4 Environmental Laboratory


HOW TO ANALYZE MICROPLASTICS AND CHOOSE THE MOST SUITABLE METHOD FOR DIFFERENT SAMPLES AND RESEARCH QUESTIONS?


The fi eld of microplastic analysis is still in the relatively early stages of development, with the fi rst 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 identifi ed 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 signifi cant 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 diffi cult 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 fi eld 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 diff erent 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 fi nish with the most complex project type (biological samples).


Table 1: Comparison of microplastic analysis techniques Technique Obtained information


Raman Particle size distribution by polymer type


Smallest detectable particle size


~ 1 μm Advantages


+ Detects a wide variety of plastics + No interference from water + Cost-effective


Disadvantages - No direct quantifi cation


- Fluorescence from inorganic materials


py-GC/MS Mass fraction by polymer type


+ Direct quantifi cation ~ 0.4 μm*


+ Can also detect rubber particles


FTIR


Particle size distribution by polymer type


SEM


Particle size distribution + images showing particle size, shape, and morphology


~ 5 to 10 μm


+ Detects a wide variety of plastics


+ No fl uorescence from inorganic materials


~ 0.1 μm**


+ Detailed information on shape and morphology


+ High-resolution imaging


- No particle size or shape information


- Limited capability to detect different polymers


- No direct quantifi cation - Interference from water


- Limited ability to identify plastics


- Tricky to obtain a representative sample


- High cost


* Theoretically, there is no lower size limit, but analyzed particles are obtained through fi ltering, and conventional fi lters have a minimum mesh size of 0.4 μm. ** Even though the theoretical maximum resolution of SEM is higher, there are practical constraints on studying smaller particles.


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 fi ltration 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 fi ltering, 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.


IET SEPTEMBER / OCTOBER 2024


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