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).
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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 firstname.lastname@example.org. Son works in technical and application sales at Thermo Fisher Scientific in Korea. Reach her at email@example.com. Dr. Ilitchev is a content specialist in materials and structural analysis at Thermo Fisher Scientific. Reach him at firstname.lastname@example.org.