Identification of microplastics with Raman microscopy

Importance of the Topic

Microplastics, defined as plastic particles smaller than 5 mm, represent the most abundant form of marine debris and pose emerging risks to ecosystems and human health. Their small size and complex sources complicate routine monitoring and impact assessments. Reliable identification of polymer type and additives in environmental microplastics is critical to trace their origin and predict biological effects.

Objectives and Study Overview

This study evaluates the performance of a portable Raman microscopy system for rapid identification of microplastics recovered from estuarine surface waters. Key goals include:

• Assessing the system’s ability to distinguish common polymers (polyethylene, polypropylene, polystyrene)
• Demonstrating identification of primary (beads, fibers) and secondary (irregular fragments) microplastics
• Exploring detection of colorants and pigments in plastic particles

Methodology

Water samples were collected from the surface of Delaware Bay (USA), preserved with 4 % formaldehyde and size-fractioned using stainless steel sieves (5 000, 1 000, 300 µm). The 300 and 1 000 µm fractions were dried, oxidized with peroxide and density-separated to isolate microplastics. Recovered particles were sorted by morphology (fragment, fiber, bead, film, foam, rubber) under a stereomicroscope before Raman analysis.

Instrumentation

A portable i-Raman® EX spectrometer with 1 064 nm CleanLaze® laser excitation and InGaAs detector was used, coupled to a BAC151C video microscope with a 50× objective (42 µm spot size, 9.15 mm working distance). Acquisition parameters included laser power <165 mW (<50 % full power) and integration times from 30 s to 3 min. Spectral identification was performed using BWID® software against a polyethylene, polypropylene and polystyrene library, calculating a hit quality index (HQI) from 0 to 100.

Key Results and Discussion

Analysis of 22 particles yielded:

• 11 polyethylene identifications (HQI >95)
• 4 polypropylene identifications (HQI >90)
• 2 polystyrene beads (spherical primary particles, HQI >98)
• 5 inconclusive results, primarily dark-colored fragments due to elevated fluorescence or altered spectra

Example findings included clear differentiation of irregular secondary fragments (blue PE, HQI 95.7) and identification of copper phthalocyanine green pigment in colored PP fibers. Lower HQI scores highlighted the impact of dyes and sample degradation, underscoring the need for moderate laser power to prevent burning.

Benefits and Practical Applications

• Rapid, on-site polymer identification without extensive sample preparation
• Detection of pigment additives to trace plastic sources
• Portable solution for environmental monitoring in remote or field locations
• Complementary alternative to FTIR microscopy for sub-100 µm particles

Future Trends and Opportunities

Advancements in portable Raman technology, enhanced spectral libraries and multivariate algorithms will improve identification accuracy for dark or heavily pigmented microplastics. Integration with automated imaging and machine learning promises higher throughput screening. Expanding databases to include weathered and composite plastics will further extend environmental forensics capabilities.

Conclusion

This application note demonstrates that portable Raman microscopy with 1 064 nm excitation provides a robust and expedient approach to identify environmental microplastics down to tens of micrometers. High HQI matches for common polymers and pigment detection confirm its utility for field-based environmental monitoring and source tracking.

Reference

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