Overview of ISO 16094 standards for microplastic analysis of water samples

Standardization efforts in the field of microplastic analysis have been ongoing for several years, but are still in the relatively early stages. The upcoming ISO 16094 standards will further the process by introducing more specific analysis protocols than was the case with ISO 24187, which was published in 2023.

Part 2 of the new standard will provide guidelines for microplastic analysis of water samples with vibrational spectroscopy methods (Raman and Fourier-transform infrared spectroscopy, or FTIR), while Part 3 will focus on thermoanalytical methods (thermal extraction and desorption with gas chromatography-mass spectrometry, or TED-GC-MS, and pyrolysis-GC-MS).

ISO 16094 is currently in the draft stage, with official publication expected at the end of 2024 or the beginning of 2025. This article is based on the draft versions of Parts 2 and 3 of the standard (ISO/DIS 16094-2 and ISO/DIS 16094-3, respectively), which means that some of the details may still change. The information will be checked and updated once the final versions are published.

ISO 16094-2: Microplastic analysis with FTIR and Raman

Part 2 of ISO 16094 outlines the testing procedure for analyzing microplastic concentrations in clean water samples with FTIR and Raman, combined with optical microscopy. With these methods, it is possible to obtain the following information: 

• Size (1 μm to 5,000 μm) and number of microplastic particles

• Classification of particles by size range

• Determination of plastic composition (most notably, identifying PE, PP, PET, PC, PS, PTFE, PVC, PA, PMMA, and PU particles)

• Identification of other particles, such as minerals, pigments, proteins, and cellulose

During the analysis, a specified volume of water is passed through filters with a pore size that can retain the microplastics of interest. The filters are then analyzed with the chosen spectroscopy method coupled with microscopy to identify the polymers present and obtain the particle size distribution.

Differences between spectroscopic methods

According to ISO 16094-2, the most notable differences between Raman and FTIR in microplastic analysis are minimum measurable particle size and main interferences. In terms of size, Raman can measure particles as small as 1 to 10 μm, while FTIR cannot reliably go below the 10 to 20 μm size range, depending on the instrument.*

Both techniques are subject to interference from mineral particles, surface alterations by biofilms or weathering, proteins, and carbohydrates. In addition, the Raman spectrum suffers from interference from colored particles, pigments, fatty acids, and amides, while the main interferences unique to FTIR are carbon black and water.

ISO 16094-3: Microplastic analysis with TED-GC-MS and py-GC-MS

The thermoanalytical methods outlined in Part 3 of the ISO 16094 standard differ from spectroscopy methods in terms of obtainable information. With TED-GC-MS and py-GC-MS, it is possible to determine the mass fraction of plastic and types of polymers present in the sample, but particle size, shape, or numbers cannot be identified.

The array of polymers identifiable with thermoanalytical methods is also more limited than with spectroscopic methods. While both TED-GC-MS and py-GC-MS can be used to reliably identify PE, PP, PS, and PET, the possibilities to analyze further polymer types depend on the method. Generally, py-GC-MS can detect a wider range of additional polymers, including PVC, PC, PMMA, ABS, PA6, and PA66.

Analysis procedure with thermoanalytical methods

Thermoanalytical methods are suitable for solid samples, which is why microplastic analysis with TED-GC-MS and py-GC-MS is performed on water filtration residue, obtained using plastic-free filters with an appropriate mesh size. Alternatively, manually identified individual particles can be analyzed directly.

When analyzing filtrate residue, ISO 16094-3 recommends drying the sample to obtain its dry weight and to prevent biological growth during storage. Homogenization with cryogenic milling may also be appropriate in some cases. Once a suitable sample has been obtained, the following steps are followed:

1. A pyrolysis step to decompose the sample in an inert atmosphere.

2. A transfer/separation step to move the decomposition products to the chromatographic system. 

3. A detection/quantification step to identify and quantify the polymer-specific decomposition products.

General sampling and sample handling precautions

Both Part 2 and Part 3 of ISO 16094 apply to ultrapure water, drinking water, and raw groundwater with a low concentration of organic and other suspended matter (1–100 mg/l or lower). Due to the generally very low microplastic concentrations in such waters, avoiding contamination at every stage of sampling and analysis is crucial. Some of the steps that should be taken include the following: 

• Not wearing clothing that contains synthetic fibers or using cosmetic products that may generate microplastics when handling samples.

• Ensuring that the laboratory environment is as plastic-free as possible and cleaned regularly with wipes and detergent that do not contaminate the space.

• Avoiding plastic laboratory equipment, instead using glass and metal instruments and containers.

Microplastic testing by ISO 16094 standards

Measurlabs offers microplastic analyses for clean water samples and a wide range of other sample materials, from wastewater to challenging biological matrices. Testing is already conducted following sample handling best practices similar to those outlined in ISO 16094, and using the analytical methods it specifies. Once the final versions of the standards are officially published, we aim to promptly expand our service selection with accredited methods.