Thermal desorption and analysis of atmospheric nanoplastics via PTR-MS

Nanoplastics (NP) represent a global anthropogenic pollutant. Aerosolized NP enter the atmosphere and are eventually deposited in precipitation and surface waters, facilitating their entry into the food chain. Due to their small size, these particles can translocate within organisms and penetrate tissues and cells. While the health effects of environmental exposure levels remain poorly understood, the continuous increase in global concentrations is a growing concern. However, the characterization and analysis of aerosolized or deposited NP are analytically challenging.

In this study, we present a newly developed low-volume thermal desorption (TD) solution that is directly interfaced to a proton-transfer-reaction mass-spectrometer (PTR-TOF 6000X2, IONICON Analytik GmbH, Austria) for real-time detection of volatile organic compounds at lowermost concentrations. The TD unit allows for precise temporal temperature control up to 400°C. These temperatures are sufficient to efficiently thermolyze a large fraction of common NP into PTR-MS detectable volatile organic compounds (VOCs). In most cases, the released VOCs serve as specific markers for plastic identification: for instance, styrene for polystyrene (PS), methyl methacrylate for polymethyl methacrylate (PMMA), and terephthalic acid for polyethylene terephthalate (PET). Polyvinyl chloride (PVC) is identified via aromatic compounds such as benzene and naphthalene, while polyethylene (PE) exhibits a characteristic homologous series of alkenes and alkanes.

To validate this TD method, commercial monodisperse solutions of PS and PMMA were prepared with concentrations ranging from 0 to 60 ng in HPLC-grade water that was prefiltered through a 0.2 µm PTFE syringe filter. These samples were contained in precleaned headspace vials baked in a vacuum oven at 150°C and 10 mbar for >5 h to eliminate potential contaminants. After an evaporation step in a vacuum desiccator, the dry samples were heated in the TD unit and the thermolysis products were transferred in a controlled carrier gas (Air or N₂) to the PTR-MS for quantitative analysis. Several replicates were prepared for each sample, along with laboratory blanks. Respective signals were integrated, and linear regressions were calculated. We achieved an R² of 0.99 with a 3σ limit of detection (LOD) of 2.8 ng for PS, and an R² of 0.98 with an LOD of 9.2 ng for PMMA.

We further present initial results from samples containing deposited NP and explore data analysis methods based on matrix factorization.