Identification of Microplastics with Portable Raman Microscopy

Significance of the Topic

Marine microplastics pose ecological hazards due to their persistence, ubiquity, and potential to transport contaminants. Analytical confirmation of microplastic identity is essential for reliable environmental monitoring and risk assessment.

Objectives and Study Overview

This application note demonstrates the use of a portable 1064 nm Raman microscopy system for the characterization of microplastics collected from Delaware Bay surface waters. Aims include validating polymer identification, assessing sample preparation workflows, and highlighting the system’s in-field capabilities.

Methodology and Instrumentation

• Sample Collection and Preparation:
  1. 5-minute surface tows with 200 μm mesh plankton net.
  2. Sample fixation in 4% formaldehyde, followed by size fractionation (300 μm to 5 mm).
  3. Wet peroxide oxidation and density separation to remove organic matter.
  4. Drying on 200 μm mesh in aluminum foil at 90 °C.
• Visual Pre-Screening: Identified and categorized particles under a stereomicroscope into fragments, fibers, beads, films, foams, and rubber.
• Raman Analysis:
  1. Instrument: i-Raman® EX portable system, 1064 nm excitation to suppress sample fluorescence.
  2. Video microscope assembly with 50× objective (9.15 mm working distance, 42 μm spot).
  3. Laser power <165 mW, integration times 30 s–3 min, spectra intensity corrected with NIST 2244 standard.
  4. Software: BWSpec® for data capture; BWID® library matching with hit quality index (HQI).

Main Results and Discussion

• Polyethylene (PE): 11 samples identified (HQI > 95).
• Polypropylene (PP): 4 samples identified; fiber analysis revealed additional peaks from copper phthalocyanine pigment, indicating colored additive presence.
• Polystyrene (PS): 2 beads reliably matched (HQI ~98).
• Inconclusive: 5 black fragments due to strong absorption and fluorescence interference.

Laser-induced damage was noted on delicate fibers at higher power, highlighting the need for power optimization.

Benefits and Practical Applications

• Rapid, in-field polymer identification to reduce sample misclassification.
• Non-destructive analysis preserving sample integrity for further tests.
• Library-based matching enables source tracking and pigment detection.

Future Trends and Potential Uses

• Integration with automated particle recognition for high-throughput screening.
• Expanding reference libraries to include a wider range of polymer blends and additives.
• Combining Raman with complementary techniques (e.g., FTIR, hyperspectral imaging) for comprehensive microplastic profiling.
• Development of standardized protocols to enhance inter-laboratory comparability.

Conclusion

Portable 1064 nm Raman microscopy offers a robust solution for microplastic identification, overcoming fluorescence challenges and enabling rapid polymer confirmation in the field. Its deployment supports improved environmental monitoring and pollutant source elucidation.

References

1. K.L. Law, Annu. Rev. Mar. Sci. 9, 205–229 (2017).
2. T.S. Galloway et al., Nat. Ecol. Evol. 1 (2017).
3. J.R. Jambeck et al., Science 347, 768–771 (2015).
4. R.C. Hale et al., J. Geophys. Res. Oceans 125 (2020).
5. J.R. Clark et al., Front. Ecol. Environ. 14, 317–324 (2016).
6. P. Vermeiren et al., Mar. Pollut. Bull. 113, 7–16 (2016).
7. J.H. Cohen et al., Environ. Sci. Technol. 53, 14204–14211 (2019).
8. J. Masura et al., NOAA Technical Memorandum NOS-OR&R-48 (2015).
9. A. Duran et al., J. Raman Spectrosc. 42, 48–55 (2011).