Fast, Automated Microplastics Analysis Using Laser Direct Chemical Imaging

Importance of the Topic

Microplastics are pervasive in marine environments as a result of large scale production and improper disposal of plastic waste. These particles pose risks to marine life and human health. Accurate monitoring of microplastic size shape and polymer type is critical for assessing environmental impact meeting regulatory requirements and guiding mitigation strategies.

Objectives and Study Overview

The study aimed to develop and demonstrate a fast automated workflow for characterizing and quantifying microplastics in high volume marine water samples. Water was collected in the Indian Ocean along a transect from Sumatra to Madagascar. The focus was on particles between 20 and 5000 micrometers with detailed analysis of particles below 300 micrometers.

Methodology and Instrumentation

Sampling

• High volume filtration using the Geesthacht Inert Microplastic Fractionator to separate suspended particulate matter into >300 micrometers and 10 to 300 micrometers fractions.
• Sampling system constructed from stainless steel and PFA seals to minimize contamination and backflushed between stations.

Sample Preparation

• Enzymatic and oxidative digestion with Proteinase K chitinase hydrogen peroxide and iron catalyst to remove organic matter.
• Density separation with zinc chloride solution and vacuum filtration onto PTFE and polycarbonate membranes.
• Strict contamination control using laminar flow cabinets and air purifiers.

Instrumental Analysis

• Agilent 8700 LDIR Chemical Imaging System employing a quantum cascade laser and MCT detector for rapid single wavenumber scans and full spectral acquisition.
• Automated particle detection size measurement and real time spectral matching to a library of over 420 reference spectra.
• Manual transflection and micro ATR-FTIR validation for particles above 300 micrometers using diamond or germanium crystals.

Main Results and Discussion

Particle concentrations ranged from 10 to 226 particles per cubic meter across stations. A total of 30 471 natural particles and 635 synthetic particles were identified. Synthetic polymers represented only 2.6 percent of total particles with acrylate polyurethane varnish PET PE chloride and PVC being the main types. Most microplastics were under 100 micrometers in size. Automated LDIR analysis matched manual ATR-FTIR results with high accuracy and enabled clear differentiation of particle aggregates fiber attached to diatoms and polymer blends. Comparison with earlier studies yielded similar concentration ranges confirming method robustness.

Benefits and Practical Applications

The presented workflow offers high throughput and reduced operator bias compared to manual FTIR or Raman methods. Automated imaging and spectral identification enable comprehensive analysis of entire filter areas. This approach supports large scale monitoring regulatory compliance and quality assurance in environmental laboratories.

Future Trends and Potential Applications

• Expansion of spectral libraries to include a wider range of anthropogenic and natural particles.
• Depth profiling of microplastic concentrations using LDIR at different water layers.
• Development of standardized procedures and interlaboratory comparisons.
• Integration with other imaging modalities for multimodal particle characterization.

Conclusion

LDIR imaging combined with an optimized sample preparation protocol provides a rapid reliable and automated method for microplastic analysis in marine water. The technique reduces contamination risk offers detailed particle characterization and is suitable for large scale environmental monitoring.

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