles up to 500 nm in size. In addition, the complete wavelength region can be used for identification.

Particles larger than 500 μm can be identified by a single Raman spectrum, and the smaller particles by Raman imaging and consecutive comparison of the measured spectrum with a spectral library. All measurements in this study were done using the alpha 300R Raman microscope (WITec, Ulm, Germany), which has a 532-nm laser, an electron-multiplying charge-coupled device (EMCCD) camera as a detector and objectives with magnifications from 20 to 100×.

In Raman imaging, every point of the sample is measured based on the sample’s complete Raman spectrum. The measuring time for one spectrum is between 20 and 500 msec.

Figure 1 shows an image from a microphotograph and Raman images of a microplastics model system with polyethylene (PE), polystyrene (PS) and sand particles (particle sizes between 1 and 100 μm). At the top is a microphotograph with all particles, while the bottom Raman images show the size, density and distribution of the three different particles, each of which has a specific Raman band in the spectrum. The measuring point distance was 10 μm; an 1800 × 2000 μm area was measured at 250 msec for each spectrum. The recording time for the entire Raman image with 36,000 Raman spectra was 2.5 hours. As can be seen, the red and yellow particles are larger than the blue particles.

Figure 1 – Image from a microphotograph and Raman images of a microplastics model system consisting of PE, PS and sand.

An overlay of the three Raman images (Figure 2) shows the spatial distribution of the three particle species.

Figure 2 – Raman images of the microplastics model system consisting of PE, PS and sand.
Figure 3 – Image from a microphotograph of a coastal sediment sample from the North Sea (courtesy of Dr. Martin Löder, AWI Bremen, Biologische Anstalt Helgoland), 11 × 11 mm.

Figure 3 depicts a sample from coastal sediment from the North Sea on an 11 × 11 mm filter. To reduce the measurement time, a     6 × 4 mm (red rectangle) area was chosen for the Raman image with a measuring distance of 10 μm; the measurement period was 500 msec for each spectrum.

Figures 4 and 5 demonstrate how the method works. An initially unknown Raman spectrum was measured for every particle, e.g., the Raman spectrum (lower left spectrum) of particle number 6 in the upper-left Raman image in Figure 4. This spectrum was searched in a spectral library and identified as polypropylene (PP, lower right green spectrum, Figure 4). In cases in which the quality of the spectrum is not high enough to allow identification, it is possible to return to the particle after the Raman image processing and collect a single Raman spectrum with a longer measurement period (lower-right red spectrum in Figure 4).

Figure 4 – Raman image (top) of a coastal sediment sample from the North Sea, extracted spectrum (bottom, left) from the Raman image of particle 6 and single Raman and Raman library spectra (bottom, right) of particle 6, identified as PP; particle size about 20 μm.

The same measured sample is shown in Figure 5. The unknown particle here is number 2, upper left in the Raman image. The Raman spectrum of this particle is the lower left spectrum. The spectral library search for this spectrum showed a PS particle (lower-right green spectrum). The spectrum was of sufficient quality that an additional measurement for this particle was not necessary.

Figure 5 – Raman image (top) of a coastal sediment sample from the North Sea, extracted spectrum (bottom, left) from the Raman image of particle 2 and single Raman and Raman library spectra (bottom, right) of particle 2, identified as PS; particle size about 10 μm.

Conclusion

Microplastics in the marine environment are an increasingly serious problem for marine animals, birds and human life. These microparticles can be investigated using Raman microspectroscopy and imaging, which is able to identify the type of microplastics and their particle size and distribution in aquatic systems.

References

  1. Jambeck, J.R.; Geyer R. et al. Plastic waste inputs from land into the ocean. Science 2015, 347(6223), 768–71.
  2. Woodall, L.C.; Sanchez-Vidal, A. et al. The deep sea is a major sink for microplastic debris. R. Soc. Open Sci. 2014, 1, 140317.
  3. Rochman, C.M.; Hoh, F. et al. Ingested plastic transfers hazardous chemicals to fish and induces hepatic stress. Sci. Rep. 2013, 3, 3263.
  4. Wright, S.L.; Rowe D. et al. Microplastic ingestion decreases energy reserves in marine worms. Curr. Biol. 2013, 23(23), 1031–3.
  5. Rios, L.M.; Jones, P.R. et al. Quantitation of persistent organic pollutants adsorbed on plastic debris from the Northern Pacific Gyre’s “eastern garbage patch.” J. Environ. Monit. 2010, 12(12), 2226–36.
  6. Zettler, E.R.; Mincer, T.J. et al. Life in the “plastisphere”: microbial communities on plastic marine debris. Environ. Sci. Technol. 2013, 47(13), 7137–46.

The authors are with the Leibniz Institute of Polymer Research, Hohe Str. 6, 01069 Dresden, Germany; tel. : +49 351 4658268; e-mail: [email protected]www.ipfdd.de/en

Comments