FTIR Microscopy: Identifying the Type of Polymer in Microplastics

By definition, microplastic particles (MPP) are polymer particles ranging in size from 5000 to 1 µm. They originate from polymer beads added to personal care and cosmetic products as well as abrasion of macroscopic objects, for example  car tires, plastic bottles and synthetic textiles.

Nowadays, MPPs can be ubiquitously detected around the globe in aquatic environments. Rivers, arctic ice, lakes, oceans, tap water and bottled water are all contaminated with microplastic particles. They threaten the health of aquatic animals like crustaceans, shells and fish. Microplastic are also good transporters for persistent organic pollutants (POP) and pathogenic bacteria that adhere to the surface of the particles. Furthermore, they can release toxic plasticizers that were used in the manufacturing process. [1]

These particles ultimately make their way into the human body, either by direct intake through drinking water or through the food chain. The smallest particles can accumulate in organs and are suspected to also enter the blood stream. [2]

The method of choice to locate MPP is visual microscopy. However, identification is limited to particles down to 100 mm. This often causes incorrect estimates of the number of plastic particles and does not give the possibility of identifying the type of polymer.

To overcome this problem, Bruker offers analytical solutions for reliable particle identification down to 5 µm, by combining visual microscopy with the ATR-FTIR technique. ATR is a contact based measurement method that stands for attenuated total reflection. It is applicable to a broad range of samples, easy to learn and quick to master. Two examples are given in this article.

Unlike reflection or transmission measurements, micro ATR analysis generates IR spectra of the highest quality, regardless of the sample dimension and shape. Furthermore, MPPs can be analyzed on almost any filter material and even without filter separation using ATR (e.g. on complex matrices like sediments).

Dedicated IR transparent or reflective filter materials need to be utilized when applying transmission or reflection and the spectral data quality is very dependent on the size of the MPP, making ATR the straightforward analytical choice.

Figure 1: LUMOS FTIR microscope.

Automated FTIR ATR Microscope

A fully automated FTIR microscope like the LUMOS, guarantees a convenient workflow and easy handling for the analysis of MPP. As a stand-alone microscope, it has all components integrated in a compact package. This includes a software controlled, fully motorized sample stage and ATR crystal within the objective.

It is used by microscopic laboratories, research institutes and industry partners around the globe, e.g. in France [3], the United States [4], China [5] and Turkey [6]. It should be mentioned, that the LUMOS is also capable of state-of-the-art FTIR imaging by the use of a focal plane array detector.

Visual Identification

The combination of FTIR and visual microscopy is a powerful tool in the analysis of MPP. It offers all the advantages of optical microscopy for particle localization while providing reliable identification of the polymer.

Crossed Visual Polarizers

Transparent items are easily overlooked because they are hard to distinguish from the filter material. By highlighting particles that shift the plain of polarization, the automated visual polarizers in the LUMOS FTIR microscope can overcome this issue (see Fig. 2, top).

Dark Field Microscopy

A darkfield image recorded with the LUMOS FTIR microscope is shown in Figure 2. By using darkfield illumination, light directly reflected from the surface is blocked and only light that is reflected diffusely is collected. Therefore, transparent particles are lighting up on an otherwise dark background, greatly improving contrast (see Fig. 2, bottom).

Figure 2: Visual enhancement options offered by the LUMOS. The contrast improving effect of crossed visual polarizers can clearly be seen (top), while in the darkfield image, the particles light up on a gold filter for easy localization (bottom).

Precise Measurement with ATR

The LUMOS FTIR microscope is equipped with fully automated, freely adjustable knife edge apertures that enable the user to define specific measurement areas that only target the particle itself. This helps to avoid background influence.

Figure 3: Example for the adjustment of the knife edge aperture (red box) to the desired measurement area.

