Characterizing Microplastics with Raman Imaging Spectroscopy

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In marine environments, plastic pollution is extremely harmful and is currently widespread all over the world. The enormous, 299 million tons/year, global plastic production,¹ which is linked to a growing use of disposable goods, coupled with low degradability of polymers have led to the accumulation of plastic debris in natural habitats.² Although synthetic polymers are durable, large plastic items can undergo fragmentation processes, mainly as a result of mechanical breakdown caused by wave action or abrasion by sand and other materials and is promoted by photochemical processes triggered by UV-B light.^3,4,5 Microplastics are the smaller plastic fragments, whose diameters are 5 mm or less.^6,7 This category has been further divided in small microplastic particles (S-MPP ≤ 1 mm) with diameters of 1 mm or less and large microplastic (L-MPP), ranging between 1 and 5 mm.⁸

Microplastics can absorb both heavy metals and persistent organic pollutants (POPs) from sediments and water, and the tiniest particles can penetrate the food web,^9,10 posing a severe health risk to wildlife and ultimately to humans.

Identification and quantitation of microplastics are significant analytical challenges, and the lack of official analytical methods makes it difficult to compare different studies. The majority of the research studies performed thus far start by visually sorting particles under a stereo-microscope to isolate potential microplastics from other debris.¹¹

The major restriction of visual sorting is particle size, because as the size decreases, the difficulty of differentiating microplastics from interfering particles increases.^{12, 13} Hence, analyzing sorted particles with techniques that allow proper identification of plastics is highly recommended; for instance, spectroscopic techniques or pyrolysis GC/MS (py-GC/MS), even if these methods are less efficient and miss the microplastic particles discharged by visual sorting.

According to a recent study,¹⁴ spectroscopic techniques such as Raman, NIR, and FTIR in microplastics identification and quantification maximize the accuracy and the sensitivity of the analysis; however, there are only a few methods that facilitate quick and reliable analysis. Spectroscopic approaches are usually single-point analyses that are not automated; only a small number of semi-automated filter analysis studies have been carried out, employing single MCT detector-μFTIR-chemical mapping^15,16 to analyze just a few sub-areas of the filter surface and, more recently, using Focal Plane Array (FPA)-based imaging μFTIR to scan the entire surface of a small filter with diameters exceeding 10 mm.¹⁷

While this last approach offered very good results, displaying high lateral resolution and enabling the detection of particles sizes as low as 20 μm, analysis of smaller microparticles and sub-microparticles is not possible by μFTIR techniques, because of diffraction phenomena which takes place below 10 μm in FTIR. Another disadvantage is that the analysis is time-consuming, and can be on the order of tens of hours.¹⁸ These drawbacks can be overcome by the most promising technique, Raman imaging microscopy, which combines the speed of an imaging technique with high spatial resolution, typical of Raman microscopy technique.

This article discusses a simulation of the analysis of microplastic particles by using reference materials to give an ideal analytical model of potential environmental samples.

Acknowledgement

Thermofisher Scientific records its gratitude towards National Research Council, Padova Unit (CNR-IDPA, Venice) for the provision of the microparticle standard and for beneficial scientific discussions.

