High-Speed Measurement of Microplastics Smaller than 100 μm Collected on a Filter and Efficient Analysis

Importance of the Topic

Microplastics smaller than 100 µm are increasingly detected in aquatic environments and drinking water, raising concerns about their environmental fate and potential health impacts. Rapid and reliable characterization of these particles in terms of identity, size distribution, and mass is essential for environmental monitoring, regulatory compliance, and risk assessment.

Objectives and Overview of the Study

This study demonstrates a high-throughput analytical workflow combining Fourier transform infrared (FTIR) microscopy with a high-speed mapping program and a dedicated particle analysis software. The primary goal is to reduce measurement time and automate data processing for microplastics collected on filters, using a standard reference sample to validate performance gains and data accuracy.

Instrumentation Used

• FTIR Microscope System: IRTracer-100 coupled with AIMsight infrared microscope
• Sample Holder: Particle filter (PF) holder for 13 mm diameter Si filters (10×10 mm, 5 µm pores)
• Detector: Two-dimensional T2SL focal plane array
• Software: High-Speed Mapping Program; Particle Analysis Program

Methodology

A tablet of reference microplastics was dispersed in purified water and deposited onto an Si filter, which was secured in a PF holder to ensure a flat analysis surface. Transmission spectra were acquired across a 1.7 × 2.1 mm area using 8 cm⁻¹ resolution, 30 scans per point, SqrTriangle apodization, and a 20 × 20 µm aperture with 20 µm step size. The high-speed mapping algorithm monitored C–H stretching peaks between 3200 and 2800 cm⁻¹ in an initial scan at each pixel. Points lacking detectable peaks were skipped, cutting overall mapping time to approximately one-eighth of conventional approaches.

Key Results and Discussion

The automated map revealed distinct spectral fingerprints corresponding to polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), and protein. The particle analysis software color-coded each polymer type, counted particles, and extracted quantitative metrics including major and minor axes, Feret diameter, and projected area. In the selected region, one PE particle, one PET particle, and seven PS particles were identified. PS particles ranged from under 50 µm up to clusters between 150 and 400 µm, consistent with the enlarged tiled images. Individual particle tables also provided estimated volumes and masses using a log–log relationship between surface area and mass.

Benefits and Practical Applications of the Method

• Measurement time reduced by a factor of eight compared to standard mapping techniques.
• Automated polymer identification and color-coded visualization facilitate rapid data interpretation.
• Comprehensive size distribution, volume, and mass estimates support detailed microplastics profiling.
• CSV export of particle data enables seamless downstream statistical analysis and reporting.

Future Trends and Potential Applications

Advances may include expanding spectral libraries to cover a wider range of polymers and additives, and incorporating machine learning for more robust peak detection and classification. Improved detector sensitivity and filter materials could extend analysis to sub-micron particles. Applications span environmental monitoring, water treatment validation, food and beverage safety, and quality control in manufacturing processes.

Conclusion

The integration of a high-speed FTIR mapping program with automated particle analysis offers a powerful, efficient solution for characterizing microplastics smaller than 100 µm. This workflow significantly accelerates data acquisition while delivering detailed information on particle identity, size, volume, and mass, thereby enhancing the reliability of microplastics assessments.

References

1. Tomoya Kataoka, Yota Iga, Rifqi Ahmad Baihaqi et al. The geometric relationship between the projected surface area and the mass of a plastic particle. Water Research. 2024;61:122061.
2. Ministry of the Environment. River and Lake Microplastics Investigative Guidelines. Water Environment Management Division, Environmental Management Bureau. March 2024.