Plastic waste ranges from large, easily visible macro-plastics down to micro-plastic particles that may be too small to detect with normal vision. The European Chemical Agency (ECHA) defines microplastic particles as particles with dimensions from 5 mm to 1 nm.
Microplastics can come from primary sources where the plastics have been produced as small particles found in some cosmetic products and cleaners, those generated in ship-breaking industry and industrial abrasives in synthetic ‘sandblasting’ media consisting of beads of acrylic plastics and polyester. Or microplastics are derived as secondary products that arise from the mechanical, photochemical, or thermal break down of larger plastic products. Another primary source is from plastic pellets, sometimes called nurdles, that are lost to the environment during the plastic production.
While the polymer materials in plastics are essentially inert, in marine environments these polymers can act as sponges that collect persistent organic pollutants (POPs) that occur universally in sea water at very low concentrations. These contaminated plastics can be ingested by marine species and enter the food web from fish, bivalves and eventually humans.
On the inverse, residual monomer and additives compounded into plastics as well as partially degraded plastics can slowly leach out a small fraction of POPs. In contrast to ‘cleaning’ of sea water by virgin plastics these tend to add POPs into seawater.
Evidence of microplastic particles in marine biota has been well documented. Considering that runoff being plastics generally leads to rivers, which lead to oceans, and considering the size of the oceans themselves, it’s not surprising that the oceans and beaches are the largest collection point of waste plastics and source of microplastics. And although human consumption of marine biota is a concern, microplastic and accompanying POP contamination in freshwater sources likely poses a larger risk.
Koelmans et al. reviewed a number of studies on microplastics in drinking water and freshwater, concluding that, “To understand human health implications, more high-quality data is needed.”
Collection methods, sample curation and handling, extraction and purification methods, and the use of positive controls varied among these studies, and resulted in dissimilar and difficult to compare data sets. However, the dominant analytical techniques used to identify microplastics were FTIR and Raman microspectroscopies. Both techniques are widely recognized for their ability to characterize polymeric compounds.
The information found in the vibrational spectra of polymer materials provides a molecular fingerprint that can be used with spectral databases to positively identify different types of polymer materials. Infrared spectroscopy (micro-FTIR) depends on changes in dipole moments and is particularly useful for detecting polar functional groups found in many different types of polymer materials (hydroxyl, amines, amides, carbonyls, etc.). Raman spectra depend on a change in polarizability and thus are good for looking at polymer backbone structures as well as where there are delocalized electrons such as alkenes and aromatics.
Both FTIR and Raman spectroscopy can be used to analyze microscopic sized samples. Used in conjunction these techniques deliver complementary data that is more useful than either technique alone. This makes vibrational spectroscopy an indispensable tool for the identification and analysis of micro-plastic particles.
In a newly published paper, “Identification of Micro-Plastic Particles in Bottled Water using FTIR & Raman Spectroscopy,” Thermo Fisher Scientific authors discuss the analysis of microplastic particles using a targeted discrete point analysis from a “stitched” image of multiple fields of view used to select possible particles for analysis and provide information on particle size and shape. The process eliminates the need to image a large sample area, such as a silicon filter, and dramatically reduces the amount to spectral data collected. Although bottled water was used as a sample for microparticle detection, the same method would be application to samples collected from lakes, streams, wastewater or other sources.