A Closer Look into Microplastics

[Daniela-C]

Courtesy of IS-X.Why is plastic — having a lifespan of several hundred years or more — widely used for disposable tools and containers?  This is a paradoxical question that is hard to answer, but it projects recycling to the forefront. Sadly, we are all familiar with images of the big accumulation of plastics in our oceans, but we are much less aware of what we cannot see. Contamination from microplastics and nanoplastics, ranging from a few millimeters to microscopic particles, is everywhere, impacting not only the entire marine environment, but reaching us through fresh water, air and food.

Microplastics, defined as particles smaller than 5 mm, are considered a new type of emerging contaminant generating great concern for public and government authorities because of the severe threat to human and animal health. Microplastics are not only dangerous themselves, especially for filter-feeding organisms in the marine environment, but they can adsorb other persistent, highly toxic organic compounds that accumulate in the food chain through those microparticles. The role of microplastics as vectors of persistent organic pollutants (POPs) such as pesticides, polychlorinated biphenyls (PCBs) and additives was presented for the first time at the Dioxin Conference in 2019.

Therefore, having an in-depth understanding about size distribution and the composition of microplastics is a big challenge for scientists involved in environmental pollution studies.

The level of microplastics contamination is assessed based on particle number information through spectroscopic techniques like Fourier-transform infrared (FTIR) and Raman spectroscopy, or mass information through thermoanalytical methods where the sample is thermally decomposed through pyrolysis and then analyzed via GC-MS. Both approaches produce key information on the chemical identity of the different polymers and are often used as complementary techniques in risk assessment studies.

If spectroscopic techniques provide particle-related information, Py-GC-MS allows for mass-quantitative information for specific polymers identified through characteristic decomposition products, generating fingerprint chromatograms known as pyrograms.

The relevance of thermal methods for microplastics analysis is growing and Py-GC-MS is applied to identify and quantify polymer types of microplastic particles as well as associated organic plastic additives.

Most microplastic particles are composed of the six major polymer types: polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyamide (nylon) (PA) and polyethylene terephthalate (PET).

An interesting Application Note developed in collaboration with the Institute of Marine Research in Bergen, Norway, reports the use of the Pyrolysis-GC-Orbitrap MS as a powerful tool for the identification and quantitation of microplastic in biological samples, highlighting the importance of high-resolution accurate mass (HRAM) mass spectrometry for selectivity in case of complex matrix, as well as the capability for untargeted and retrospective analysis.

IS-X (www.is-x.com), a Chromatography Expert Center with laboratories in Belgium and the Netherlands, recently implemented a Pyrolizer-GC-MS system based on the Thermo Scientific ISQ 7000 Single Quadrupole GC-MS for rapid identification of microplastics, with full control and data processing achieved with the Thermo Scientific Chromeleon CDS.

IS-X has a team of 10 scientists and engineers who provide consultancy services, detailed feasibility studies of new methods and technologies as well as contract research activities on a global scale. We asked Dr. Joeri Vercammen, Managing Director at IS-X, to provide us some insights on the challenges of this application and the potential of this technique for assessing microplastic pollution:

Q: What type of samples did you analyze with the Py-GC-MS system and which polymers could you identify?
A: As a contract research organization, it is always our goal to develop generic methods that merge flexibility, versatility and robustness. In that respect, we have carried out several projects in the context of microplastic analysis. Obviously, prime focus is on the analysis of microplastics in water samples, albeit drinking water, surface water, wastewater and even sea water, but we have also validated a sample preparation method that allows extraction of residual microplastics from mussels. Within our method, we are able to quantify all of the most common plastics, i.e., polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinylchloride (PVC), polymethylmethacrylate (PMMA), Nylon 6, polyethylene terephthalate (PET) and polyurethane (PU).

Q: Environmental and biological matrices are complex. What are the most critical aspects in the sample preparation?
A: The impact of unwanted matrix interferences should indeed not be underestimated, particularly considering the complexity of biological matrices, such as the mussel tissue mentioned above, and the minute sample amounts amenable to achieve complete pyrolysis. In order to achieve accurate results, we apply oxidative digestion of organic matter by means of Fenton’s reagent. Residual inorganic material, which remains intact after pyrolysis, is subtracted from the initial sample weight.

Q: How is the calibration for quantitative determination achieved?
A: We construct calibration curves using neat polymer standards, which are usually applied to calibrate GPC (gel permeation chromatography) methods. We have identified unique tracer components for each polymer, which are conveniently included in a target processing method in Chromeleon software. The ISQ 7000 GC-MS is applied in full-scan/SIM mode to achieve ultimate sensitivity while maintaining full flexibility regarding data interpretation. Throughout our method development process, we have identified several factors that have a significant impact on method reproducibility and accuracy. The use of an internal standard (perylene-D12), for example, proved to be indispensable in that respect, but we also grind standards and samples with a cryogenic ball mill. Finally, calibration standards include the entire sample preparation process.

Q: Which detection limits can be reached?
A: Typical detection limits are situated around 0.1 µg per polymer, although matrix interferences might give rise to higher detection limits, particularly when wastewater samples are involved.

Q: How difficult is interpretation of the data and how can the software help?
A: Data interpretation, indeed, is challenging but Chromeleon CDS proved to be an excellent tool. Identification of polymers in complex samples, for example, is greatly simplified using standard eWorkflow templates and functionalities, such as ion-ratio confirmation capability. Interactive charts are used to monitor method performance on a sample-to-sample basis for extensive sample batches.

An increasing number of studies are expected concerning microplastic pollution, to go toward method standardization, which is still missing. Py-GC-MS represents a promising approach for surveillance, enabling time-saving detection of bulk amounts of micro- and nanoplastics below the lower size limit of the microscopy techniques.

Joeri Vercammen, Ph.D.

Joeri Vercammen, Ph.D. has more than 20 years of expertise in GC&MS method innovation and validation. In addition to his role at IS-X, he is also visiting professor at Ghent University (INCAT research group), where he aims to inspire the next generation of scientists regarding the exciting ecosystem of chromatography.