How to analyze microplastics and choose the most suitable method for different samples and research questions?

The field of microplastic analysis is still in the relatively early stages of development, with the first international standards, ISO 4484-2 on microplastics released by textiles and ISO 24187 on microplastics present in the environment, being introduced only last year. Furthermore, even ISO 24187 does not explicitly specify which analysis method should be used for any given environmental sample, but rather lists an array of available methods and sets guidelines for sampling.

With the current degree of standardization – or the lack thereof – microplastic analysis requires a high degree of expertise from the analyzing laboratory. In this article, we will go over the most important factors that should be considered when selecting the most suitable analysis method. In addition to general guidelines, we will provide practical examples of microplastic-related research questions in increasing order of complexity, explaining how they can be addressed through a combination of diligent method selection and sample preparation steps.

Common analytical challenges

Various factors from a complex sample matrix to extensive requirements on the types of plastic particles to be identified can complicate microplastic analysis. The process is most straightforward when analyzing clean water samples and detecting relatively large particles (> 10 μm) composed of common industrial polymers. Conversely, sample matrices that contain significant quantities of non-plastic particles will complicate sample preparation, while the need to identify rare polymer types or detect very small particles (especially nanoplastics) will limit the array of available analysis techniques. Particle shape is also a variable that can be difficult to study.

Overview of the most common analysis methods

Based on our experience of conducting dozens of microplastic analysis projects, the three most effective analytical techniques in the field are Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and pyrolysis-gas chromatography-mass spectrometry (py-GC/MS). In addition, complex research questions may require further analysis with scanning electron microscopy (SEM). These techniques are summarized in Table 1.

Choosing the most appropriate method for different types of environmental microplastic research projects

The following three case studies are examples of the types of microplastic projects Measurlabs has successfully facilitated for customers. We will start with the most straightforward project type (clean water), advance to a more challenging sample matrix (wastewater), and finish with the most complex project type (biological samples).

Example 1: Microplastics in clean water

Typical reasons for analyzing microplastics in clean water include comparing concentrations in tap vs. bottled water and ensuring that filtration systems are effective in removing plastic
particles. These kinds of samples require little preparation and can be effectively analyzed using several analytical methods, which means that method selection can be largely based on convenience factors, such as price and availability.
Our recommendations for microplastic analysis of clean water samples:
•   Recommended method: micro-Raman. Raman offers excellent value for money in routine water analysis. It detects a wide variety of plastic types, and water does not cause interferences in the spectrum. The smallest detectable particle size is 1 μm, which is considerably better than that of FTIR. Results are expressed as the number of plastic particles by plastic type and size range.
•    Secondary method: py-GC/MS. Analysis with py-GC/MS is more expensive than with Raman and does not provide information on particle size distribution. Still, it can provide extremely useful data on the concentrations of the most common industrial plastics in the sample, with the results expressed in μg per liter. Different size ranges of particles are obtained through filtering, and it is possible to analyze particles as small as 0.4 μm.
•   Sampling: contamination should be avoided throughout sample collection and shipping.
This includes using glass containers, not wearing cosmetics or synthetic clothing when collecting or handling samples, and avoiding plastic laboratory equipment. These principles apply to microplastics sampling regardless of the matrix.

Example 2: Microplastics in wastewater and natural water

Microplastic analysis of wastewater is commonly conducted to evaluate the environmental impact of discharges, primarily targeting municipal and industrial treatment plants. In contrast, analyses focusing on natural water are often commissioned by researchers wishing to compare the extent to which different bodies of water have been exposed to microplastic pollution.
Our recommendations for analyzing wastewater and natural water:
•   Recommended methods: micro-Raman and py-GC/MS. As with clean water, Raman is a cost-effective and reliable method for determining the particle size distribution of microplastics in typical wastewater and natural water samples. If information on mass concentration is required, or if nanoplastics are to be detected, py-GC/MS should be used instead.
•   Sample preparation: due to the presence of inorganic and organic solids in wastewater and natural water, sample preparation needs to include a digestion step and/or density separation step to separate plastic particles for analysis. Digestion is performed by adding an appropriate digestion solution (acid, base, or oxidizing agent), chosen based on the interfering matter and the types of plastic likely to be present, ensuring that plastic particles are preserved while interferences are effectively removed.

Example 3: Microplastic analysis of biological samples

Detecting and quantifying microplastics in biological samples, such as blood, feces, and animal tissues, is complicated by several factors, starting with the difficulty of reliably extracting plastic particles from the complex matrix. Fortunately, advanced extraction methods have been developed recently to address these issues, making it possible for Measurlabs to offer analyses for almost any biological matrix. The purpose of such projects is usually academic, such as assessing the extent to which microplastics make their way into fish or game animals that people in turn consume.
Our recommendations for analyzing biological samples:
•   Recommended method: micro-FTIR. Due to its relatively long history of use in microplastic analysis, dedicated sample preparation procedures have been developed for the analysis of complex biological matrices with FTIR, making it the preferred method for these kinds of samples. Also, while the FTIR spectrum can suffer from interferences caused by water, it is not as susceptible to interference from fluorescence, which can originate from certain polymer dyes and additives, as well as some components in biological samples.
•   Sample preparation: biological samples usually require several digestion, extraction, and filtering steps to separate plastic particles for analysis. The most appropriate procedure is determined on a case-by-case basis, but some examples of steps that can be taken to remove interferences include heating the sample to denature proteins and adding Proteinase K to digest them.
The more complicated the sample matrix, the more extensive the experience required to effectively overcome challenges inherent to microplastic analysis. Choosing a laboratory with a proven track record of employing different analytical methods and handling a range of sample matrices is crucial for consistently achieving reliable and accurate results.

About Measurlabs

Measurlabs is a one-stop solution for testing services, offering a comprehensive range of analyses through a network of over 900 accredited laboratories. In addition to microplastic testing, their service selection for environmental samples includes PFAS, VOC, PAH, persistent organic pollutant, and heavy metal analyses, among others.
Read more at https://measurlabs.com/