References and Further Reading

1. PlasticsEurope. Plastics – The Facts 2014/2015. An Analysis of European Plastics Production, Demand and Waste Data. (accessed July 26, 2015).
2. Barnes, D.K.; Galgani, F.; Thompson, R.C.; Barlaz, M. Accumulation and Fragmentation of Plastic Debris in Global Environments. Philos. Trans. R. Soc. London, Ser. B. 2009, 364 (1526), 1985-1998.
3. Andrady, A.L. Microplastics in the Marine Environment. Mar. Pollut. Bull. 2011, 62 (8), 1596-1605.
4. Cooper, D; Corcoran, P.L. Effects of Mechanical and Chemical Processes on the Degradation of Plastic Beach Debris on the Island of Kauai, Hawaii. Mar. Pollut. Bull. 2010, 60 (5), 650-654.
5. Corcoran, P.L.; Biesinger, M.C.; Grifi, M. Plastics and Beaches: A Degrading Relationship. Mar. Pollut. Bull. 2009, 58 (1), 80-84.
6. Moore, C.J. Synthetic Polymers in the Marine Environment: A Rapidly Increasing, Long-Term Threat. Environ. Res. 2008, 108 (2), 131-139.
7. Thompson, R.C.; Olsen, Y.; Mitchell, R.P.; Davis, A.; Rowland, S.J.; John, A.W.G.; McGonigle, D.; Russell, A.E. Lost at Sea: Where is All the Plastic? Science. 2004, 304 (5672), 838.
8. Imhof, H.K.; Schmid, J.; Niessner, R.; Ivleva, N.P.; Laforsch, C. A Novel, Highly Efficient Method for the Separation and Quantification of Plastic Particles in Sediments of Aquatic Environments. Limnol. Oceanogr.: Methods. 2012, 10 (7), 524-537.
9. Farrell, P.; Nelson, K. Trophic Level Transfer of Microplastic: Mytilus edulis (L.) to Carcinus maenas (L.). Environ. Pollut. 2013, 177, 1-3.
10. Setälä, O.; Fleming-Lehtinen, V.; Lehtiniemi, M. Ingestion and Transfer of Microplastics in the Planktonic Food Web. Environ. Pollut. 2014, 185, 77-83.
11. Hidalgo-Ruz, V.; Gutow, L.; Thompson, R.C.; Thiel, M. Microplastics in the Marine Environment: A Review of the Methods Used for Identification and Quantification. Environ. Sci. Technol. 2012, 46 (6), 3060-3075.
12. Dekiff, J.H.; Remy, D.; Klasmeier, J.; Fries, E. Occurrence and Spatial Distribution of Microplastics in Sediments from Norderney. Environ. Pollut. 2014, 186, 248-256.
13. Fries, E.; Dekiff, J.H.; Willmeyer, J.; Nuelle, M.-T.; Ebert, M.; Remy, D.; Identification of Polymer Types and Additives in Marine Microplastic Particles using Pyrolysis-GC/MS and Scanning Electron Microscopy. Environ. Sci.: Processes Impacts. 2013, 15 (10), 1949-1956.
14. Song, Y.K.; Hong, S.H.; Jang, M.; Han, G.M.; Rani, M.; Lee, J.; Shim, W.J. A Comparison of Microscopic and Spectroscopic Identification Methods for Analysis of Microplastics in Environmental Samples. Mar. Pollut. Bull. 2015, 93 (1-2), 202-209.
15. Harrison, J.P.; Ojeda, J.J.; Romero-González, M.E. The Applicability of Reflectance Micro-Fourier-Transform Infrared Spectroscopy for the Detection of Synthetic Microplastics in Marine Sediments. Sci. Total Environ. 2012, 416, 455-463.
16. Vianello, A.; Boldrin, A.; Guerriero, P.; Moschino, V.; Rella, R.; Sturaro, A.; Da Ros, L. Microplastic Particles in Sediments of Lagoon of Venice, Italy: First Observations on Occurrence, Spatial Patterns and Identification. Estuarine, Coastal and Shelf Science. 2013, 130, 54-61.
17. Löder, M.G.J.; Kuczera, M.; Lorenz, C.; Gerdts, G. Focal Plane Array Detector-Based Micro-Fourier-Transform Infrared Imaging for the Analysis of Microplastics in Environmental Samples. Environ. Chem. 2015, 12 (5), 563-581.
18. Shi, H.; Magaye, R.; Castranova, V.; Zhao, J. Titanium Dioxide Nanoparticles: A Review of Current Toxicological Data. Part. Fibre Toxicol. 2013, 10 (15), 1-33.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

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