Microplastics analysis methodologies – selecting the right approach

Microplastic analysis is still a relatively new field, with the first international standard published only recently in September 2023. Even this standard – ISO 24187 – does not specify a set testing procedure but rather outlines the general principles that should be followed in sampling, sample preparation, and choice of test methods. How to choose the most suitable microplastic analysis method for different samples and research questions? 
As matrix-specific standard methods are still under development, the most suitable microplastic detection and quantification technique for any given situation needs to be selected on a case-by-case basis. This article aims to guide researchers and industry professionals through the key factors to consider during the method selection process.

What is method selection based on?
Different microplastic analysis methods have different strengths and limitations when it comes to the type of particles they can detect and the types of information they produce on the plastic content of the sample. When selecting a suitable technique, the following should be considered:

•  An approximate size range of particles the sample is likely to contain. Very small microplastics (< 10 μm) and nanoplastics (< 1 μm) are tricky to detect with many of the most common methods.
• The polymer types likely to be present. The capabilities of different techniques to reliably identify specific polymers vary.
• The sample matrix, including the possible interferences it might cause or preparation it will require.
•  The goal of the analysis. Depending on the method, the obtainable results may include the number of particles by size range and polymer type, or the mass fraction of microplastic by polymer type. Some advanced methods offer additional information on particle shape and morphology.
  

Popular microplastic analysis techniques
Among the various methods used for microplastic analysis, Raman spectroscopy, FTIR spectroscopy, and pyrolysis-GC/MS are most widely available through commercial laboratories. The ideal use cases, advantages, and limitations of these methods are summarised below.

Raman spectroscopy
With micro-Raman spectroscopy, it is possible to identify microplastics by polymer type and particle size. The method is capable of reliably identifying all of the most common industrial polymers, including polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polycarbonate (PC), polystyrene (PS), polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), polyamide (PA), polymethyl methacrylate (PMMA), and polyurethane (PU) and even more rare types using a reference library.

Compared to FTIR, Raman offers improved spatial resolution, being able to detect microplastics as small as 1 μm. The method is especially well-suited for analysing different water samples, as water does not cause interferences in the Raman spectrum. On the other hand, interference may be caused by fluorescence from inorganic components, such as clay. The effects can be minimised through diligent sample preparation.

Fourier transform infrared spectroscopy (FTIR)
Regarding plastic identification capabilities, FTIR matches and surpasses Raman, as it can be used to identify an even larger array of rare polymers. The smallest detectable particle size is approximately 5 to 10 μm, depending on the equipment and the sample matrix. This makes Raman a better solution for analysing the smallest microplastics.
FTIR has a relatively long history of being applied to microplastic detection, during which solutions have been developed for analysing a variety of complex sample matrices. These include biological samples, such as blood, faeces, and tissues, for which FTIR is the recommended microplastic analysis method when information on particle size distribution is required.

Pyrolysis-gas chromatography-mass spectroscopy (py-GC/MS)
Py-GC/MS can be used to detect extremely small microplastics and even some nanoplastics. In principle, there is no lower limit for detectable particle size, but in practice, filters with a minimum pore size of 0.4 μm are used in sample preparation, making that the de facto lower limit. The method does not provide information on the particle size distribution of microplastics within the sample but is instead used to measure the combined mass of particles belonging to certain size ranges, obtained through filtering.
The array of plastics that can be identified with py-GC/MS is more limited than with spectroscopic methods, but it can detect the most common industrial plastics and certain rubber particles. Like FTIR, py-GC/MS is suitable for complex matrices. To ensure optimal detection capability, nonplastic organic and inorganic matter should be removed before analysis.

Method selection is key to success in complex microplastic research projects
Choosing a suitable microplastic detection method can be relatively straightforward when analysing routine samples, such as clean water, as multiple methods will produce accurate and reproducible results. With increasingly complex matrices, the importance of selecting an optimal technique increases, often necessitating input from experienced microplastic testing professionals.

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