Unveiling Microplastic Pollution in the Air: Optimizing filter material and Work-up in PM10 studies

Microplastics (MP) are prevalent environmental pollutants found in urban, rural, and remote locations worldwide. Although aquatic and soil samples are extensively studied, the examination of aerosol samples is a relatively new area of research. The investigation of inhalation exposure to nano- and microplastic particles is particularly noteworthy, given that the portion of particulate matter (PM) with an aerodynamic diameter less than 10 µm (PM10) is believed to exert the most significant impact on human health. 

Analyzing MP involves the application of mass-balanced or particle-related methods. To allow the applicability of both methods, the election of filter material during sampling is crucial. Membrane filters like quartz fibre filters (QFF) are the material of choice used for various types of analytes due to their high uptake capacity of PM. The direct analysis of the sampled QFF with particle related methods is impeded without particle extraction, since microscopic methods require all analytes to be detectable on the surface.

The mechanical stress induced on the filters during extraction leads to an extensive fibre loss of the filter material that aggravates the following evaluation. A pre-sampling filter treatment for QFF with potassium silicate solution (K₂SiO₃ ⋅ n H₂O, also known as water glass) as inorganic adhesive is presented. The advantages of the improved filter properties during post-sampling processing for microscopic analysis are shown and results are compared to non-treated filters.

A multiple step QFF work-up is applied including particle extraction, digestion (removal of organics by employing chemical treatment) and density separation. Since every processing step is prone to particle loss, the evaluation of recovery rates is essential. We present two approaches for quality assurance within the particle size range of interest. The optical approach allows the assessment in terms of absolute particle numbers, while mass spectrometry method provides the particle loss specified in mass.

After successful work-up, we are combining both method types by using Raman Microscopy (µRaman) and Pyrolysis Gas Chromatography-Mass Spectronomy (py-GC/MS) for the characterisation of MP. This combination raises the possibility to compare the results of both method types. We will show that the combination of both methods can be a significant contribution to the analysis of MP in the atmosphere.