Introduction
Microplastics (MPs) have emerged as one of the most critical emerging environmental contaminants due to their persistence, ability to adsorb other pollutants, and adverse effects on human and ecosystem health. Defined as solid, water-insoluble polymer particles smaller than or equal to 5 mm, MPs encompass a wide range of compositions, densities, shapes, and sizes. They are classified as primary (manufactured at micrometer dimensions, such as cosmetic microbeads and synthetic fibers) and secondary (resulting from the degradation of macroplastics). These particles have been identified in all ecosystems, including marine waters, freshwater, soil, air, and even in deep oceans and polar regions. Annually, between 19 and 23 million tons of plastic waste enter aquatic systems, with projections reaching 53 million tons by 2030. Despite the widespread contamination, our knowledge regarding the toxicity of these particles, particularly nanoplastics and their effects on human health remains very limited. Accurate identification and quantification face significant challenges, including the lack of global standardized methods, particle diversity, and technical limitations. A quantitative assessment of 50 studies revealed an average quality score of only 8.41 out of 18, indicating generally poor study quality. The lack of standardized and harmonized protocols for sampling, preparation, identification, and quantification across diverse environmental matrices remains the primary obstacle to accurate risk assessment and global data comparability. This review critically examines the entire analytical workflow, from field sampling to chemical characterization, across aqueous, solid, biological, and atmospheric matrices, providing a comprehensive decision-making framework for method selection based on matrix type, particle size range, and study objectives.
Sampling Methodologies
Sampling is the most sensitive and impactful step in the microplastic analysis chain, as any error at this stage compromises the quality of final results. Method selection depends entirely on matrix type. Aqueous matrices utilize grab sampling (simple and low-cost), plankton nets (suitable for large volumes but losing particles smaller than mesh size), or automated pumping with in-line filters (capable of collecting very fine particles). Solid samples like soil and sediment employ surface sampling with simple tools and coring to examine vertical particle distribution. Wastewater requires 24-hour composite sampling using automatic samplers (gold standard method). Activated sludge presents the most challenging matrix due to exceptionally high viscosity and organic matter content. Biota sampling involves dissection of soft tissues or digestive tracts of aquatic organisms. Air and dust sampling use high-flow pumps with appropriate filters. Contamination control is critical across all methods, including using non-plastic containers, cotton clothing, HEPA-filtered hoods, and preparing blank samples. Conventional sampling methods (plankton nets, filtration with 1-5 μm meshes) are ineffective for nanoplastics, as these particles pass through filter pores and are dispersed in very large water volumes (tens to hundreds of liters) at extremely low concentrations (nanograms per liter). Advanced techniques like ultrafiltration with membranes of pore sizes <0.1 μm or ultracentrifugation at >100,000×g are required for nanoplastic collection. Surface adsorption onto container walls can lead to >50% particle loss during transfer and storage, necessitating silanized glassware or non-ionic surfactant addition.
Sample Preparation
Sample preparation critically impacts final result quality, accuracy, and reliability. Physical pre-treatment includes sieving, drying, and mass determination. Chemical pre-treatment involves organic matter digestion, physical separation, staining, and filtration. Fenton oxidation (Fe²⁺+H₂O₂, 1:10 molar ratio, H₂O₂ dose equivalent to 5× initial COD, pH 2.5–3.0, 30 minutes) achieves up to 97% COD removal in activated sludge without degrading PE, PA, or PET. However, practical implementation faces three fundamental limitations: scalability issues (peroxide costs escalate significantly for volumes above several liters, and neutralization of Fe(II)/Fe(III) salts is time-consuming and expensive); formation of iron hydroxide precipitates that trap fine particles (<100 μm), requiring ultrasonication or mild acid washing for release; and strong dependency on initial COD-to-H₂O₂ ratio, making control difficult in samples with variable composition. Physical separation methods include density separation (NaCl for low-density polymers like PE and PP; ZnCl₂ or sodium polytungstate for high-density polymers like PVC and PET), oil extraction (up to 99% recovery), electrostatic separation, and magnetic separation (up to 92% efficiency). Density separation is ineffective for nanoplastics (<30% recovery), as Brownian motion dominates over Stokes' law at submicron dimensions. Alternative methods like asymmetric flow field-flow fractionation (AF4) and size-exclusion chromatography (SEC) are more efficient for nanoplastic separation but require specialized equipment and long analysis times. Nile Red staining (10 mg/L in acetone, 37°C, 15–30 min) enables rapid screening, with polycarbonate track-etched (PCTE) filters (1 μm pore size) commonly used due to smooth surface and high contrast.
Identification and Polymer Characterization
Polymer identification techniques are classified into non-destructive spectroscopy, destructive mass spectrometry, and advanced microscopy. FTIR spectroscopy in ATR, micro-, and imaging modes provides non-destructive chemical fingerprinting but suffers from spectral interference and diffraction limits (particles <10 μm). Raman spectroscopy offers superior spatial resolution for sub-10 μm particles, with CARS and stand-off variants enabling intracellular and remote imaging. LC-MS detects additives at ng/L levels, providing sensitivity for plasticizers, stabilizers, antioxidants, and flame retardants. Py-GC-MS and TGA-MS enable sensitive mass quantification of all polymers (including sub-20 μm particles) but are destructive and matrix-sensitive. Advanced microscopy (SEM-EDS, TEM, Cryo-SEM) provides morphology information but requires combination with spectroscopy for chemical identification. The selection of appropriate methods depends on particle size, sample type, required accuracy, destructive/non-destructive nature, cost, and equipment availability.
Challenges and Future Perspectives
Key challenges include the lack of global protocols and method inconsistencies; cross-contamination and QA/QC deficiencies; economic barriers to advanced instrumentation; and unique nanoplastic issues including diffraction limits, density separation inefficiency, digestion risks, and lack of reference standards. International standardization efforts such as ISO 16094-2:2020, ISO 24187:2023, ISO/TR 21960:2020, EPA initiatives, and ASTM D8401 represent important progress, but knowledge gaps remain regarding nanoplastics, severe matrix interference, high costs, and development of rapid in-situ methods. Future perspectives emphasize deep learning for automated spectral identification (95-96% accuracy), portable sensor design, hyphenated techniques (SEM-EDS + μ-FTIR/μ-Raman + Py-GC-MS/LC-MS), global protocol development for nanoplastics (ISO, OECD, UNEP, EPA), and green chemistry approaches. Key recommendations include rigorous blanks and contamination controls throughout all stages; spike recovery tests with multiple polymers (PE, PA, PET) across size ranges in real matrices targeting ≥80% recovery; method selection aligned with matrix, size range, and objective using the decision framework; stringent QA/QC with transparent reporting; special attention to nanoplastics using advanced hyphenated methods; and continuous updating with latest international standards and inter-laboratory proficiency tests.
Conclusion
Interdisciplinary synergy among analytical chemistry, environmental science, polymer engineering, toxicology, data science, and economics is essential to address the complex questions of fate, transport, environmental impacts, and human health effects of micro- and nanoplastics, and to develop effective management strategies. This review provides a comprehensive roadmap for researchers and regulatory bodies to navigate the complex landscape of microplastic analysis, emphasizing the critical importance of quality control, method harmonization, and international collaboration to achieve accurate, reliable, and comparable global data for effective environmental monitoring and policy development.