###### Abstract
In the area of PET recycling for new food contact applications such as water bottles, a similar situation exists: in 2011, the EFSA established evaluation safety criteria for PET recycling processes when the intended use is the manufacture of FCMs [23]. As unknown contaminants may be introduced during the re-collection and recycling steps, the EFSA requires that it must be ensured that even potentially genotoxic substances are sufficiently removed in the recycling process. The cleaning efficiency is tested in a so-called challenge test, and the finally recovered PET material must not contain any unknown (and therefore potentially genotoxic) contaminants which could lead to consumer exposure above the lowest level of the threshold of toxicological concern (TTC) concept, which is 0.0025 \(\upmu\)g/kg bw/d for genotoxic substances [24; 25]. Depending on the population age groups (infants, toddlers, and adults) to be considered and the related food consumption specifications, the migration limit to be controlled will therefore be in the range from 0.017 \(\upmu\)g/kg food (infants) to 0.15 \(\upmu\)g/kg food (adults) [23]. Again, such low levels can hardly be analytically controlled in non-targeted measurements to seek for any potential contaminant of unknown identity in food simulants and in foods. Therefore, the EFSA applied an indirect \"method of analysis\", more specifically \"migration modeling\", to back-calculate from these low maximum allowable migration levels to their corresponding concentrations in the PET polymer, denoted C\({}_{\text{mod}}\)[23]. For comparison with C\({}_{\text{mod}}\), in the challenge test, the cleaning efficiency is determined using surrogate substances by measuring their decrease in the PET material during the recycling process. The final residual concentrations in PET, denoted C\({}_{\text{res}}\), are compared with C\({}_{\text{mod}}\), and when C\({}_{\text{mod}}\) > C\({}_{\text{res}}\), the reprocessed PET is considered to be safe [23]. The motivation for and objectives of this paper are the following: Although BPA is not a monomer used in the production of PET, it cannot be excluded that some BPA impurities may be non-intentionally present in rPET-based FCMs. This will trigger the analytically extremely challenging question of whether rPETs may contain BPA at levels exceeding the level implied by the new TDI or SML requirements. As mentioned above, verification of such low BPA residues in foods or food simulants is hardly any longer directly accessible by analytical means. We see a solution for this problem in proceeding in analogy to the migration modeling approach as described above for evaluation of PET recycling processes [23; 26]. Starting from the level implied by the new TDI or SML, back-calculations can be made by migration modeling to establish maximum concentrations in rPET for different food contact applications [27; 28]. These maximum concentrations can be compared with actual concentrations in rPET, which are analytically much more accessible than those in food (or food simulants). Such a procedure, however, requires precisely determined diffusion coefficients for BPA in the PET polymer matrix, which are not yet available in the scientific literature. Therefore, our first objective was to measure the diffusion coefficients and the activation energy of the diffusion of BPA in PET polymer. Our second objective was to describe an analytical method based on LC-MS (i) to verify the back-calculated levels in rPET and (ii) for sensitive BPA determination in rPET samples at very low limits of detection. This method was used to determine the concentration in various rPET samples from the European market between 2019 and 2023. Our working hypothesis is the following: by combining this analytical method with migration modeling parameters based on established BPA-related parameters, migration levels of BPA from rPET into foods will be accessible at, and even below, the new EFSA TDI [17] or the new proposed threshold limit of the European Commission [22].