## 3 Results  
In the present study, styrene migration levels in yogurt and dairy products were measured using purge-and-trap (P&T) in combination with gas chromatography (GC). This technique eliminates the need for organic solvent extraction and laborious cleanup steps to separate target compounds from complex matrices such as foodstuffs: volatile organic compounds (such as styrene) are purged out of the sample matrix (solid or liquid) by an inert gas stream and collected onto a cooled sorbent trap, where they are concentrated and later introduced by rapid heating into the gas chromatograph (GC)/mass spectrometer (MS) for analysis. Since very low detection limits can be achieved, this is a suitable method for the trace analysis of volatiles, such as styrene, in food. The main parameters of the calibration curves for styrene, the limits of quantification (LOQ) and detection (LOD) (expressed in ug/kg, i.e., part per billion or ppb) in each investigated food matrices obtained with the used purge & trap gas chromatography method are reported in Table 3. The squares of the correlation coefficients (\(R^{2}\)) were higher than 0.999 for all of the matrices tested within the range of interest. For the foods with lower fat content (3.5-10%), lower detection limits were obtained; the higher levels were obtained for the fatty foods with 24% and 30% fat. In the \"reference food matrixes\" (plain yogurt and dairy products taken from the market packed in PP or glass) styrene was not detectable; the styrene levels measured in the similar products packed in PS can therefore strongly assumed as migrating from the packaging and not due to its natural presence in the food. The recovery rates of styrene in spiked food matrices which were stored with the investigated samples in the freezer were comparable with the one obtained from freshly spiked foods and were in the range 88-111%. Overall, the results showed that the method is sensitive and accurate, suitable for its intended use. The measured styrene levels in the PS packaging as well as its migration quantified in the respective food at best before date (for some products also before and after it) are summarized in Table 4. The migrated amount is directly expressed as measured in the food considering the real ratio of the PS surface area of the package to the quantity of the food packed for each pot (see S/F ratio reported in Table 2). Since the packages containing the food samples were stored until BBD of the food, it should be noted that the measured residual levels of monomer styrene may underestimate the initial concentration in unfilled packages. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Matrix** & **Range [ug/kg]** & \(R^{2}\) & **Recovery 1 [\%]** & **LOQ [ug/kg]** & **LOD [ug/kg]** \\ \hline Yogurt 3.5\% fat & 4–61 & 0.9999 & 98 & 1.7 & 0.5 \\ Cream 30\% fat & 5–61 & 0.9992 & 98 & 10.0 & 3.0 \\ Sour cream 24\% fat & 12–61 & 0.9996 & 101 & 3.8 & 1.1 \\ Sour cream 10\% fat & 5–49 & 0.9998 & 88 & 2.7 & 0.8 \\ Coffee cream 10\% fat & 12–838 & 0.9993 & 111 & 1.2 & 0.3 \\ \hline \hline \end{tabular}  
* Recovery calculated as measured concentration/spike level.  
\end{table} 
Table 3: Main parameters of the calibration curves for styrene, limits of quantification (LOQ) and detection (LOD) in the investigated food matrices. Since the residual levels of monomer styrene in the different PS pots were different, in order to compare the migration levels between the products, the styrene relative migration was determined. It represents the percentage of substance that migrates from the polymer to the food and was calculated by dividing the mass of styrene recovered in the food by the initial mass determined in the packaging. In Table 5, the relative migration of styrene at BBD and the main characteristics of the investigated products (food and packaging) are summarized.