KEY POINTS
- The size of microplastics and the fact that they are often mixed with other materials make them difficult to observe, isolate, and identify.
- Depending on the method used, the amount of microplastics detected in a sample can vary by a factor of 15.
- This wide variability underscores the need to fully understand what is being measured in order to better interpret the results.
Try this experiment: observe a piece of supposedly biodegradable plastic film in a compost pile. After a few months, there’s no visible trace left—the material seems to have evaporated. It’s tempting to conclude that the material has undoubtedly degraded.
But has it really disappeared? Has it been converted into carbon dioxide? Has it been broken down by microorganisms or fragmented into particles too small to be seen? This is where the real scientific problem begins.
We hear a lot about microplastics, but much less about how they’re measured. Yet, statistics on microplastics only make sense if we know how they were obtained. In certain sediments, studies have shown that an appropriate extraction protocol can recover more than 15 times as many particles as a less effective method. In other words, depending on the method used, some particles will be detected, while others will remain invisible.
To help you understand this better, we must first understand exactly what microplastics are and how we can track their evolution.
Microplastics are generally defined as plastic particles smaller than 5 millimeters. Their size makes them difficult to observe, isolate, and identify, especially when mixed with soil, compost, sewage sludge, or organic waste.
Before you start searching, you need to sort through the results
The first step in identifying them is often the least dramatic: extracting the particles from their matrix.
Even in a bottle of filtered water, this is already a delicate process. In soil or compost, it becomes much more complicated, because an environmental sample is never “clean.” It contains mineral grains, plant fibers, pieces of organic matter, microorganisms, and sometimes biofilms—that is, living layers formed by microbial communities. All of these can mask or mimic plastic fragments.
The protocols therefore combine several steps. The sample may first be dried, then sieved to separate the different particle size fractions. Some of the organic matter can then be selectively removed or degraded using mild reagents, before proceeding to density separation.
The principle is fairly simple: certain plastics float in saline solutions that are denser than water, while many mineral particles sink. The recovered particles are then filtered and examined. This preparation step is one of the major challenges in analyzing microplastics in complex matrices, as noted in several methodological reviews on microplastic analysis methods.
Sample preparation is therefore a critical step. If the treatment used to remove organic matter is too aggressive, it can damage certain plastics, especially when they are already aged or biodegradable. The particles may then fragment, lose some of their material, or have their surface altered, which complicates their counting and identification. Conversely, a preparation method that is too gentle can introduce other biases: a separation solution that is not dense enough may fail to recover the heaviest plastics, and plant residues may remain in the sample. However, these organic debris—particularly fibers or colored fragments—can resemble microplastics when observed visually.
It is also essential to avoid contaminating the sample in the laboratory. A textile fiber that has fallen off a piece of clothing, plastic dust, or a poorly controlled filter can skew the analysis. This is why laboratories use contamination “blanks”: control samples—which are expected to be free of microplastics—that undergo the same steps as the actual samples in order to detect any potential contamination during handling. They also use rinsed glassware, clean tweezers, and sometimes non-synthetic clothing.
Seeing a particle doesn't prove everything
Visual observation remains invaluable. It allows us to identify fibers, fragments, films, and colored particles. It reveals their shape, size, and, at times, their likely origin: a textile fiber, a piece of a bag, a chip of paint, or a fragment of material.
But seeing a particle is not always enough to prove that it is plastic. A blue fiber could come from a synthetic textile, but it could also be the result of contamination during handling. A brown particle could be a plant fragment, an organic aggregate, or heavily oxidized plastic.
To avoid these biases, researchers therefore often use complementary methods that identify the material’s chemical signature. Infrared spectroscopy and Raman spectroscopy (named after the Indian physicist Chandrashekhara Venkata Râman, who won the Nobel Prize in 1930) work much like a molecular fingerprint scan.
They illuminate the particle and analyze how its chemical bonds vibrate. Polyethylene, PET, PLA, and PBAT do not produce the same signals and can thus be identified.
Understanding the Acronyms
- Polyethylene, or PE, is one of the most common plastics: it is found, for example, in plastic wrap, bags, and flexible packaging.
- Polyethylene terephthalate, or PET, is well known for its use in beverage bottles and certain synthetic textiles.
- Polylactic acid, or PLA, is a bio-based polyester used in particular in certain types of packaging, cups, and 3D-printed objects.
- Poly(butyl succinate), or PBS, and polybutylene adipate terephthalate, or PBAT, are also polyesters being studied for biodegradable applications, such as in films or flexible packaging.
- Not all of these plastics behave the same way in the environment.
These methods have a significant advantage: they allow us to establish a link between a particle’s appearance and its chemical composition. This means we can determine not only that a fragment is present, but also whether it is polyethylene, polyethylene terephthalate (PET), polylactic acid (PLA), or another type of plastic.
They are therefore very useful for mapping microplastics—that is, identifying where they are located, in what form, and what material they are made of. But they also have limitations. Very small particles, for example, are difficult to analyze. Pigments, biofilms, fluorescence, or matrix residues can interfere with the signal.
For nanoplastics—even smaller particles, often less than a micrometer in size—the challenge becomes even greater: it is no longer just a matter of seeing a fragment, but of detecting a material dispersed within complex background noise.
