Aug 01, 2017
Special Issues
Volume 32, Issue 8, pg 17–23
Microplastics are particulates, roughly 20–1000 µm in size, originating from materials such as clothing, abrasive action on plastics, or engineered microbeads as found in some exfoliating cosmetics. The microplastics enter aquifers where the particles can be consumed by filter feeders. Microplastics are chemically stable, giving them a long lifetime in the environment and making excretion or digestion difficult. Analytically, the size and polymeric nature of microplastics makes Fourier transform infrared (FT-IR) microscopy an ideal tool for detection and identification. Standard analyses typically start with a filtration step, extracting the material from the matrix. The analysis can proceed directly on the dried filter without further sample preparation. This simplicity in both sampling and analysis enables the rapid assessment of microplastic encroachment and can assist in the development of remediation techniques. We show examples from both prepared and field samples using microattenuated total reflection (ATR) FT-IR.
Macroscopic plastics are showing up in many environmental systems, including mid-Pacific zones (1) and tropical islands (2). Microplastics, 20–1000 (or 5000) µm fibers or granules of polymer, are more insidious because they can’t be picked up simply as trash. Microplastics originate both from engineered materials, such as the microbeads found in some exfoliating creams, and from abrasion or wear of polymeric materials, even from laundering of synthetic fabrics (3). Microplastics live a long time in the environment and unfortunately they are exactly the right size for filter feeders to consume them. From there, they can move up the food chain and are now found in fish, birds, and other wildlife.
The microplastic materials are small enough to be highly mobile in the environment, carried most easily by flowing water. Adsorption of chemical or biological toxins on the microplastics can then enable transport of those toxins from one biome to another (mobilizing materials). More directly, the microplastics clog critical digestive pathways. The chemical environments in the digestive paths are insufficient to dissolve these clogs, resulting in incapacitation or death of the organisms (3,4).
Remediation requires answering two critical questions: What are the particles and how many particles are present (number density)? Polyethylene, polypropylene, polystyrene, and nylon are some of the more commonly found materials, from food packing, toys, and many other sources. The number density varies from negligible in isolated lakes and streams to severe in some lakes and estuaries. Changes in the microplastic population (type or number) indicate a new set of conditions, such as those generated by flooding which augments transport of these materials over a large area.
In the aquifer, simple filtration (sieves or simple filters) is typically sufficient to isolate representative populations of microplastics. Sampling within an organism requires separation of the microplastics from the organism, such as the digestive tract of fish (4). This generates a solution that is then filtered. Observation of these samples under a standard light microscope can lead to particle counts, but identification of the materials using visible microscopy alone is problematic at best. Microplastics have been analyzed by gas chromatography–mass spectrometry (GC–MS), scanning electron microscopy with energy dispersive spectroscopy (SEM–EDS), and combustion analysis. Each of these techniques has strengths and weaknesses—sensitivity, specificity, time for analysis, and destruction of the sample being considerations. However, vibrational spectroscopy, both Fourier transform infrared (FT-IR) and Raman, can provide insights quickly and nondestructively with a high degree of confidence.
FT-IR microscopy is an excellent tool for the analysis of these materials. Filtration of a known volume of liquid (river water, for instance) followed by infrared identification and particle counting provides a thorough picture of the material present. Modern software automates many of the steps, enabling an analyst to answer those critical questions quickly and efficiently. This article starts with the basics of sample collection and single-particle identification and then examines techniques for the analysis of larger regions. We present data from both model systems using manufactured microbeads and real world filtrates.
Experimental
Samples prepared from both reference materials and environmental sources were examined. The primary goal of the work was to demonstrate the utility of FT-IR for this analysis; no extrapolation in a particular environment was made. Recently, we examined samples from the automotive, food, and cosmetics industries as well as environmental samples; here only a brief overview is provided..
Results and Discussion
Most environmental studies of microplastics involve filtration of a liquid sample (typically but not always aqueous), followed by drying. This results in a sample that can be placed directly into the microscope—there is no need to pick off pieces for analysis. The visible capabilities of FT-IR microscopes enable location and selection of a particle or a region for analysis.
The fiber or particle was located visually and then centered in the aperture. The Ge-ATR accessory was then brought into contact with the particle. With Ge-ATR, there is a fourfold increase in magnification (because of its high index of refraction). The FT-IR microscope has a 10× magnification; with the Ge-ATR accessory the magnification increases to 40×. The standard 1-mm circular pinhole thus results in an effective aperture at the sample of 25 µm. As most microplastics are this size or larger, this ensures good spectral purity.
The spectrum matches with acrylic (the strong nitrile peak at 2243 cm-1 because of -C≡N is strongly indicative of this), suggesting a fabric source as the likely origin. The two particles in Figure 2 were separated from another (different origin) water sample. Again, the ATR results are unambiguous, with the identification as polyethylene and polypropylene, respectively. These are common from many sources. Excluding sample preparation time, the analysis of these three samples took less than 2 min each even with a user not trained in microscopy.
