Raman and infrared spectroscopy are highly complementary analytical tools that are sensitive to the vibrational modes of molecular species or materials. Understanding the behavior of microplastics means being able to characterize their dimensions as particles and also evaluate their chemical composition. Raman spectroscopy and Fourier Transform Infrared (FTIR) have successfully been used to characterize some particles from environmental samples.
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Plastic pollution is a particularly insidious problem due to the long environmental persistence of many plastic-based items.
Plastics are among the world’s most used materials, with over 300 million tons being produced in 2017.
Worldwide demand for these materials also shows no sign of slowing, though there are encouraging signs that recycling of plastics is becoming more commonplace, with a 92% increase in the quantity of plastic, post-consumer, being sent to recycling.
With plastic use and demand exceeding disposal capabilities, there have been efforts to try and develop biodegradable plastics and encourage further uptake of plastic recycling.
Alongside this, there have been growing concerns about the global coverage of small plastic fragments, known as microplastics, and their accumulation in the food chain.
Microplastic refers to any plastic particle that is between 1–5000 µm in size.
They vary in shape and construction, from spheres to fragments and fibers, and are generally formed from the breakdown of large plastic species. In turn, microplastics further degrade to form nanoplastics.
While microplastics have been detected worldwide, even in remote oceanic regions, there are still many questions as to their exact environmental and toxicological impacts.
Part of the challenge with resolving these questions comes from the fact that not all microplastics exhibit the same toxicological or degradation behavior. This is because the chemical composition of the plastic determines much of this.
Raman and FTIR Spectroscopy
Fourier Transform Infrared (FTIR) and Raman spectroscopy have been used to characterize particles from environmental samples, but which of these spectroscopic approaches is best suited for microplastic analysis?
FTIR is already a commonly used technique for plastics analysis, including monitoring of live processes. Generally, FTIR spectra can be recorded very quickly, and as with Raman spectroscopy, the vibrational fingerprints observed in the spectrum can be used for qualitative analysis.
In comparison to standards of known concentration, both FTIR and Raman can also be used for quantitative analysis.
One of the challenges of working with microplastics with FTIR spectroscopy is that the diffraction limit of the infrared light required means that particles that are < 10 µm are not detectable.
The study also reported issues with misidentification of small (~ 20 µm) microplastics with FTIR. However, the fast analysis time means large sample areas can be investigated, and polyesters and aliphatic compounds are typically straightforward to identify.
While Raman spectroscopy proved more reliable for identifying the size distribution of particles than FTIR, particularly for smaller species, performing Raman measurements on microplastics also comes with its challenges.
One of the inherent issues in Raman spectroscopy is that the excitation laser often creates a large fluorescence background which can overwhelm the relatively weak Raman signals. This can lead to poor contrast and make certain species challenging to analyze.
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For microplastics, a combination of FTIR and Raman spectroscopy is ideal due to the sensitivity to different chemical bonds in the samples, meaning a larger total number of types of plastics can be identified, as well as the ability to combine more rapid measurements from FTIR with the greater accuracy for monitoring size distributions from Raman spectroscopy.
The complementarity of the techniques makes it possible to perform full analysis and identification on even complex particles, such as paints and pigments.
Future Optical Developments
For microplastics analysis, both Raman and FTIR would benefit from greater technique automation to perform tasks such as automated particle finding.
One of the challenges of using Raman imaging to measure particles of different sizes is that the optimal focusing conditions for these can vary and so some type of automated adjustment and optimization would be needed. Faster analysis times are also always desirable.
One of the main challenges for optical techniques to study microplastics is dealing with the often complex mixtures and environments the microplastic samples are found in.
One advantage of Raman and FTIR is that minimal additional sample preparation is needed for measurements.
This means that the techniques have great potential for being able to measure microplastics in situ, rather than the need for sampling and returning these to the lab for analysis.
As well as improvements in hardware, such as faster detector readouts and lower dark counts to improve signal to noise, one area that will benefit both techniques will be the development of better algorithms for noise filtering and fluorescence background rejection (for Raman).
This will enhance the areas that can be mapped with imaging versions of the techniques.
These developments will help provide greater insights into both the chemical composition and size of microplastics in the environment and their exact roles and behavior.
References and Further Reading
PlasticsEurope (2017) Plastics: The Facts 2017, https://plasticseurope.org/, accessed September 2021
PlasticsEurope (2020) Plastics: The Facts 2020, https://plasticseurope.org/, accessed September 2021
Hale, R. C., Seeley, M. E., La Guardia, M. J., Mai, L., & Zeng, E. Y. (2020). A Global Perspective on Microplastics. Journal of Geophysical Research: Oceans, 125(1), 1–40. https://doi.org/10.1029/2018JC014719
Käppler, A., Fischer, D., Oberbeckmann, S., Schernewski, G., Labrenz, M., Eichhorn, K. J., & Voit, B. (2016). Analysis of environmental microplastics by vibrational microspectroscopy: FTIR, Raman or both? Analytical and Bioanalytical Chemistry, 408(29), 8377–8391. https://doi.org/10.1007/s00216-016-9956-3
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. https://doi.org/10.1016/j.watres.2018.05.060
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