Impact of sample pretreatment on analytical representativity in mechanical plastic recycling: A case study on brominated flame retardants

The EU has restricted the use of certain brominated flame retardants (BFRs) in recycled plastics, yet standardized analytical workflows for their determination remain lacking. Mechanical recycling of plastics often produces heterogeneous material streams due to imperfect sorting, making sample pretreatment essential for obtaining representative results for legislative compliance. This study evaluated the impact of different sample pretreatment strategies on analytical representativity using a model feed of blue and white high-impact polystyrene (HIPS) flakes. The evaluated pretreatments included (1) direct injection molding, (2) melt-homogenization followed by injection molding, (3) cryogrinding followed by injection molding, and finally (4) cryogrinding combined with melt-homogenization and injection molding. The distribution of the blue-colored HIPS in the samples was assessed visually and using direct insertion probe mass spectrometry (DIP-MS). Additionally, BFR-containing samples were obtained from a mixture of HIPS and tetrabromobisphenol A (TBBPA). The distributions of total bromine and TBBPA in the specimens were evaluated using X-ray fluorescence (XRF) and DIP-MS, respectively. Direct injection molding of the flake mixtures resulted in heterogeneous distributions, while cryogenic grinding or melt-homogenization prior to molding produced homogeneous samples. XRF analysis of BFR-containing samples revealed that melt-homogenized samples exhibited lower and more consistent bromine content compared to directly molded ones. DIP-MS was effective not only for additive identification but also for evaluating their spatial distribution. Overall, this study emphasizes the need for a standardized sample pretreatment protocol, which would enable reliable analysis of entire recycled plastic batches, ultimately improving the accuracy and effectiveness of plastic recycling.