Identification of Plastics/Microplastics (MPs) Using FTIR

Importance of the topic

The identification of plastics and microplastics (MPs) in environmental samples is essential for research, monitoring, recycling and risk assessment. Fourier transform infrared (FTIR) spectroscopy provides a rapid, nondestructive way to determine polymer composition by measuring vibrational signatures of functional groups. Accurate assignment of spectral features and use of spectral libraries—including libraries specific to UV‑degraded plastics—improves identification reliability for environmental samples that commonly contain weathered polymers and interfering organic matter.

Objectives and overview of the study

This application study measured and compared FTIR spectra of nine representative plastics before and after accelerated UV degradation. The aims were to (1) document characteristic IR peak positions for each polymer, (2) evaluate spectral changes caused by UV aging (appearance of carbonyl/hydroxyl bands, peak broadening), and (3) demonstrate how spectral-library searching combined with peak-attribute checks supports higher-confidence identification of plastics and MPs in environmental matrices. National guidelines (Japanese MOE) and ISO 16094‑2 were used as reference frameworks for peak selection and operator decision criteria.

Methodology

Measurements were performed on representative bulk polymer specimens subjected to accelerated UV exposure using an ultra-accelerated weathering tester. FTIR spectra were acquired in transmission/ATR modes (including QATR accessories) before and after controlled UV irradiation periods specific to each polymer. Major diagnostic peaks were annotated and tabulated. Library search scores (similarity, hit quality index) were used to generate candidate identifications; when automated indices were marginal, operator judgment using the documented characteristic wavenumbers was recommended.

Used instrumentation

• FTIR spectrometers: IRSpirit‑X, IRXross, IRTracer series (Shimadzu models used in the study).
• ATR accessory: QATR‑S / QATR 10 for surface/solid measurements.
• UV accelerated weathering tester: Iwasaki Electric ultra‑accelerated device (SUV‑W262/161) with metal halide light source.

Main results and discussion

The study documents characteristic peak wavenumbers for nine common polymers and reports systematic spectral changes induced by UV exposure. Key observations across polymers include:

• Carbonyl formation: UV aging commonly produced new or intensified carbonyl bands in the 1700–1760 cm−1 range (e.g., PE, PP, PS, PC, PMMA), indicative of oxidative chain scission and photo‑oxidation products.
• Hydroxyl formation and broadening: Hydroxyl bands near ~3500 cm−1 and general peak broadening were observed in readily degraded polymers (PET, PC, PVC, PU), reflecting chain oxidation and increased hydrogen‑bonded species.
• Polymer‑specific diagnostic bands: Distinct peak sets that remain useful for ID even after weathering were identified—for example:

1. Polyethylene (PE): strong CH2 stretches ~2915 and ~2845 cm−1; bending around 1460–1472 cm−1; low‑frequency skeletal features near 730–718 cm−1. Density (HDPE vs LDPE) affects peak positions/intensities.
2. Polypropylene (PP): CH stretches near 2953 and ~2919 cm−1 and multiple CH deformation bands; carbonyl bands appear quickly on UV exposure.
3. Polystyrene (PS): aromatic C–H stretches around 3027 cm−1 and characteristic ring vibrations near 1600 and 1490 cm−1; fingerprint bands near 1027 and 756 cm−1.
4. Polyethylene terephthalate (PET): ester carbonyl ~1713–1715 cm−1; C–O and skeletal bands near 1410, 1342, 1241, 1094 cm−1; hydroxyl growth and band broadening after short UV exposure; reduced crystallinity lowers certain peak intensities.
5. Polyamide/nylon (PA): N–H stretch ~3298 cm−1, CH stretches ~2930/2857 cm−1, amide I at ~1630–1634 cm−1 and amide II at ~1538 cm−1; crystalline markers observed in certain bands.
6. Polyurethane (PU): N–H bands ~3290 cm−1, C–H stretches and urethane C=O near 1700–1735 cm−1, plus diagnostic urea/urethane fingerprint bands (≈1538, 1414, 1312, 1223 cm−1).
7. Polyvinyl chloride (PVC): CH stretches ~2911–2919 and 2862 cm−1; plasticizer‑derived ester peaks (phthalate esters) around ~1730 cm−1 more intense in soft PVC; UV aging yields hydroxyl bands and peak broadening.
8. Polycarbonate (PC): aromatic and carbonate features including strong bands near 1770 cm−1 (carbonate C=O), and fingerprint peaks ~1500, 1217, 1159, 1081, 1013 cm−1; rapid UV‑induced broadening and OH formation were observed.
9. Polymethyl methacrylate (PMMA): ester carbonyl ~1722–1729 cm−1 and multiple fingerprint peaks (1430–1254, 1193–1186, 1098, 990–965 cm−1); PMMA showed limited UV degradation under the test conditions.

