Technique for Measuring Microplastics Collected on Various Filters Using a Particle Filter Holder

Importance of the Topic

Microplastics ranging from a few micrometers up to 5 mm pose significant environmental challenges in marine ecosystems. Accurate identification of fine microplastics below 100 µm is essential for monitoring and mitigating their impact, leading regulatory bodies like Japan’s Ministry of the Environment to issue guidelines for sampling and measurement using infrared spectroscopy.

Objectives and Study Overview

This study demonstrates a streamlined methodology for measuring microplastics collected on various filter types using a particle filter holder (PF holder) combined with Fourier transform infrared (FTIR) spectroscopy and infrared microscopy. Key aims include evaluating filter materials, optimizing sample preparation with PF holders, and applying high-speed mapping and particle analysis software to improve accuracy and efficiency.

Used Instrumentation

• IRXross Fourier transform infrared spectrophotometer
• AIMsight infrared microscope
• Particle filter holders for 13 mm and 25 mm diameter filters
• High-speed mapping program and particle analysis program

Methodology

Standard microplastics of polystyrene (PS), polyethylene (PE), and polypropylene (PP) (20–40 µm) were dispersed in purified water and collected by suction filtration through filters made of PTFE, aluminum oxide (Al2O3), gold-coated polycarbonate (Au/PC), and stainless steel. Filters were dried flat using PF holders to prevent wrinkling. Infrared measurements employed transmission mode for PTFE and Al2O3, and reflection mode for Au/PC and stainless steel, with an aperture size of 20 µm×20 µm, 8 cm⁻¹ resolution, 50 scans, and SqrTriangle apodization. A mapping interval of 20 µm covered areas up to 780×560 µm depending on filter type, and particle detection focused on C–H signals between 3 400 and 2 400 cm⁻¹.

Main Results and Discussion

Filter material influenced spectral quality and detection efficiency: PTFE absorbed near 1 200 cm⁻¹, overlapping key peaks; Al2O3 absorbed in the 1 200–700 cm⁻¹ range, masking aromatic signals; Au/PC introduced interference fringes; stainless steel offered the broadest transmittance range. PF holders maintained filter flatness, yielding sharp, in-focus images and facilitating particle identification. Particle analysis detected 26 PS, 21 PP, and 1 PE particles, with major axis diameters predominantly between 20 and 40 µm.

Benefits and Practical Applications

• Consistent sample flatness and focus enhance measurement accuracy.
• High-speed mapping combined with particle analysis accelerates microplastic quantification.
• Adaptable filter selection allows tailoring to target polymer and size ranges.

Future Trends and Potential Applications

Advancements may include full automation of mapping and analysis, integration with complementary imaging modalities, expansion of detectable polymer libraries, and real-time field-deployable systems. Standardized workflows could further harmonize microplastic monitoring across laboratories and regulatory programs.

Conclusion

Implementing PF holders with FTIR spectroscopy and infrared microscopy provides a robust approach for accurate, efficient microplastic analysis on diverse filter substrates. Understanding filter-specific spectral behaviors and employing dedicated mapping and analysis software ensures reliable detection of fine microplastics.

References

1. Ministry of the Environment. FY2020 Comprehensive Assessment of the Current Status, Biological Impacts, and Other Considerations Regarding Marine Debris. 2025.
2. Kataoka T., Iga Y., Baihaqi R.A., et al. Geometric relationship between the projected surface area and mass of a plastic particle. Water Research. 2024;61:122061.