Identification of Microplastics using the Nicolet RaptIR FTIR Microscope

Importance of the topic

Microplastics are pervasive environmental contaminants detected in water, air and food chains and are the subject of emerging regulatory requirements. Reliable identification and characterization of microplastics (size, polymer type, morphology, degradation state) are essential for environmental monitoring, source attribution, risk assessment and to support standardization of analytical protocols. FTIR microscopy combined with automated image analysis addresses key analytical challenges posed by particle heterogeneity, small size ranges and the need to minimize sample handling artifacts.

Objectives and study overview

This application note demonstrates how the Thermo Scientific Nicolet RaptIR FTIR Microscope together with OMNIC Paradigm software can be used to identify and characterize microplastics on filters. The main goals were to: establish automated workflows for particle detection and/or chemical mapping; evaluate reflectance-mode spectral collection and matching against a purpose-built microplastics reflectance library; and produce comprehensive reports that include particle identity, size, shape and spatial distribution.

Methodology

- Sample preparation: Water/wastewater samples were processed following the SCCWRP/California SWRCB protocols to isolate particles in the ~1–50 μm range. Final concentrates were deposited onto reflective substrates such as silicon, gold-coated polycarbonate, Al2O3 or stainless steel filters. Air-deposited particles were collected by exposing filters/slides to outdoor environments.
- Data acquisition strategies: Two complementary approaches were used:

• Particle analysis: Visual/optical image processing identifies particles on the filter, the microscope stage is automatically positioned to each particle, apertures are set according to particle form factors (size/shape), background spectra are collected and reflectance spectra are measured for each particle.
• Chemical mapping: A full-area or multi-region map is acquired where every pixel contains an IR spectrum; pixel-wise correlations and chemometric tools (correlation analysis, MCR, PCA) are applied to find and identify particles regardless of visual contrast with the substrate.

- Spectral acquisition parameters: Reflectance spectra were collected with a liquid-nitrogen-cooled MCT detector at 8 cm-1 resolution. Particle-mode spectra were typically co-added from 8–16 scans (examples cite 16 scans); other examples referenced 32-scan acquisitions for library generation.

Used instrumentation

- Thermo Scientific Nicolet RaptIR FTIR Microscope equipped with high-quality visible and IR optics for particle imaging and aperture control.
- OMNIC Paradigm software for automated particle finding, aperture optimization, spectral acquisition and reporting.
- Single-point LN2-cooled MCT detector for reflectance spectral collection.

Microplastics reference library

A reflection-mode microplastics reference library was compiled from the CMDR Polymer Kit (Hawaii Pacific University), Polysciences microbead standards and Sigma-Aldrich polymer standards. The library represents approximately 30 common polymer types (polyethylenes of various densities, polypropylene, polyesters, PET, polyvinyl chloride variants, polystyrenes, nylons, polycarbonate, PTFE, PVDF, PMMA, PU, silicone, epoxy, etc.) plus several common laboratory contaminants (cellulose, skin cells, hair, soil/silica, nitrile glove residue). The library was specifically created in reflectance/FTIR mode to improve spectral matching when data are acquired in reflection rather than ATR or transmission.

Results and discussion

- Particle analysis workflow: Automated particle finding on a 10 × 10 mm silicon filter successfully identified particles across a broad size range (~25 μm to 1 mm). The system automatically selected apertures, collected spectra and produced per-particle identifications with associated size and shape metrics. Results were delivered in a comprehensive report including counts, size distributions and per-material maps.
- Chemical mapping workflow: Full-area reflectance maps provided pixel-level correlation to target polymers (example: polyethylene highlighted as correlated regions). Mapping was particularly useful for low-contrast particles, dense particle clusters, fibers and films, and for detailed investigation of laminated or environmentally degraded particles.
- Reflection vs ATR considerations: Reflection-mode acquisition avoids physical contact of an ATR tip with multiple particles, eliminating cross-contamination and adhesion artifacts inherent to automated micro-ATR when moving between particles. Because most commercial libraries are ATR or transmission-dominated, the availability of a reflectance library significantly improves match quality for reflectance-collected spectra.
- Data processing: Once spectra/maps are acquired, automated library matching, correlation analyses and multivariate methods (MCR, PCA) enable robust particle identification even in complex samples. Software-generated reports summarize particle identity, shape, size distribution and spatial distribution for downstream interpretation.

Benefits and practical applications

- Throughput and automation: Automated particle finding and stage control accelerate analyses of hundreds to thousands of particles with minimal operator input, enabling high-throughput monitoring.
- Reduced contamination risk: Non-contact reflectance measurement reduces cross-sample contamination compared with ATR tip approaches.
- Comprehensive outputs: Integrated imaging and IR data provide combined physical (size, shape) and chemical (polymer identity) characterization useful for environmental forensics, QA/QC, regulatory compliance and research into degradation pathways.
- Flexibility: Ability to analyze single particles or perform full-area mapping suits a range of sample types from sparse atmospheric deposition to dense filter loads and fibers/films.

Limitations and considerations

- Mapping trade-offs: Full-area chemical mapping can produce many empty (particle-free) pixels, increasing acquisition time and data storage requirements; region selection can mitigate this but requires prior knowledge or iterative analysis.
- Degraded polymers: Environmental weathering (oxidation, UV exposure) alters spectral features and can complicate library matching; reflectance libraries that include environmentally altered spectra or advanced chemometric approaches improve identification fidelity.
- Library dependence: High-quality reflectance-mode reference spectra are essential; legacy ATR/transmission libraries are suboptimal for reflectance data without appropriate conversion or reflectance-specific references.

Future trends and potential uses

- Enhanced spectral libraries: Expansion of reflectance-mode libraries to include weathered/degraded polymers and common contaminants will increase identification reliability for environmental samples.
- Machine learning and automated classification: Integration of supervised/unsupervised ML methods can improve particle detection, spectral deconvolution and classification in complex matrices.
- Standardization and inter-laboratory workflows: Harmonization of sample preparation, mapping/particle-analysis protocols and reporting formats (consistent with regulatory bodies like SCCWRP/SWRCB) will facilitate comparability and regulatory adoption.
- High-throughput monitoring networks: Automated FTIR microscopy combined with optimized sample handling can support routine monitoring of water treatment, wastewater effluent, atmospheric deposition and food-chain studies.

Conclusion

The Nicolet RaptIR FTIR Microscope with OMNIC Paradigm software provides an efficient, automated reflectance-based workflow for microplastics identification and characterization. By combining automated particle finding, aperture optimization, reflectance-mode spectral acquisition and purpose-built reference libraries, the system delivers reliable polymer identification along with particle size and morphology metrics while minimizing contamination risks inherent to contact-based ATR approaches. The approach supports both targeted particle analysis and full-area chemical mapping, making it adaptable to diverse sample types and research or monitoring needs.

References

1. Rochman C. & Hoellein T. 2020. Science, 368, 1184-1185.
2. World Health Organization. 2019. WHO calls for more research into microplastics and a crackdown on plastic pollution. [web notice]
3. California State Water Resources Control Board. Microplastics method (SCCWRP/SWRCB) for drinking water. [protocol]
4. De Frond H. et al. 2021. Analytical Chemistry, 93(48), 15878–15885.
5. Cowger W. et al. 2020. Analytical Chemistry, 93(21), 7543–7558.
6. Hawaii Pacific University CMDR Polymer Kit information.