Microplastics Analysis Using FT-IR, IR Laser and Raman Spectroscopy

Microplastics analysis is the scientific process of detecting, identifying, counting, and characterizing plastic particles smaller than 5 mm in our environment, food, water, or in biological samples. A complete analysis determines not just whether microplastics are present, but how many, what size, what shape, and especially, what polymer type they are.

Visual inspection alone cannot distinguish a synthetic polymer particle from a natural one like cellulose or a mineral fragment. However, this information is very important for identifying, for example, the sources and origin of the particles. Chemical identification, for example using spectroscopy, is essential for confirming polymer identity and distinguishing plastic particles from visually similar natural or mineral particles

Microplastics have been detected in ocean water, deep-sea sediments, Arctic ice, agricultural soil, indoor air, tap water, bottled water, human lung tissue, blood, and placental tissue. Microplastics have been reported in marine and freshwater environments, soils, air, food and drinking water, as well as in biological samples.

Nanoplastics are commonly described as plastic particles below 1 µm. Their small size makes characterization particularly challenging. Raman microscopy offers higher spatial resolution than conventional IR microscopy and is therefore particularly useful for analyzing very small plastic particles.

The two main approaches are mass-driven analysis and particle-driven analysis. Mass-driven methods such as pyrolysis-GC/MS quantify polymer concentration by mass but destroy the sample, so they cannot provide particle counts, sizes or morphology. Particle-driven methods, such as IR or Raman microscopy, identify and characterize individual particles while preserving particle-level information.

For drinking-water microplastics analysis, Commission Delegated Decision (EU) 2024/1441 recognizes vibrational spectroscopy methods including μ-FTIR, μ-Raman and equivalent approaches such as QCL-IR. This means laboratories can select the technique that best matches their requirements for particle size, throughput and analytical detail.

The three techniques are FT-IR microscopy, IR Laser Microscopy, and Raman microscopy. All three methods combine a microscopic approach with a spectroscopic technique to measure a molecular vibration spectrum of a particle. Each offers different trade-offs in spatial resolution, throughput, spectral range, and sample compatibility.

Subsampling means analyzing only a representative fraction of a sample or filter instead of the complete filter. It is often used to reduce measurement time in particle-based spectroscopic workflows, especially when particle loads are high. However, because particles may be distributed unevenly across a filter, subsampling can introduce additional uncertainty through spatial bias, handling losses, overcrowding, or measuring too few particles.

Subsampling can reduce measurement time, but the measured fraction may not fully represent the complete filter. Uneven particle distribution, clustering, edge effects and handling losses can affect reported particle counts, polymer composition and size distributions. Bruker’s full-filter IR imaging workflows avoid this additional sampling step. With ILIM, complete 25 mm filters can be analyzed in less than 30 minutes, including spectral acquisition and automated data evaluation, enabling high-throughput analysis without relying on subsampling.

FT-IR microscopy combines infrared spectroscopy with microscopy to identify and characterize individual microplastic particles. It is an established choice for routine microplastics analysis, offering reliable polymer identification, particle sizing and automated filter analysis. μ-FTIR is explicitly recognized for drinking-water microplastics analysis in EU Commission Delegated Decision (EU) 2024/1441.

IR Laser Imaging Microscopy (ILIM) uses a tunable quantum cascade laser (QCL) for rapid hyperspectral IR imaging. Its key advantage is exceptionally fast chemical mapping of large filter areas, making it particularly attractive for high-throughput microplastics screening. QCL-IR is explicitly referenced in EU Commission Delegated Decision (EU) 2024/1441 as an equivalent vibrational spectroscopy approach.

Raman microscopy combines Raman spectroscopy with optical microscopy to identify and characterize particles based on their molecular composition. In microplastics analysis, it is particularly useful for very small particles because of its high spatial resolution, reaching around 1 µm and, under optimized conditions, into the submicron range. Raman can also provide valuable information on pigments, fillers and inorganic components, making it a strong choice when small-particle analysis or detailed compositional information is required. μ-Raman is explicitly recognized for drinking-water microplastics analysis under EU Commission Delegated Decision (EU) 2024/1441.

Sample preparation is the greatest source of variability between laboratories. Inadequate removal of organic and inorganic matrix material leads to false identifications and unreliable results. Contaminants can be tolerated to a certain extent and detected as such by the software. However, if the microplastic particles are obscured or masked by these contaminants, even the software cannot compensate for what arrives on the filter.

Filter choice depends on the measurement technique. Anodisc (aluminum oxide) filters are widely used for IR transmission. Silicon membrane filters provide broad mid-IR spectral access and are also compatible with Raman. Gold-coated polycarbonate filters are well suited to Raman and IR transflection measurements. PTFE, metal mesh, and nitrocellulose filters have significant limitations for imaging workflows but could be used for single point measurements with ATR.

Silicon membrane filters are compatible with both IR transmission and Raman measurements. Gold-coated polycarbonate filters work for Raman and IR transreflectance measurements. Anodisc filters fluoresce under laser excitation, show a rough surface at high magnifications, and are therefore not suitable for Raman. 

For imaging approaches, automated software like Bruker's MPID converts millions of raw spectra into a particle list with identity, size, shape, and count for every detected particle. Manual evaluation is not feasible at scale. can improve classification robustness compared with conventional library matching, particularly for weathered or contaminated samples.

Environmental particles are degraded by UV exposure, chemical weathering, and contamination. These changes shift spectra away from clean reference standards in ways that simple template matching cannot reliably accommodate. A neural network trained on real-world degraded spectra maintains accuracy where library matching fails.

A confidence score measures how closely the spectral feature vector of an identified particle correlates to a verified reference spectrum for that polymer class. It allows analysts to set a threshold, accepting only high-confidence identifications, to control the trade-off between sensitivity (detecting more particles) and specificity (avoiding misclassification).

The key regulatory and standardization documents are EU Directive 2020/2184, which establishes the EU drinking-water framework; Commission Delegated Decision (EU) 2024/1441, which specifies vibrational spectroscopy methods including μ-FTIR, μ-Raman and equivalent approaches such as QCL-IR; ISO 24187:2023, which establishes general principles for microplastics analysis across environmental matrices; and ISO 16094-2:2025, which covers vibrational spectroscopy methods for drinking water and waters with low suspended-solids content. ASTM WK87463 is currently under development as a test method for identifying and quantifying microplastics in water using IR spectroscopy.

EU Commission Delegated Decision (EU) 2024/1441 does not require one specific vibrational spectroscopy technique. It permits methods such as μ-FTIR, μ-Raman, and equivalent techniques such as QCL-IR (IR Laser Imaging) for the compositional analysis of microplastic particles and fibres. The most suitable technique therefore depends on factors such as particle size, sample type, throughput, and analytical requirements.

ISO 24187:2023 establishes general principles and minimum requirements for microplastics analysis across environmental matrices. ISO 16094-2:2025 provides more specific requirements for particle-based microplastics analysis using vibrational spectroscopy in drinking water and waters with low suspended-solids content. Laboratories working with these water samples should therefore consider both standards.