Detecting Microplastic Contamination in Drinking Water with Raman Spectroscopy

Microplastics were first seen in the environment in the early 1970s in the Atlantic Ocean. Since then, these environmental pollutants have been reported in an alarming number of freshwater and saltwater sources.1 Their levels have drastically increased over time with an estimated 5.25 trillion plastic particles now present in the seas across the world.2

Significant research in this field has prompted serious concerns around the accumulation of plastic particles in marine animals, leading to an outright ban of microplastic sources (for example, microbeads used in a variety of cosmetic products), in the UK in 2018.3

It is still not fully known what impact the consumption of contaminated marine animals can have on human beings, but research suggests that it could result in an accumulation of microplastics in the body, which in turn leads to chemical toxicity and an immune response activation.2

Analytical Techniques to Identify Microplastics

While there has been a great deal of research into the impact of microplastics in bodies of water, research into microplastics in drinking water has been limited. Most studies in this field have employed micro-Fourier transform infrared (µ-FT-IR) spectroscopy.1

Micro-Fourier transform infrared (µ-FT-IR) spectroscopy is a rapid, accurate technique which involves minimal sample preparation.1,4 However, µ-FT-IR is only capable of detecting microparticles down to a size of 20 µm.

A recent study conducted at the Chemical and Veterinary Examination Office Münsterland-Emscher-Lippe (CVUA-MEL) in Münster, Germany, discovered that 80% of all the microplastics found in drinking water were less than 20 µm in size.1

This discovery confirmed that techniques with a higher resolution than 20 µm are required to effectively detect microparticle pollutants. Raman spectroscopy is one such technique, able to detect particles as small as 2 µm while offering rapid, accurate detection with minimal sample preparation, comparable to µ-FT-IR.

Laser-induced breakdown spectroscopy (LIBS) offers a further alternative, providing elemental analysis which allows clarification of the presence of inorganic metal particles.4,5

Hound from Unchained Labs Offers Microscopy, Raman, and LIBS in a Single Instrument

Historically, Raman spectroscopy, microscopy, and LIBS have been performed using different, specific instruments. Unchained Labs has successfully combined all three techniques into a single integrated device: Hound.

Hound offers particle analysis and characterization, enabling researchers to accurately count, size and identify the elemental and chemical composition of particles.5

Analysis can be performed both on a dry sample and in suspension, with samples in a suspension being prepared by simply pipetting the solution to be analyzed onto a coated glass surface. Meanwhile, dry samples are prepared by placing visible, large particles directly onto the filter surface, or through filtration of the sample solution via a suitable mesh filter round.

A single filter can be used with suspensions ranging from microliters to liters, allowing substantial volumes of suspected contaminated solutions to be analyzed in a single-stage.5

Gold-coated surfaces are Raman inactive for both dry and wet sample analysis, facilitating high-quality data collection by improving the signal-to-noise ratio and reducing background interference.6

Darkfield and/or brightfield imaging may be used to count particles in the sub-visible to the visible range of many materials; for example, organics, inorganics, proteins, and metals.

Automated or manual counting processes can be employed, allowing researchers to efficiently determine the morphology, size, and fibrosity of all particles within a matter of minutes. When utilizing the automated process, researchers are able to accurately identify thousands of particles in an unattended run.5

Hound captures all particles’ size, shape, image, and coordinates, with operators able to either manually identify particles or pre-define selection parameters designed to automatically analyze and confirm identification by LIBS and/or Raman.5,6

The instrument is equipped with two Raman lasers (785 nm and 532 nm). This enables greater flexibility – a vital consideration as materials often possess varying signal efficiency at different wavelengths.

Various plastic types have differing Raman spectral signatures, meaning that collected data can be evaluated against a database of more than 150 known common contaminants and ensuring that operators can be confident in their particle identification.

Additionally, researchers can add their own custom signatures to this reference database, facilitating accurate identification of materials found specifically in their area of interest.

Raman Spectroscopy can Identify Microparticles from Different Plastic Sources

By employing Raman spectroscopy, the team at CVUA-MEL successfully analyzed 38 drinking water samples with various types of packaging materials such as single-use plastic bottles, reusable plastic bottles, plastic cartons, and glass bottles.

The scanning and counting procedure was automated, and the darkfield imaging mode was utilized to make certain that even the most minute particles were captured for analysis. Darkfield imaging was chosen because these small particles are commonly missed in brightfield imaging, instead of appearing transparent or white.1

The researchers were able to confirm that every sample contained microparticles, with most of these originating from their packaging.

For example, the plastic bottle samples contained a large amount of polyethylene terephthalate and polypropylene particles, which are used to make the bottles and caps, respectively. This key finding indicated that the packaging itself was releasing microplastics into the drinking water.1

The reusable plastic bottle was found to release the largest amount of microplastics, suggesting that increased stress resulting from repeated use was causing more microparticles to leach into the drinking water.

As mentioned earlier, the researchers discovered that most of the microplastics present were under 20 µm in size.1 This discovery confirmed that microparticle analysis should always be conducted using instrumentation capable of identifying these small particles. Raman spectroscopy provides an ideal method for researchers looking to understand the full effect of microplastics on human health.

References and Further Reading

  1. Schymanski D., et al. (2018). Analysis of microplastics in water by micro-Raman Spectroscopy: Release of Plastic Particles from Different Packaging into Mineral Water. Water Research. https://doi.org/10.1016/j.watres.2017.11.011.
  2. Wright S.L., et al. (2017). Plastic and Human Health: A Micro Issue? Environmental Science and Technology. doi: 10.1021/acs.est.7b00423.
  3. www.gov.uk. (2018). World Leading Microbeads Ban Comes into Force. https://www.gov.uk/government/news/world-leading-microbeads-ban-comes-into-force
  4. Lee K., et al. (2017). Identification of Particles in Raw Materials. AAPS PharmSciTech. doi: 10.1208/s12249-017-0823-0.
  5. www.unchainedlabs.com. (2020). Hound. The Unrivaled Particle Characterization and ID Platform. https://www.unchainedlabs.com/hound/
  6. www.unchainedlabs.com. (2020). Hound Seminar: Identification of Particle Contaminants with Hound. https://www.youtube.com/watch?v=fHFl0DD7POM

This information has been sourced, reviewed and adapted from materials provided by Unchained Labs.

For more information on this source, please visit Unchained Labs.

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