When the average consumer is asked to picture plastic waste, they will probably imagine a crumpled water bottle, eternally unchanging in some forgotten ditch. Plastic does, however, break down, but not in the way we might hope. Just as ocean waves erode rock into sand, mechanical forces (along with UV radiation and oxidation) break down plastic litter into smaller and smaller pieces, eventually resulting in microplastics, or plastic particles less than half a centimeter in diameter. These minute pieces of debris are spread across the world, appearing everywhere, from the deepest sea trenches to Antarctica. In fact, even bottled water, often advertised for its purity, is not immune to the scourge of microplastics, with some commercial products containing thousands of particles per liter. A study at the State University of New York at Fredonia even found that 93 percent of tested bottled water had microplastic contamination.
The concern is not just one of physical pollution—there is increasing evidence that these microplastics have a detrimental effect on the environment as well as on human life. This is largely because they suffuse a variety of hazardous chemicals, such as bisphenol A, phthalates, and persistent organic pollutants into their surroundings. As research continues into the ramifications of this ubiquitous plastic litter, scientists are faced with a unique hurdle: accurately finding and identifying microplastics, either among the countless other microparticles in the environment or at the scant concentrations found in consumer goods.
Bottled water, in particular, poses a significant challenge as contaminants can be few and far between. The particles that are identified must be characterized without ambiguity in order to trace their origin. That is why techniques that are capable of analyzing individual particles, and not just the bulk sample, are so important. Both infrared (IR) and Raman spectroscopy are well suited to do so, as plastics and polymers all have a distinct “fingerprint” signal between 500 – 1500 cm-1 in their infrared spectra. While it is difficult to directly interpret the information in this region, it can easily be correlated against a reference spectrum. Advanced systems are even capable of automatically comparing and matching against an entire library of standard spectra, providing clear, nearly unambiguous identification in seconds.
Automated FTIR Microparticle Analysis
Up until recently particles were visually pre-sorted with a stereo-microscope prior to analysis; this fundamentally restricted the minimum size the particles could be, as, even with magnification, the smaller the particles, the more difficult it is to distinguish between them. Additionally, thorough sorting was an arduous process, taking substantial human labor to perform.
A combination of automated analysis with high-quality Fourier transform infrared (FTIR) spectroscopy circumvents both these issues, increasing the sensitivity, scope, and speed of analysis. A mosaic combination of optical images is initially taken of the entire surface on which the microplastics are distributed, an area that could be as large as 50 mm in diameter. Regions of interest are then identified, and automated software obtains spectra of each individual particle contained therein. These spectra are compared against a library of reference signals from a range of polymers and plastics, generating a clear side-by-side comparison of experimental signal and standard. An added benefit of this approach is that the analysis of low particle concentrations is equally facile, as the regions of the optical image containing the scarce sample are quickly located and characterized.
Analysis of Bottled Water
As an example of what this technique is capable of, three different commercial bottled water samples (A – 500 mL, B – 300 mL, and C – 300 mL) were analyzed using FTIR. These were obtained and used without sample pretreatment. A Whatman Anodisc filter membrane was used to capture any particulates in the water via pressure filtration; the filters were subsequently air dried.
The surface of each filter was then imaged, in this instance using a Thermo Scientific Nicolet iN10 MX Infrared Microscope with a 25 x 25 µm step size in ultrafast mapping mode and a 10 mm x 10 mm total imaged area, creating a mosaic optical image (see Figure 1A). Proprietary software recognizes any particles on the surface and obtains FTIR spectra of each (see Figure 1B).
These are then compared against the spectral library for identification (see Figure 1C). One single microparticle was identified on each filter, even for sample A, whose particle was a mere 58 x 124 µm in size (see Figure 1C, Figure 2). These findings can subsequently be interpolated back for a general contaminant concentration.
These experimental results clearly demonstrate the utility of high-quality FTIR, in combination with a spectral library and analytical algorithms, for unambiguous identification of trace microplastic contaminants in bottled water. The critical aspect of this robust analytical technique is throughput; as concentration can vary greatly, the method must be capable of analyzing comparatively large surfaces for potentially dozens of different microparticles. With their unique spectral signatures, IR spectra are a practical identifier for this process.
Additionally, while not discussed in detail here, Raman spectroscopy is equally important for the analysis of particles smaller than 10 µm in diameter. Raman is also capable of identifying inorganic plastic additives, such as titanium dioxide, whose characteristic wavelengths are outside the mid-IR range.
Together, these two techniques offer a highly specific and efficient toolset for microparticle and microplastic identification, giving researchers the tools they need to investigate this increasingly concerning pollutant.
Dr. Rocchia is a business development manager for molecular spectroscopy at Thermo Fisher Scientific. Reach him at email@example.com. Son works in technical and application sales at Thermo Fisher Scientific in Korea. Reach her at firstname.lastname@example.org. Dr. Ilitchev is a content specialist in materials and structural analysis at Thermo Fisher Scientific. Reach him at email@example.com.