## 4 Discussion  
Based on the ICP-MS established LOD for total silver we did not find any detectable silver release from the silver nanocomposite in this study. So, in addition to the previously [1] observed non-migratability of silver nanoparticles under Fickian diffusion conditions, the results of this study support the assumption that also under the foreseeable food contact material stress conditions silver nanoparticles would not be released. When we apply the LOD for total silver of 16 ng/dm\({}^{2}\) to the conventional food packaging dimensions of 6 dm\({}^{2}\) per 1 kg food--this translates to a detection limit or maximum migration of 0.1 ug/kg food. Also in case of TiN it is already known that these nanoparticles do not migrate out of the polymer based on Fickian diffusion. It should be noted here that the former migration tests were done in a model study on LDPE samples with an extremely high loading (1000 mg/kg) of TiN which would not find any application in practice. Therefore, the stress test conditions used in this study can be seen as a worst-case for the release of TiN as well. However, even under the assumption that the detected signals in abrasion tests after additional preceding heating, swelling or stretching of the nanocomposites were solely caused by release of TiN, the released amount of TiN onto the abrasive food simulant would be less than 1 ug/kg (assuming a food packaging with 6 dm\({}^{2}\) per kg food and 77.3% Ti in TiN). It must be noted that release of Ti was only found in samples that showed impairment caused by the additionally applied prestress conditions. After heating TiN nanocomposites tend to stick together and need to be pulled apart. That way it cannot be excluded that the surface might get slightly damaged. Though swelling in isooctane did not cause migration in our previous migration study, Ti could be found after the nanocomposite was swollen and abraded. Nanocomposites were stickier after storage in isooctane wherefore a higher friction of the dry food simulant can be assumed during the abrasion test. During stretching of the films the film itself becomes thinner and might also be partially damaged (cracks in the surface) whilst TiN particles remain unchanged. Therefore, a direct contact of the simulant with the TiN particles might become possible. Although those stress conditions took obviously influence on the nanocomposites integrity, the test films were not damaged in a way that allows the conclusion that free, unbound TiN particles were abraded from the nanocomposites surface. It seems rather likely that the above-described test scenarios weakened the polymer matrix in a way that allowed abrasion of small polymer flakes in which TiN was still embedded and then measurable by ICP/MS. Figure 9: AF4/MALLS/ICP-MS fractograms of a test sample that was stretched first and then abraded: The same sample that caused a signal via MALLS detection (black) at elution times relevant for laponite did not cause a signal for Mg via ICP-MS (online) detection (red). Via AF4/ICP-MS detection no difference between abrasion from test samples and the reference sample withoutlaponite (blue, shifted by \(-2.5\times 10^{4}\) CPS for better differentiation) could be found. In the case of laponite nanocomposites signals in the AF4/MALLS fractogram, signals were only detected after additionally applied prestress conditions. Since AF4/MALLS does not allow differentiation between laponite and other matrix components in the redispersed abrasion, element-specific ICP-MS measurements were carried out on identical samples. Though magnesium is a ubiquitous element ICP-MS allows sensitive detection of laponite when coupled online to particle-sensitive AF4, because differentiation between magnesium based particles and dissolved magnesium becomes possible. This was demonstrated by comparison of the MALLS signal and the simultaneous recorded ICP-MS signal for an injection of a laponite dispersion into the AF4.