Microplastics are tiny plastic particles defined as being smaller than five millimeters in size. These particles are ubiquitous, found everywhere from deep ocean trenches to the air we breathe, raising concerns about their potential effects on ecosystems and human health. Addressing microplastic pollution requires a systematic testing process to determine their quantity, type, and source. This testing involves careful collection, isolation from the surrounding environmental material, and sophisticated instrumental analysis for chemical confirmation and quantification. The primary goal of this multi-step process is the accurate detection, identification, and measurement of these contaminants.
Sample Collection and Initial Preparation
The first step in microplastic analysis is gathering a representative sample from the specific environment being studied. Because microplastics are found in surface water, deep water, sediment, air, and biological tissue, the collection tools must be specialized. For collecting particles floating on the water surface, a Manta Trawl is commonly used, which is a net towed behind a vessel to filter large volumes of water and concentrate the microplastics. For bulk water samples, researchers may use pumps or simple grab samples, while sediment sampling often involves tools like shovels, scoops, or Van Veen grabs.
Initial preparation of the collected sample ensures consistent and comparable results. Environmental samples, especially sediment or soil, are first dried to obtain a consistent dry weight before analysis. The dried sample is then homogenized to ensure the microplastics are evenly distributed and often sieved to remove particles larger than the 5-millimeter upper limit. This initial size sorting and drying reduces the overall sample volume and prepares the material for the isolation steps that follow.
Isolating Microplastics from Environmental Matter
Before the plastic particles can be chemically analyzed, they must be separated from the bulk of the environmental matrix, such as sand, organic debris, or tissue. This isolation is accomplished through two primary techniques: density separation and organic digestion. Density separation exploits the fact that most common plastics have a lower density than inorganic materials like sand and minerals. The sample is mixed with a concentrated salt solution, such as sodium iodide (NaI) or zinc chloride (ZnCl2), which creates a liquid denser than the plastic, causing the microplastics to float to the surface for collection.
For samples rich in organic material, such as sewage sludge, biological tissue, or soil, organic digestion is required to dissolve the non-plastic matter. This process uses strong chemicals like hydrogen peroxide (H2O2) or oxidizing agents to break down biological substances without damaging the plastic polymers. Enzymatic digestion is increasingly favored as it minimizes the risk of altering the structure or mass of the plastic particles. The goal of both density separation and digestion is to yield a clean, concentrated sample ready for instrumental identification.
Identifying Microplastic Material and Type
The final stage involves confirming the isolated particles are plastic and determining their specific polymer type. Initial screening uses microscopy, which allows for visual counting and sizing of the particles, as well as classification of their shape, such as fragments or fibers. Microscopy alone is insufficient because it cannot chemically confirm the particle identity, making spectroscopic analysis necessary.
Fourier-Transform Infrared (FTIR) Spectroscopy is a primary technique that shines infrared light onto the particle. Each polymer type absorbs infrared light at unique wavelengths, creating a distinct spectral “fingerprint” that identifies the plastic, such as polyethylene (PE) or polyvinyl chloride (PVC). Advanced FTIR instruments using Attenuated Total Reflectance (ATR) or microscopy allow for the analysis of particles down to about 10 micrometers. The lower limit of detection for FTIR is constrained by the wavelength of the infrared light used.
Raman Spectroscopy serves as a complementary technique, utilizing a laser light source instead of infrared light. This method is often more effective for identifying smaller microplastics, able to characterize particles down to one micrometer. Raman analysis is less affected by water contamination and can be quicker than traditional FTIR, although some plastics may exhibit a fluorescence that interferes with the reading. After chemical identity is confirmed, quantification reports the number or mass of identified particles, typically as particles per volume (e.g., per liter of water) or per mass (e.g., per kilogram of sediment).