Spectroscopic Analysis of Microplastics in the Environment

Spectroscopic technologies are proving increasing vital to analysis of plastics contamination in the environment

The origin of plastic

A word originally meant to indicate “pliable and easily shaped”, plastics have arisen as synthetic polymer materials used in great abundance today. Polymers - “of many parts” - are made up of variably-size chains of repeating chemical units. There is a wide abundance of natural polymers, such as cellulose and some forms of spider silk for example, with properties of high tensile strength, toughness, and density.

These same attributes of natural polymers eventually led to the development of synthetic polymers and many of the plastics seen today. Synthetic plastics stem largely from petroleum sources and include long chains of repeating units different than those found in nature. Unlike those from natural origins, synthetic petroleum-based plastics degrade at a slow and variable rate – some thought to persist for 5000-10000 years or more.

An epidemic of plastic

As concerns over environmental pollution have risen, the issues surrounding plastics have floated to the top. The initial idea that plastics were disposable eventually led to cheaper manufacturing and widespread use of so-called single-use plastic products, e.g. shopping bags, straws, coffee stirrers, cotton swabs, etc. Despite modern recycling efforts, many of these materials cannot or simply are not recycled or reused.

Effects of plastic pollution

Ocean debris is perhaps the most profound consequence now observed regarding plastic in the environment.  There are many outlets and references for the reporting of damage to the natural world at the hands of plastic pollution. Perhaps the most humbling effects are those seen in marine (and terrestrial) animals, after ingesting what was thought to be legitimate food.

What are microplastics?

Despite our best efforts to ensure food, beverage, and water safety, we are seeing evidence that plastics have invaded our resources as well.  Depending on the chemistry of plastic polymers (polypropylene, polystyrene, polycarbonate, etc), these materials are prone to breakdown (not biodegradation) as a result of heat, sunlight, mechanical forces, and chemical reactivity. Microplastics are the resulting next phase of plastic de-evolution -- small, micron-scale, semi-detectable (or undetectable) souvenirs of what once was.

How do you detect microplastics?

A recent study made international news and ignited a firestorm around the presence of microplastics in 90% of bottled water for human consumption. This followed a previous finding that a large percentage of the world’s tap water is contaminated with low levels of microplastics. These developments and others have led to the World Health Organization (WHO) announcement of a comprehensive review of plastics in drinking water.

How were these results collected and what were the methods for microplastics detection? Filtration, density separation, and other volume-reducing methods are now routinely used along with fluorescence detection incorporating Nile Red dye, which preferentially adheres to polymers in aqueous samples over common organic materials (e.g. algae, wood, feathers).

FTIR spectroscopy for plastics identification

Fourier Transform Spectroscopy offers high analytic power for confirmation of polymer identity, beyond just detection. Most microplastics can be visualized, either with out without microscopy, fluoresced and typed with respect to morphology. FTIR provides chemical identity-level information through generation of distinct spectra indicative of polymeric species, e.g. polycarbonate aromatic hydrocarbon chains versus polypropylene aliphatic hydrocarbon chains. IR Spectral Databases are good references for study of unique spectroscopic characteristics of different polymers. As mentioned, FTIR is a very capable tool for analysis of materials and identification not just of chemical constituents, but impurities, fingerprints, and other features as well.  

Attenuated total reflectance (ATR) is a modification of FTIR that can provide further insight into a variety of solid samples in their native state. The advantages with ATR and FTIR in general is the potential of accurate identification of plastics, and combined with morphological data and location etc., may produce very valuable information as to the source and the combined factors that lead to (micro)plastic pollution.

Raman spectroscopy for plastics analysis

Raman can provide further insight into the chemical composition of microplastics contamination – in essence adding another layer of information on origin and lifecycle of the material(s). Again, spectral databases provide valuabe information regarding unique Raman characteristics of different polymers.

Outlook

For all the good plastics have provided, medical devices and health care, food and beverage safety – the fact is that plastics persist in the environment long after intended use and are therefore detrimental to all biological life -- over 80% of all plastics ever produced are still present in our world. As such, evolving analytical methods and technologies will continue to be vital investments for our present and future ecosystem.

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Article updated July 2021