## 4 Materials and Methods  
### Sample Materials  
The GPPS and HIPS grades used in the present study are standard industrial materials used in the European market for food contact applications. The used GPPS grade has a molecular weight of around 240.000 g/mol, melt flow index of 12 cm\({}^{3}\)/10 min at 200 \({}^{\circ}\)C/5 kg load, glass transition temperature of 100 \({}^{\circ}\)C and density of 1040 kg/m\({}^{3}\). The used HIPS grade has a molecular weight of around 190.000 g/mol, melt flow index of 4 cm\({}^{3}\)/10 min at 200 \({}^{\circ}\)C/5 kg load, glass transition temperature of 100 \({}^{\circ}\)C and density of 1030 kg/m\({}^{3}\). The pellets (gently provided by the Styrenics Steering Committee of Plastics Europe) were processed using a Collin Lab & Pilot Solutions co-extruder (Maitenbeth, Germany) to produce sheets with a thickness of approx. 350 \(\upmu\)m. For both polymers, the temperature in the extruder was approx. 190 \({}^{\circ}\)C; the screw speed was approx. 100 rpm.  
A mixture of five model substances, with a variety of chemical and physical properties, were spiked homogeneously during polystyrene sheets production as already described in [7]. They were toluene (CAS No. 108-88-3), chlorobenzene (CAS No. 108-90-7), phenyl cyclohexane (CAS No. 771-98-2), benzophenone (CAS No. 119-61-9) and methyl stearate (CAS No. 112-61-8). All chemicals used in the experiments were of analytical grade: toluene, styrene and benzophenone were purchased from Sigma-Aldrich (St. Louis, MO, USA); chlorobenzene from Merck (Darmstadt, Germany); phenyl cyclohexane from Alfa Aesar (Ward Hill, USA) and methyl stearate from Fluka (Seelze, Germany).  
Determination of the Initial Concentration (C\({}_{p,0}\)) of Spiked Substances, Styrene Monomer and Styrene Oligomers  
The initial concentrations of the spiked substances as well as of the styrene monomer and styrene oligomers were determined in the GPPS and HIPS spiked sheets by extraction with acetone (purchased from Chemsolute, Th.Geyer GmbH, Renningen, Germany). 1.0 g of the material was extracted with 10 mL acetone and stored at 60 \({}^{\circ}\)C for 3 days. Triplicate experiments were performed. A subsequent extraction (under the same condition) and recovery experiments were performed to check for exhaustiveness.For recovery experiments, reference sheets (not spiked during extrusion) were used. They were fortified at three levels with the model substances and available commercial standards for styrene dimers (PSS-266) and trimers (PSS-370) purchased from PSS Polymer Standards Service (Mainz, Germany), directly in the vial, extracted and analyzed as the samples.  
The extracts were analyzed by gas chromatography with flame ionization detection (GC-FID). The Agilent 6890 GC/FID system (Agilent Technologies, Santa Clara, CA, USA) was used with the following conditions: column: DB 1 (length 20 m, inner diameter 0.18 mm, film thickness 0.18 \(\upmu\)m); GC temperature program: 50 \({}^{\circ}\)C (2 min), followed by heating at 10 \({}^{\circ}\)C min\({}^{-1}\) to 340 \({}^{\circ}\)C (15 min); pre-pressure: 50 kPa hydrogen; split: 10 mL min\({}^{-1}\). The injection volume was 5 \(\upmu\)L with a split ratio of 1:20. An internal standard solution of di-(tert-butyl)-hydroxyanisol (BHA purchased from Fluka, Seelze, Germany) and Tinuvin 234 (Ciba, Basel, Switzerland) was used for all GC analyses to check the stability of the retention times and the performance of the GC analysis.  
Quantification of spiked substances and monomer styrene was achieved through calibration with respective standard substances, while styrene oligomers were semi-quantified using the response factor of BHA.