FTIR Letter Vol. 45

Significance of Topic

Fourier transform infrared (FTIR) spectroscopy and infrared microscopy have become essential tools in analytical chemistry for environmental monitoring, industrial quality control, and materials characterization. Advances in sample pretreatment and imaging software enable the reliable identification and quantification of microscopic particles such as tire and road wear particles (TRWPs) and other elastomers in complex matrices. Parallel innovations in FTIR instrumentation—especially compact, moisture-resistant spectrophotometers and liquid-nitrogen-free detectors—expand operational environments and simplify analyses. In-depth spectral interpretation of functional groups, such as carbonyls, further enhances compound identification and quantitation.

Objectives and Study Overview

Three linked topics were addressed:

• Develop a transmission-mode FTIR microscopy method to identify and quantify elastomer particles (≥20 µm) and TRWPs in sediment.
• Introduce the IRSpirit-X series FTIR spectrophotometers—particularly the moisture-resistant IRSpirit-ZX with a ZnSe beam splitter—and compare performance to KBr-based models.
• Detail infrared spectral analysis approaches for carbonyl-containing compounds, focusing on characteristic absorption bands beyond the main C=O stretching vibration.

Methodology and Instrumentation

• Sample Pretreatment for Sediment Analysis
  • Digestion: 10 % KOH in methanol (50 °C, 24 h) to remove organics, followed by flotation and high-density centrifugation to eliminate inorganics.
  • Filtration: 5 µm pore silicon membrane to collect elastomer particles while excluding fine impurities.
• FTIR Microscopy and Software
  • Transmission-mode FTIR microscopy (30×30 µm aperture, 15 µm step) for initial mapping and particle sizing using YCALOS software.
  • Reference Spectral Library: Transmission spectra of eight elastomer types were generated from pulverized environmental and tire tread samples; ATR spectra were used for high-carbon-black TRWPs.
  • Particle Identification and Quantification: Combined HQI thresholding, key-band checks, visual inspection, and ATR re-analysis for ambiguous spectra.
• IRSpirit-X Series FTIR Spectrophotometers
  • Models: IRSpirit-TX and ‑LX (KBr beam splitter) vs. IRSpirit-ZX (ZnSe beam splitter) for high humidity tolerance.
  • Detectors: DLATGS standard vs. optional LiTaO₃ (IRSpirit-LX); transmission range 7,800–350 cm⁻¹ for KBr, 6,000–550 cm⁻¹ for ZnSe.
  • TEC MCT Detector Option: Peltier-cooled MCT detector for AIMsight and AIRsight microscopes, enabling µFTIR without liquid nitrogen.
• Spectral Analysis of Carbonyls
  • Carboxylic Acids: Broad OH stretch at 3,300–2,500 cm⁻¹ due to hydrogen-bonded dimers.
  • Fatty Acid Band Progressions: Regularly spaced weak peaks (1,350–1,180 cm⁻¹) indicate methylene chain length.
  • Esters: C–O–C asymmetric (1,300–1,200 cm⁻¹) and symmetric (1,150–1,000 cm⁻¹) stretching bands.
  • Ketones: C–C–C asymmetric stretch at 1,250–1,025 cm⁻¹ (aliphatic) or 1,325–1,215 cm⁻¹ (aromatic).
  • Aldehydes: Weak Fermi resonance band near 2,720 cm⁻¹ from CH stretch overtone coupling.

Main Results and Discussion

• Tokyo Bay Sediment Analysis
  • 1.70×10⁵ elastomer particles per gram of dry sediment detected; NR+NBR (63.1 %), IIR (14.8 %), and SBR (10.7 %) dominated.
  • TRWPs accounted for 9.8 % of total particles (1,770 µg/g dry sediment), consistent with Py-GC/MS data from other Japanese waterways.
  • Median particle diameter was 45.8 µm, smaller than previously reported ranges.
• Instrument Comparison
  • IRSpirit-ZX with ZnSe beam splitter maintained stable baseline and high transmittance under 40 °C/70 % RH, unlike KBr models prone to deliquescence.
  • TEC MCT detector produced high-quality µFTIR spectra (25×25 µm) without liquid nitrogen, matching standard T2SL performance.
• Spectral Interpretation
  • Demonstrated practical identification of carbonyl subclasses and quantitation of fatty acid methyl esters (FAMEs) in diesel blends via C=O peak calibration.

Benefits and Practical Applications

• Comprehensive FTIR microscopy workflow enables reliable detection and quantification of elastomer and TRWPs for environmental monitoring and regulatory compliance.
• IRSpirit-ZX offers robust field and laboratory operation in humid environments without frequent desiccant replacement.
• TEC MCT detector simplifies µFTIR analyses by eliminating liquid nitrogen requirements, reducing operational complexity.
• Carbonyl spectral diagnostics sharpen compound classification in materials research, quality control, and forensic analysis.

Future Trends and Potential Applications

• Extending FTIR microscopy protocols to wastewater, atmospheric particulates, road runoff, and biological tissues for broader microplastic and elastomer monitoring.
• Integrating ATR and transmission spectra into unified spectral libraries and machine-learning identification algorithms for automated particle classification.
• Developing standardized global methods combining FTIR microscopy and Py-GC/MS for cross-validated quantitation of TRWPs and related pollutants.
• Advancing compact spectrophotometer design with on-board humidity control, cryogen-free detectors, and AI-driven spectral advisors for real-time analysis.

Conclusion

A synergistic suite of FTIR microscopy methods, enhanced instrumentation, and detailed spectral interpretation has been established to tackle environmental and materials-analysis challenges. The transmission-mode µFTIR workflow with optimized sample pretreatment and software delivers accurate elastomer particle quantitation in sediments. The IRSpirit-X series, especially the moisture-resistant IRSpirit-ZX and liquid-nitrogen-free TEC MCT detector option, expands usability across diverse environments. Advanced carbonyl band assignments further refine compound identification. Together, these developments pave the way for standardized, high-throughput analyses in environmental monitoring, industrial QA/QC, and research applications.

References

1. Scholz N.L. et al. Recurrent Die-Offs of Adult Coho Salmon in Urban Streams. PLoS ONE 2011, 6:e28032.
2. Tian Z. et al. A ubiquitous tire-derived chemical induces acute mortality in coho salmon. Science 2020, 371:185–189.
3. Yamamoto K., Furumai H. Method for Identification of Black Microplastics by ATR-FTIR. Jpn. Soc. Civil Eng. G 2022, 78(III):349–358.
4. ISO/TS 20596:2017b. Determination of Mass Concentration of TRWP in Soil and Sediments – Pyrolysis-GC-MS.
5. Jeong S. et al. Quantification of tire wear particles using pyrolysis-GC-MS. Sci. Total Environ. 2024, 942:173796.
6. Rauert C. et al. Challenges with Quantifying Tire Road Wear Particles. Environ. Sci. Technol. Lett. 2021, 8:231–236.
7. Kameda Y., Yamada N., Fujita E. Source- and polymer-specific size distributions of microplastics. Environ. Pollut. 2021, 284:117516.
8. JIS K6230:2018. Rubber—Identification—Infrared Spectrometric Methods.
9. Rachi S., Kameda Y., Fujita E. Automated identification and quantification software for environmental microplastics. Proc. 56th Annual Meeting Jpn. Soc. Water Environ. 2022, 460.
10. Japan Rubber Manufacturers Assoc. Statistics on rubber products. (2025).
11. Unice K.M. et al. Comparison of TRWP concentrations in sediment by pyrolysis-GC/MS. Environ. Sci. Technol. 2013, 47:8138–8147.