When Chemistry Replaces Counting
Another family of methods takes a completely different approach. Instead of observing and counting each particle one by one, it analyzes a fraction of the sample to search for molecules characteristic of a given polymer.
These molecules may come from particles that are too small, too numerous, or too thoroughly mixed with the matrix to be identified individually. This is the realm of chromatographic methods coupled with mass spectrometry.
The principle can be summarized simply: the plastic is broken down into small, characteristic molecules, which are then separated and measured. In some cases, the material is heated until it fragments—this is known as pyrolysis-GC/MS.
In other cases, it is chemically transformed before analysis. One can then use gas chromatography (GC/MS) or liquid chromatography coupled with mass spectrometry, often referred to as LC-MS.
These methods therefore do not directly answer the question “How many particles are there?”, but rather another: “How much of a given polymer is present in the sample?” In other words, they do not necessarily provide the number, shape, or size of the fragments, but they can measure the mass of plastic based on characteristic chemical markers. This is very useful when the fragments are too small, too numerous, or too mixed with the matrix to be counted one by one.
For certain biodegradable polyesters, such as poly(butyl succinate) [PBS] or polybutylene adipate terephthalate (PBAT), it is also possible to track molecules derived from their building blocks. For PBS, one can look for succinic acid derivatives. For PBAT, certain approaches track markers linked to terephthalic acid after the polymer has been processed. A recent study, for example, developed a method for quantifying PBAT and its degradation products in soil using mass spectrometry, following hydrolysis of the polymer.
Nuclear magnetic resonance (NMR) complements this picture. It is not primarily used to count particles in a raw sample. Rather, it provides information about the chemical environment of atoms, such as hydrogen or carbon. It can help identify a polymer, verify a structure, track degradation, or quantify certain mixtures after extraction.
These chemical methods are powerful. However, they often destroy the sample: after heating or chemical transformation, the particles can no longer be observed directly. Consequently, information about their shape, size, and number is lost, even though a more precise measurement of the plastic present is obtained. They thus provide another piece of the puzzle.
Gone doesn’t always mean broken down
This discrepancy between what we see and what actually remains in the environment is particularly significant for biodegradable plastics. A fragment may become invisible to the naked eye and disperse into the environment without the material having been completely transformed.
The real question, then, is not just “Is the plastic still visible?” but rather: “Where has the carbon that made up the plastic gone?”
Plastics are largely made up of carbon. When they biodegrade in the presence of oxygen, a portion of that carbon is converted into carbon dioxide, or CO₂. This is what many biodegradation tests measure. But this CO₂ tells only part of the story.
The portion that isn’t converted into CO₂ can take several paths. It can remain in the form of plastic residues. It can break down into smaller particles. It can also be utilized by microorganisms and end up in their cells, in biological residues, or in organic compounds produced during degradation.
This issue is crucial because it determines how we evaluate so-called biodegradable plastics. If a material disappears visually but leaves behind persistent fragments, it can contribute to pollution that is difficult to detect. Conversely, if part of its carbon is actually converted into CO₂ or temporarily incorporated into microbial biomass, its environmental fate is different.
Understanding these different pathways therefore helps us avoid two opposing errors: assuming too quickly that a plastic has disappeared, or assuming that all carbon not converted into CO₂ necessarily corresponds to persistent microplastics.
In our research on biodegradable polyesters, we aim precisely to distinguish between these possibilities. We combine measurements of CO₂ produced (through biodegradation), analysis of the total organic carbon remaining in the environment, and chromatographic methods capable of quantifying polymer residues.
To this end, we studied two plastics used in biodegradable applications: PBS, which can be found in certain films or packaging, and PBAT, a more flexible material often associated with compostable materials. These materials were tested alone or mixed with plant matter, such as orange peels or brewer’s spent grain (solid residue, mainly barley or other grains, left in the vat—editor’s note), at a moderate temperature of around 28 °C, under conditions similar to those of household or community composting, whether at home or in a community setting, rather than high-temperature industrial composting.
We also studied the effect of bioaugmentation—that is, the addition of selected microorganisms to promote biodegradation.
This research shows that biodegradable plastics do not all follow the same trajectory: some produce more CO₂, others fragment or leave residues, and some of the carbon may also be temporarily incorporated into microbial biomass or into the organic matter in the environment.
The absence of visible fragments therefore does not necessarily mean complete disappearance. But what does not turn into CO₂ does not always remain in the form of persistent plastic.
Some of it may have been incorporated into microbial biomass or associated organic matter.
This is why detecting microplastics and nanoplastics is less like taking a photograph and more like conducting an investigation. Extraction sets the stage. Microscopy provides visual clues. Infrared and Raman spectroscopy identify molecular fingerprints. Chromatographic methods measure chemical signatures. NMR sheds light on certain transformations.
No single method tells the whole story. Some methods detect particles. Others measure mass. Still others track the polymer’s chemical fate. The challenge, therefore, is not merely to detect small pieces of plastic, but to ask the right question before choosing the right tool.
In a field where numbers circulate rapidly in public debate, this caution is essential. Microplastics are not just a pollution problem; they are also a measurement problem.
Jules Bellon, Scientific Project Manager, UniLaSalle
With contributions from Feriel Bacoup, Research Fellow, UniLaSalle, and Richard Gattin, Head of the Agri-Food and Agribusiness Team, UniLaSalle.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
