# Simply Applicable Method for Microplastics Determination in Environmental Samples
## 3 Materials and Methods  
### Microplastic Samples: Type, Shape, and Dimension  
Simulated MP samples were prepared from commercially available plastic products of the most abundant polymer types, as suggested by [PERSON] et al. [4]: PET (mineral water bottle), PS (granulate for laboratory supplies production), PVC (tablet pharmaceutical packaging), PP (granulate for laboratory supplies production), and PE (high density; mineral water bottle caps). PET, PVC, and PE (sizes of 1.9 \(\pm\) 0.6 mm, 3.7 \(\pm\) 0.9 mm, and 2.4 \(\pm\) 0.6 mm, respectively) were manually cut into smaller pieces, whereas PS and PP (sizes of 3.5 \(\pm\) 0.2 mm and 4.7 \(\pm\) 0.1 mm, respectively) were left as granules. These represented the so-called \"coarse fractions\". The fine fractions of individual polymer type were obtained by grounding coarse fractions using liquid nitrogen (LN; Messer, Germany) and a ball mill MillMix 20 (Tehntica, Slovenia) and the following protocol: 6 min cooling in LN, 4 series of milling for 2 min at 35 Hz for PET and at 25 Hz for PS, PVC, PP, and PE with intermediate cooling in LN for 1 min. The obtained particles were sieved through 1 mm and 100 \(\upmu\)m stainless steel sieves (Retsch, Germany and Fipis, Slovenia, respectively) to obtain the MP particles ranging between 1 mm and 100 \(\upmu\)m, which fit within the general classification terminology of MPs (size range between 1 and \(<\)100 \(\upmu\)m; [48]). Experimental Design for Head Space Microextraction of Volatile Organic Compounds Produced from Melted MPs  
Sorption of VOCs on the SPME fiber in the headspace (HS) of the sample depends on numerous parameters. The three most important of these are: the type of fiber, the extraction time, and the extraction temperature [49; 50]. As the formation of polar VOCs was expected during thermal decomposition of polymers, the mixed phase SPME fiber DVB/CAR/PDMS (50/30 \(\upmu\)m) with a length of 20 mm (57299-U, Supelco, Bellefonte, PA, USA) was used. Prior to first use, the fiber was preconditioned at 250 \({}^{\circ}\)C for 30 min following the producers' instructions. The thermal decomposition process depends on the size, load, and type of the MPs. The amount of MPs sample load was tested in ranges between 10 and 100 mg of MPs, being valid throughout the whole range. Of PET, PS, PVC, PP, or PE 20 \(\pm\) 5 mg was used in the case of individual polymers and a total of 94 \(\pm\) 9 mg in the case of the polymer blend was placed into a 20 mL HS vial and sealed with caps with butyl/PTFE septum (Macherey Nagel, Duren, Germany). The coarse fraction (1-5 mm) samples were placed on the 1 st hot plate (IKA, Germany), within the vial, at a temperature of 220 \(\pm\) 3 \({}^{\circ}\)C (testo 926; Testo SE&Co. KGaA, Lenzkirch, Germany) for 3 min (PS, PVC, PP, and PE) and 15 min (PET) to melt. Whereas the fine fraction samples (1 mm-100 \(\upmu\)m) were left to melt on the 1 st hot plate for 1 min (PS, PVC, PP, and PE) and 3 min (PET). Analyses of individual MP types were conducted in five replicates. Immediately after thermal decomposition, the vials were transferred in a sand bath (2 nd hotplate) with an extraction temperature of 50 \(\pm\) 2 \({}^{\circ}\)C for 20 min. The temperature profiles observed in the HS of the vial during polymer thermal decomposition and VOC's adsorption onto SPME fiber could be found in the supplementary information (Figure S1). Afterwards, the fiber was inserted into the inlet of the GC with a temperature of 250 \({}^{\circ}\)C for 10 min. To prevent any carry-over between analyzed samples, a cleaning procedure with the desorption of compounds from a blank fiber was conducted after each run. As an internal standard, a solution of deuterated dimethyl phthalate (DMP-46; Sigma-Aldrich, St. Louis, MO, USA) was used [51].