Pyrolysis-GC-MS: A Powerful Tool for Microplastics Analysis
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The dangers of microplastics
Single-use plastics have become a potent symbol of environmental degradation and feature high on the list of environmental issues to regulate. Over 60 countries have put in place bans and/or taxes on single-use plastics, with plastic plates, cutlery and cups being the latest to face a ban in England.
Plastics represent some 40% of litter on beaches, much of which eventually ends up in our oceans. In fact, 8 million metric tons of plastic enters our oceans every year. This is particularly concerning because plastics are persistent pollutants, which means they can remain in the environment for centuries.
What’s more, as plastics weather under sunlight, air, heat and moisture they generate macro, micro and nanoplastics, ranging from a few millimeters in size to microscopic particles. These micro (<5 mm) and nanoplastics (<1 µm) can adsorb other toxic compounds, including pesticides, polychlorinated biphenyls (PCBs) and phthalates. If ingested by marine life, there may be toxic side effects for liver function, reproduction and feeding behaviours.
Microplastics may also represent a hazard to humans through subsequent consumption. They are known to accumulate up the food chain and have been found in various types of seafood, as well as honey, sugar, sea salt, water and beer. It’s estimated that individuals may be exposed to up to 121,000 microplastics each year via inhalation and ingestion. Although evidence is nascent, microplastics could have adverse health impacts for people ranging from chronic inflammation and metabolic disturbances to neurotoxicity and increased cancer risk.
Clearly, action needs to be taken, with analysis of the landscape of microplastics being a key first step.
Understanding the scale of the problem
Understanding the prevalence, size and composition of microplastics in the environment is critical to tackling the issue. However, standard methods to accurately assess the extent of microplastic pollution are lacking, with regulatory agencies worldwide actively working to provide a widely accepted reference analytical procedure for microplastics.
Without a widely accepted analytical procedure, estimates of plastic loads in the oceans have a wide range (more than six orders of magnitude) and no comprehensive data exist on the extent of microplastic pollution in soil, despite its importance for agriculture. Understanding of microplastic deposition in the atmosphere is even less clear.
Available analytical techniques: limits and benefits
Many different analytical approaches are available to gather information on microplastics, such as particle number and size, the identity of polymers and additives, mass fraction, and the state of degradation. While no comprehensive method exists, different techniques can be used to piece together the whole picture, each with their limits and advantages.
Traditional methods for identifying microplastics rely on microscopy, but this approach is limited in sensitivity and may generate inaccuracies. Size limits mean microscopy cannot identify some of the smaller micro and nanoplastics, and is, thus, generally only suitable for microplastics bigger than 1 mm. A comparison of microplastics analysis methods suggests that microscopic analysis may lead to small or transparent microplastics being missed, or mean that other elements in the sample (such as natural fibers) are misidentified as microplastics.
Spectroscopic techniques, like Raman and infrared (IR) spectroscopy, are widely used to assess microplastic contamination based on particle number, particle size and shape. However, spectroscopy gives no indication of polymer composition and cannot identify additives. Spectroscopic techniques are also limited in the size and number of particles which can be collected.
These techniques are, therefore, complementary to thermoanalytical methods, which involve the thermal extraction and/or thermal degradation of the sample followed by the identification/quantitation of decomposition products. Here, gas chromatography-mass spectrometry (GC-MS) plays a key role.
Pyrolysis-gas chromatography mass spectrometry
Unlike spectroscopic techniques, thermoanalytical methods provide mass-quantitative information and identification of microplastic contamination. Pyrolysis-gas chromatography mass spectrometry (py-GC-MS) is a powerful method, whereby, the sample is thermally decomposed in an inert atmosphere before being separated and analyzed by GC-MS.
Samples, including viscous liquids, solids or organic materials, can be introduced into the pyrolyzer, where a micro furnace thermally vaporizes the sample into a gaseous state. Sample preparation typically involves isolation of the particles by filtration or density separation, depending on the particle size. Direct analysis is possible if the sample consists of cryo-milled sediments.
Pyrolysis occurs at temperatures between 600 and 1000 °C, with full decomposition of the sample before GC-MS analysis. Py-GC-MS generates information on the chemical identity of polymers through characteristic degradation products, using fingerprint chromatograms known as pyrograms. As well as identifying and quantifying the polymer types of microplastics, it can simultaneously analyze additive chemicals within microplastics. Bulk samples can also be analyzed, providing summed microplastic concentration data by weight.
Py-GC-MS can also be used to quantify microplastic polymers in complex environmental and biological samples, including soil, water and marine organisms, providing a powerful method for both qualitative and quantitative analysis of microplastics in real world samples.
Py-GC-MS creates new possibilities for microplastics analysis
Py-GC-MS offers a promising option in the field of microplastic analysis, for the identification and quantitation of polymer types of microplastic particles as well as associated organic plastic additives. It can detect bulk amounts of micro and nanoplastics below the lower size limit of traditional microscopy and spectroscopy, offering low detection limits, in the range of pg to µg, and excellent linear response (over a concentration range of 0.05 μg - 50 μg).
An additional future step for microplastic studies is the combination of pyrolysis with high resolution accurate mass (HRAM) GC-MS, a powerful tool to enhance selectivity in the case of complex matrices, and enabling untargeted and retrospective analysis. This allows scientists to screen raw data from the quantitative experiment and look for additional compounds generated during the pyrolysis process, generating valuable extra data.
As the dangers of microplastics become more clear and regulation expands (the latest ECHA restriction intends to prevent the release of 500,000 tons of microplastics over the next 20 years), precise and efficient analysis methods will become even more important. Thermal analytical methods, and in particular py-GC-MS, offer numerous advantages for microplastics analysis and would help the field to achieve standardization, an important step in the pathway to tackling plastic pollution and making the environment safer for all.
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