Fast, Automated Microplastics Analysis Using Laser Direct Chemical Imaging

Significance of the Topic

Microplastics are ubiquitous and persistent contaminants in marine ecosystems, posing risks to aquatic life and food chains. Accurate quantification and polymer identification of particles in the 1 µm to 5 mm range are critical to assess pollution levels, support regulatory measures and guide mitigation strategies. Conventional methods relying on visual sorting or manual FTIR/Raman spectroscopy are labor-intensive and prone to operator bias, underscoring the need for rapid, standardized workflows.

Objectives and Study Overview

This study presents a fully integrated workflow for automated, high-throughput analysis of microplastics in large-volume ocean water samples. Key goals included harmonizing sampling, sample preparation and spectroscopic analysis to minimize contamination, maximize representativeness and deliver reliable size, shape and polymer type distributions in marine environments.

Methodology and Instrumentation

The approach combined the following steps:

• High-volume fractionated sampling aboard the R/V Sonne using the Geesthacht Inert Microplastic Fractionator (GIMPF) to collect suspended particulate matter in two size fractions (10–300 µm and >300 µm).
• Enzymatic and oxidative digestion of the 10–300 µm fraction employing Proteinase K, H2O2/Fe2+ catalysis and chitinase, followed by density separation with ZnCl2 (ρ = 1.7 g mL⁻¹) to remove organic and inorganic matrix.
• Automated laser direct infrared imaging (LDIR) analysis with an Agilent 8700 LDIR Chemical Imaging System (QCL source, MCT detector) for the <300 µm fraction, scanning entire slide areas, locating particles and acquiring spectra in real time.
• Manual ATR-FTIR confirmation (diamond or germanium crystal) for particles >300 µm and spectral library comparison using Agilent Clarity software and additional databases.

Instrumentation

Key instruments included:

• Geesthacht Inert Microplastic Fractionator (GIMPF) with dual stainless-steel mesh cartridges.
• Agilent 8700 LDIR system featuring a tunable Quantum Cascade Laser and cooled MCT detector.
• Conventional ATR-FTIR spectrometer (diamond and germanium crystals).

Main Results and Discussion

Sampling across seven stations in the Indian Ocean (total volume ~61 m³) yielded 10–226 microplastic particles per cubic meter for sizes ≥20 µm. Automated LDIR classified 30 471 natural particles and 635 synthetic particles into 14 polymer clusters. The predominant synthetic materials in the <300 µm fraction were acrylates/polyurethanes/varnish (39 %), PET (26 %), PE-Cl (7 %), PVC (6 %), PE (5 %) and PP (5 %). Over 94 % of detected particles measured <100 µm. Extensive spectral library matching and manual checks confirmed >95 % identification accuracy for PE, PET, PP and PVDC reference materials. The workflow effectively discriminated natural detritus (97.4 % in environmental samples) from synthetic polymers (2.6 %). Multi-peak and µ-ATR modes resolved polymer blends, biofouled surfaces and microfibers with high spatial resolution.

Concentrations align with prior FTIR and Raman studies in surface and sub-surface waters, suggesting elevated microplastic loads in the sampled region. Polymer densities and the relative scarcity of low-density PE and PP hint at surface accumulation and biofouling-induced sinking dynamics.

Benefits and Practical Applications

The automated LDIR workflow offers significant advantages:

• Time-efficient processing of entire filter surfaces without manual selection.
• Reduced operator bias via real-time spectral library matching.
• Broad size range detection (10–5000 µm) with sub-100 µm sensitivity.
• Capability to analyze polymer composites, biofilms and entangled fibers.

These features make it well-suited for large-scale monitoring, regulatory compliance and rapid environmental assessments.

Future Trends and Potential Uses

Advancements may include:

• Implementation of depth-profile sampling to track microplastic vertical distribution.
• Standardization of SOPs and inter-laboratory validation for regulatory frameworks.
• Integration with automated reporting pipelines for real-time monitoring.
• Expansion of spectral libraries to cover emerging polymer formulations and additives.

Conclusion

This work demonstrates a robust, contamination-controlled workflow combining high-volume sampling, optimized digestion protocols and automated laser direct infrared imaging to characterize and quantify microplastics in marine waters. The approach delivers accurate polymer identification, size distribution and concentration data with minimal manual intervention, representing a promising standard for future environmental monitoring and research.

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