Tieteestä & Tekniikasta
Microplastics analysis with Raman microscopy
Text and picture: Maria Clara Lessa Belone, Doctoral Researcher at Plastic and Elastomer Technology Research Group, Tampere University
I n 1921 when Dr. Chandrasekara Venkata Raman saw the sea as the inspiration that led to his discovery of a new scattering effect, he could not imagine that one day this would be of great importance in the efforts of protecting his source of inspiration from microplastics (MPs). Especially now, as the European Union starts to propose and implement regulations to tackle MPs pollution. The interaction between light and matter results in different pro- cesses, such as absorption and scattering. In Raman spectroscopy, a single frequency (commonly 532nm and 785nm) of radiation is used to irradiate the sample and the radiation scattered from the molecule is detected. As this allows the analysis of molecular composition, Raman effect is very beneficial in many fields, such as medicine and space science. Recently, it is also being used in MPs research. The implementation of reliable, reproductible and effective method- ologies to study MPs is still under development. Vibrational spectros- copy techniques (such as Raman, infrared and near infrared) coupled with a microscope are a popular choice for MPs characterization due to their non-destructiveness, low sample amount, possibility for high throughput and environmental friendliness. Besides these advantag- es, Raman microscopy (Raman spectroscopy coupled with an optical microscopy) also offers high spatial resolution, wide spectral coverage, high sensitivity to non-polar functional groups and low water inter- ference. These are advantageous features for MPs characterization. However, one major drawback of Raman microscopy is the fluores- cence interference that can arise from the MPs or the medium they are inserted on. The fluorescence signal can completely overshadow the Raman signal and make the MPs identification tricky. It is also a laborious and time-consuming technique. Manual particle detection covers only a small area of the whole sample, taking, for example, hours up to days to analyze less than 10% of the sample, in cases of very organic rich samples. Automated mapping routines and library matching are developed to allow faster MP detection. However, other challenges arise from this since different MP types may require dif- ferent measurement parameters to obtain a good spectra and automat- ed routines use the same parameters for all particles to be analyzed. Different samples containing MPs have been already studied using Raman microscopy: seawater, freshwater, wastewater, sediment, living organisms, food, drinks, cosmetics. The research group of Plastics and Elastomer Technology at Tampere University, together with Tampere Microscopy Center, is using Raman microscopy for the analysis of MPs in the municipal sewage sludge of Tampere. Municipal sewage sludge has been reported to contain astonishing amounts of MPs and to be an important vehicle of MPs discharge to the environment.
In our study, the sewage sludge is subjected to a series of treatments to minimize its high organic material content, which can interfere the MPs identification by physically covering them or by producing fluo- rescence. Then the samples are filtered and the manual MPs identifi- cation of a section of the filter is performed with Raman microscopy. From our study, we can get information about the MPs such as poly- mer type, particle morphology (e.g. fiber, fragment) and particle size. It also allows the estimation of the number of MPs on the municipal sewage sludge and an understanding of the possible sources of these contaminants. The data collection and analysis are ongoing, and the results are expected to be published by the end of 2024.
The identification of two MPs on a sample obtained from sewage sludge by the typical Raman spectra of PET and PE. Credit: Maria Clara Lessa Belone
REFERENCES Smith, E., & Dent, G. (2005). Modern Raman spectroscopy: A Practical Approach. Wiley. Araujo, C. F., Nolasco, M. M., Ribeiro, A. M. P., & Ribeiro-Claro, P. J. A. (2018). Identification of microplastics using Raman spectroscopy: Latest developments and future prospects. Water Research, 142, 426-440.
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