Library searches complemented by manual confirmation of characteristic peaks were emphasized. The Japanese MOE guidance and ISO 16094‑2 recommend that when automated hit quality falls in intermediate ranges (e.g., HQI 60–80%), an operator should confirm identification by checking the presence/absence of characteristic wavenumbers. The study further notes that environmental samples often contain biological or cellulosic material that may confound searches; these interferences can be distinguished using their own characteristic bands.

Practical benefits and applications

• FTIR with ATR is a rapid, non‑destructive frontline method for identifying bulk plastics and MPs recovered from environmental samples.
• Documented characteristic wavenumbers support operator verification when library matches are ambiguous, increasing confidence in MP identification per regulatory guidance.
• Using a dedicated spectral library for UV‑degraded plastics improves match quality for weathered environmental materials.
• Knowledge of polymer-specific buoyancy (density ranges reported) assists sampling interpretation: PE and PP tend to float, PET, PA, PVC, PC, PMMA, PU tend to sink.

Future trends and potential applications

Advances likely to enhance environmental plastic identification include:

• Expanded and standardized degraded‑polymer spectral libraries (including environmentally aged spectra and common additives/plasticizers).
• Integration of micro‑FTIR imaging and focal‑plane array detectors for particle‑level mapping and automated screening of filters and sediments.
• Machine‑learning algorithms for improved spectral matching and automated classification that incorporate degradation patterns and additive signatures.
• Combining FTIR with complementary techniques (Raman, thermal analysis, GC‑MS for additives) to resolve ambiguous cases and characterize additives/chemical weathering products.
• Portable/field ATR‑FTIR systems with validated workflows for rapid in‑situ screening and QC of monitoring programs aligned with ISO/MOE frameworks.

Conclusion

Measured FTIR spectra of nine representative plastics before and after accelerated UV exposure confirm that characteristic diagnostic peaks remain useful for polymer identification even after environmental‑style degradation, although new bands (carbonyl, hydroxyl) and peak broadening complicate automated library matching. Combining spectral‑library searches—ideally including UV‑degraded reference spectra—with operator review of characteristic wavenumbers produces more reliable identifications consistent with MOE and ISO guidance. Implementation of expanded degraded libraries and advanced data‑analysis tools will further improve robustness of MP identification workflows.

References

1. FTIR Talk Letter, volumes 37 and 42–46.
2. Ministry of the Environment, Japan. Guidelines for River Microplastic Monitoring Methods. April 2026.
3. ISO 16094‑2. Water quality — Analysis of microplastic in water — Part 2: Vibrational spectroscopy methods for waters with low content of suspended solids, including drinking water.
4. The Japan Society for Analytical Chemistry Symposium on Polymer Analysis and Research. Polymer Analysis Handbook, New Edition. Asakura Publishing Co., Ltd., 2008, pp. 382–593.
5. Shimadzu Corporation. Application notes referenced: Analysis of Ultraviolet‑Degraded Plastic by Plastic Analyzer (A647); Analysis of Resin Using FTIR and Thermal Analysis (T154); Prescreening of Phthalic Esters Contained in PVC Products by FTIR (A602).