Microplastics are fragments of plastic, typically defined as having a size ranging from 1 micrometer (µm) to less than 5 millimeters (mm) in length. They originate from two main sources: primary microplastics (like microbeads or microfibers) and secondary microplastics (from the breakdown of larger items like packaging). These particles are ubiquitous, detected in air, soil, and aquatic ecosystems, and their presence in both tap and bottled water is widely documented. Detecting these minuscule particles in a complex water matrix is difficult due to their small size, low concentration, and the need to differentiate them from non-plastic debris.
Preparing the Sample: Separation and Concentration
The first step in laboratory detection is to isolate the microplastic particles from the large volume of water and the accompanying organic and inorganic matter. This process, known as sample preparation, is time-consuming and requires several sequential techniques to concentrate the particles for analysis. Initial concentration is achieved through filtration, where the water sample is passed through a membrane filter, such as cellulose nitrate or polycarbonate, with a specific pore size. Using too fine a filter slows the process, while a coarser filter risks missing smaller microplastics.
Once captured, scientists must remove interfering organic materials like algae, microbes, and natural debris that would obscure identification. This is accomplished through a chemical digestion or oxidation step, often using solutions like hydrogen peroxide or Fenton’s reagent. This aggressive treatment breaks down the organic matrix, leaving behind the chemically resilient plastic particles and inorganic matter.
To further purify the sample, density separation is employed to isolate the plastic from heavier inorganic debris like sand or silt. This technique utilizes high-density salt solutions, such as zinc chloride or sodium iodide, which are denser than many common plastic polymers like polyethylene and polypropylene. When the sample is mixed with this solution, the lower-density microplastics float to the surface, while the heavier, non-plastic materials sink. The floating layer of concentrated microplastics is then collected and transferred to a new filter for the final identification stage.
Identifying the Polymers: Spectroscopy Techniques
After physical separation and concentration, the next challenge is to chemically confirm that the isolated particles are indeed plastic polymers, a process that relies on advanced spectroscopic methods. Initial analysis involves microscopy to visually count the particles and measure their size and shape, but this alone cannot determine the chemical composition. True identification requires techniques that create a molecular “fingerprint” of the material.
Fourier-Transform Infrared Spectroscopy (FTIR) is a primary method, working by directing infrared light onto the particle. The chemical bonds within the polymer absorb and transmit the light at specific wavelengths, creating a unique spectrum that identifies the plastic type, such as polyethylene (PE), polypropylene (PP), or polyvinyl chloride (PVC). For microplastics larger than about 10 micrometers, FTIR imaging microscopes offer a reliable method for rapid chemical mapping.
Raman Spectroscopy serves as a complementary technique, utilizing a laser source to excite the sample and measure the scattered light. This process produces a unique vibrational spectrum for polymer identification. Raman is advantageous for analyzing smaller particles, with some instruments detecting particles down to 0.5 micrometers, offering better spatial resolution than traditional FTIR. It is also less affected by water interference, though fluorescence issues with certain materials can obscure the spectral signal.
Limitations of Current Detection Methods
Despite the sophistication of laboratory techniques, microplastic detection faces significant scientific and practical limitations that challenge accurate and standardized reporting. A major hurdle is the lack of globally consistent protocols for sampling, processing, and reporting results, leading to considerable variability in published data. Studies often use different minimum size cut-offs for microplastics, such as 20 µm versus 1 µm, making direct comparison of contamination levels difficult.
A profound technical limitation involves the detection of the smallest particles, known as nanoplastics, defined as less than 1 micrometer. The resolution limits of current micro-FTIR and Raman systems struggle to reliably analyze particles below this 1 µm threshold. This means the full scope of plastic contamination remains largely unknown, which is a concern because nanoplastics are hypothesized to be the most biologically active fraction.
The entire detection process is expensive and labor-intensive, requiring specialized, high-cost equipment and trained personnel. The time investment for manually counting and spectroscopically verifying thousands of particles per sample contributes to high operational costs. This limits the frequency and scale of monitoring undertaken by water utilities and researchers, restricting widespread routine testing.
What Consumers Can Do
Since laboratory-grade detection is not feasible for the average person, consumers must focus on mitigation and demanding transparency regarding water quality. One effective action is to install an appropriate home water filtration system. Reverse osmosis (RO) systems are highly effective, as their membranes have pores small enough (often down to 0.0001 to 0.001 micrometers) to remove virtually all microplastics and many nanoplastics.
Other point-of-use options, such as under-sink or countertop units utilizing ultrafiltration or fine carbon block filters, can also significantly reduce microplastic levels. Filters with a pore size of 0.5 to 1 micrometer effectively capture a large portion of the microplastic fraction found in tap water. Choosing a filter independently certified for microplastic reduction provides assurance of its effectiveness.
Consumers can also engage their local water provider by asking specific questions about their water testing protocols, including whether they currently test for microplastics and what removal efficiencies their treatment plants achieve. This inquiry can encourage utilities to adopt more rigorous testing standards. A final, indirect measure is to reduce the use of single-use plastics, which are the primary source of the microplastic pollution that ultimately enters the water supply.