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

Importance of the topic

Microplastics (MPs) measuring from a few micrometers up to several millimeters pose significant environmental challenges, especially in aquatic ecosystems. Accurate identification and quantification of MPs are vital for pollution monitoring, regulatory compliance, and risk assessment. Infrared spectroscopy coupled with microscopy offers molecular specificity, but sample preparation and filter handling often limit precision when analyzing particles below 100 µm.

Study Objectives and Overview

This study aimed to evaluate various filter materials and introduce a particle filter (PF) holder that maintains flatness during drying and infrared measurement. Using a combination of an IRXross FTIR spectrophotometer and an AIMsight infrared microscope, the work compared filter media, optimized mapping parameters, and demonstrated the accuracy and efficiency of MP detection and analysis.

Methodology and Instrumentation

Sample Preparation and Filtration:

• Standard polystyrene (PS), polyethylene (PE), and polypropylene (PP) particles (≤100 µm) dispersed in purified water.
• Suction filtration through 13 mm PTFE, Al₂O₃, gold-coated polycarbonate (Au/PC), and stainless steel filters with pore sizes from 0.2 µm to 8 µm.

Infrared Measurement:

• High-speed mapping range set to detect C–H signals (3,400–2,400 cm⁻¹).
• Resolution of 8 cm⁻¹, 50 scans per spot, and 20 µm × 20 µm aperture.
• Transmission mode for PTFE and Al₂O₃ filters; reflection mode for Au/PC and stainless steel.
• Data processed using a particle analysis program to extract count, size, area, volume, and theoretical mass.

Instrumentation Used

• IRXross FTIR Spectrophotometer
• AIMsight Infrared Microscope
• Particle Filter (PF) Holders for 13 mm and 25 mm filters
• High-speed Mapping and Particle Analysis Software Modules

Main Results and Discussion

Filter Characteristics and Spectral Interference:

• PTFE filters exhibit absorption near 1,200 cm⁻¹, overlapping key C–O–C stretching vibrations of certain MPs.
• Al₂O₃ filters absorb between 1,200 and 700 cm⁻¹, affecting aromatic ring detection in PS spectra.
• Au/PC filters produce interference fringes in reflection mode, reducing search reliability.
• Stainless steel filters allow broad transmittance (4,000–700 cm⁻¹) with minimal background, facilitating detection across polymers.

PF Holder Performance:

• The PF holder maintains a horizontal, wrinkle-free surface, improving focus uniformity and particle detection across the filter area.
• Comparative images show significantly sharper, more consistent focus when using the PF holder.

Particle Analysis Outcomes:

• Detected 26 PS, 21 PP, and 1 PE particles on stainless steel filters.
• Most common major axis diameters ranged from 20 to 40 µm.
• Mass and volume estimated via theoretical area–mass relationships, acknowledging potential limitations.

Benefits and Practical Applications of the Method

• Flattened filters provide clear fields of view, reducing measurement errors and improving throughput.
• High-speed IR mapping combined with automated particle analysis accelerates identification and quantification of MPs.
• Choice of filter material can be tailored to target polymer types and size ranges, balancing filtration time and spectral clarity.

Future Trends and Potential Applications

• Integration of machine learning for automated classification and spectral interpretation of MPs.
• Development of advanced filter materials with minimized spectral interference.
• Miniaturization and standardization of PF holders for field-deployable infrared microscopes.
• Implementation in routine environmental monitoring and quality control labs for water safety assessments.

Conclusion

Employing a specialized PF holder with FTIR microscopy enhances the reliability of microplastic analysis by ensuring filter flatness and reducing spectral artifacts. Selection of filter materials based on spectral compatibility is critical. The combined high-speed mapping and particle analysis workflow delivers accurate, efficient detection of MPs as small as 20 µm, offering a robust approach for environmental and industrial applications.

References

• Ministry of the Environment. FY 2020 Comprehensive Assessment of the Current Status, Biological Impacts, and Other Considerations Regarding Marine Debris. 2025. Available at: https://www.env.go.jp/content/900543555.pdf
• Kataoka T, Iga Y, Baihaqi RA, et al. Geometric relationship between the projected surface area and mass of a plastic particle. Water Research. 2024;61:122061.