Chemical Identification

The obtained spectrum is compared to a comprehensive polymer library in order to identify microplastic particles with the utmost reliability. Bruker and the Kunststoff Institut Lüdenscheid (Germany) provide a specialized, constantly updated library which contains all important polymers, fillers and additives. This enables quick identification of any particle found. Additionally, the Bruker “ATR-LIB-COMPLETE” database, with over 26,000 entries, can be searched to identify any non-polymer material.

Example 1: Particles in Bottled Drinking Water

In the first case study, it was suspected that water contamination was the result of a problem in the bottling process. The bottled water was passed through a filter and the collected residue subjected to FTIR microscopy analysis in order to investigate possible issues with the production batch.

A red fiber was located on the filter surface and measured with an aperture (red rectangle, Fig.5) that matched the fiber to avoid influence from the filter material. This way, the fiber was identified as a polyester, indicating possible contamination sources.

Figure 4: Library search result of the red fiber. The spectrum of the fiber is shown in red and the library spectrum in blue.

Figure 5: Red particle on a filter with indication of particle length and aperture size.

In addition, transparent particles were found and measured by ATR. The knife edge apertures (red rectangle) once again helped to avoid any influence of the filter material. The collected high-quality spectra were compared to a spectral reference library, which unambiguously identified the particles as inorganic silicates.

Figure 6: 800 x 800 μm cutout of a drinking water filter (left) and magnified particle together with its size and the fitted aperture (right; red box).

Example 2: MPP in Environmental Sediment Sample

The versatility of ATR goes far beyond analyzing micropalstic ontop of a filter. In this case, sediment samples from a river bed were collected and analyzed without prior sampling by using the LUMOS FTIR microscope.

Once more, the aperture was adjusted in order to fit a thin fiber that was found on the surface of the sediment. Figure 7 shows the visual image and resulting spectrum of the measurement. Spectra of brilliant quality were received and the fiber identified as polyamide, most likely a contamination from textiles and wastewater.

Figure 7. Fiber found in a river sediment and adjusted aperture (red rectangles, left) of the measurement. Bottom: acquired spectrum of the fiber.

Conclusion

Particles down to 5 µm are easily located by visual microscopy using the LUMOS FTIR microscope, and subsequently chemically identified by ATR FTIR microscopy. Ultimately, the approach presented is very simple to apply and results in high quality, reliable data.

The LUMOS is the analytical tool to analyze microplastic particles of any shape and dimension on virtually any filter substrate, or in complex matrices e.g. sediments.

References

1. I. L. Nerland, C. Halsband, I. Allan,K. V. Thomas, Norwegian Institute for Water Research, Microplastics in Marine Environments- Occurrence, Distribution, and Effects, 2014.
2. D. Y. Feng, Y. Zhang, B. Lemos, R. Hongqiang Tissue accumulation of microplastics in mice and biomarker responses suggest widespread health risks of exposure. Scientific Reports 2017.
3. R. Dris, J. Gasperi, M. Saad, C. Mirande and B.Tassin, Synthetic fibers in atmospheric fallout: A source of microplastics in the environment?, Marine Pollution Bulletin 2016, 104, 290 – 293.
4. A.P.W. Barrows, S.E. Cathey, C.W. Petersen,
   Marine environment microfiber contamination: Global patterns and the diversity of microparticle origins, Environmental Pollution 2018, 237, 275-284.
5. G. Peng, P. Xu, Ba. Zhu, M. Bai, D.Li, Microplastics in freshwater river sediments in Shanghai, China: A case study of risk assessment in mega-cities,
   Environmental Pollution 2018, 234,448-456.
6. O.Güven, K.Gökdag ,B. Jovanovic, A. E. Kıdeys, Microplastic litter composition of the Turkish territorial waters of the Mediterranean Sea, and its occurrence in the gastrointestinal tract of fish, Environmental Pollution 2017, 223, 286-294.

This information has been sourced, reviewed and adapted from materials provided by Bruker Optics.

For more information on this source, please visit Bruker Optics.