How Colorants Complicate Raman Spectroscopy of Microplastics: New Insights from Environmental Research

How Colorants Complicate Raman Spectroscopy of Microplastics: New Insights from Environmental Research © John -chronicles-stock.adobe.com

Introduction
Microplastics (MPs), tiny plastic particles found ubiquitously in the environment, pose increasing ecological and health concerns. To identify and characterize these particles, scientists commonly use Raman spectroscopy due to its ability to provide molecular fingerprints of materials. However, a recent study by Azari and colleagues (1) reveals that the presence of colorants—pigments and dyes incorporated into plastics—can significantly hinder Raman spectral analysis. This research, published in Environmental Pollution, outlines the mechanisms through which colorants affect Raman signals and discusses the implications for accurately detecting and studying microplastics in environmental samples (1).

This investigation was conducted by Azari A., Ronsmans S., Vanoirbeek J.A., Hoet P.H., and Ghosh M., representing institutions focused on environmental sciences and toxicology. Their collaborative work contributes to ongoing efforts to refine microplastic detection methods crucial for monitoring pollution and assessing ecological risks (1).

Colorants and Their Impact on Raman Spectra
Azari and colleagues emphasize that colorants embedded in plastics cause strong fluorescence when subjected to Raman spectroscopy. This fluorescence manifests as broad background signals that can overwhelm or mask the distinctive Raman peaks essential for identifying the polymer type (1). The team explains that fluorescence is induced when certain chromophores in colorants absorb the laser excitation light and re-emit it at longer wavelengths, creating noise that interferes with clean spectral acquisition (1) .

This interference complicates the spectral interpretation of microplastics, especially since the characteristic vibrational bands of common polymers such as polyethylene, polypropylene, or polystyrene become difficult to resolve. The researchers highlight that standard Raman setups using visible lasers (commonly 532 nm or 785 nm) are particularly susceptible to this fluorescence from colorants (1).

Methodology and Spectroscopic Analysis
The study systematically examined various colored microplastic particles, analyzing how different pigments affected Raman signal quality. Using controlled laboratory conditions, the team applied Raman spectroscopy to microplastic samples containing known colorants and compared the spectral results to uncolored controls. They documented the presence of intense fluorescence backgrounds and distortion of Raman peaks depending on the colorant type and concentration (1).

Advanced spectral preprocessing techniques were tested but were found only partially effective in mitigating fluorescence effects. The authors note that selecting excitation wavelengths in the near-infrared region could reduce fluorescence but may compromise Raman signal intensity, creating a trade-off in analytical sensitivity (1).

Implications for Environmental Microplastic Studies
The findings raise concerns for researchers relying on Raman spectroscopy to identify microplastics in environmental samples, which often include a diverse range of colors and additives. The presence of colorants can lead to false negatives or misclassification if fluorescence is not properly accounted for. Azari and coauthors call for the development of improved spectroscopic protocols or complementary methods to overcome these challenges (1).

The authors propose that integrating traditional Raman with multiple lasers and appropriate detectors, along with establishing a more comprehensive reference library of colorants and additives, would improve the use of Raman spectroscopy for qualitative analysis of MPs (1). They suggest that employing advanced techniques like time-gated or surface-enhanced Raman spectroscopy (SERS) might be considered to improve outcomes.

Other research suggests that analytical techniques, such as Fourier-transform infrared spectroscopy (FT-IR) or hyperspectral imaging, might provide more reliable identification in complex MP samples (2). Additionally, better characterization of colorant types and their fluorescence behavior can aid in designing targeted analytical strategies (1).

References
(1) Azari, A.; Ronsmans, S.; Vanoirbeek, J. A.; Hoet, P. H.; Ghosh, M. Challenges in Raman Spectroscopy of (Micro)Plastics: The Interfering Role of Colourants. Environ. Pollut. 2024, 363, 125250. DOI: 10.1016/j.envpol.2024.125250

(2) Cordeiro, R. D.; Cardoso, V. V.; Carneiro, R. N.; Almeida, C. M. Validation of an FT-IR Microscopy Method for the Monitorization of Microplastics in Water for Human Consumption in Portugal: Lisbon Case Study. Environ. Sci. Pollut. Res. 2024, 1–22. DOI: 10.1007/s11356-024-33966-8