**2.1 δ13C as a possible tool to investigate plastic polymers**

δ13C values recorded in this study for the most used petroleum-derived plastic polymers, plant-derived polymers, some commercial items made by petroleumand natural-derived polymers, which are largely found in the marine litter worldwide (i.e., food packaging items), and natural matrices are reported in **Figure 9**.

 Due to their high stability and durability [28], in the last decades, petroleumderived plastic materials have largely replaced paper and other cellulose-based products with a continuously increasing trend. At the moment, a wide variety of petroleum-based synthetic polymers are produced worldwide (approximately a total of 335 million tons in 2016), and significant quantities of these polymers end up into natural ecosystems as waste products [1].

The δ13C values of the majority of the analyzed petroleum-derived plastic polymers ranged over a wide interval, that is, between −33.97 and –25.41‰. Only a few polymers, such as PTFE, silicon, and ABS showed more negative δ 13C values (−40.70 ± 1.17, −39.37 ± 0.27, and − 35.17 ± 0.98‰, respectively), possibly due to fractionation processes during their synthesis.

 With the exclusion of PTFE, ABS, and silicon, the recorded δ13C range results are comparable to that reported for crude petroleum [29]. Petroleum is constituted by a complex mixture of organic substances, with a predominance of hydrocarbons, whose exact composition depends on the site of extraction. Petroleum usually shows negative values of δ13C, ranging between −34 and −18‰ depending on the specific extraction field. In fact, as reported by Stahl [29], petroleum could be originated from the lipid fraction of organic matter. In particular, the carbon

#### **Figure 9.**

*δ13C values determined in various petroleum- and plant-derived polymers, as well as in natural matrices analyzed in this study.* 

isotopic value of petroleum can vary in relation to the marine vs. terrestrial origin of the source, with an enrichment of 12C with respect to 13C in the marine environment compared to the terrestrial one [30].

 Interestingly, different δ13C values were recorded for some polymers as pure material and once in packaging commercial items. For instance, a significant (p < 0.05) more negative δ 13C value was determined in the HDPE shopping bag for food use with respect to the original HDPE polymer. This could be related to the addition of some organic additives (i.e., stabilizers) in the final materials used for food packaging. In fact, depending on the commercial use, plastic formulations can be enriched with monomeric ingredients to improve their processing, end-use performance, and appearance (e.g., colorants, photostabilizers, etc.). Among these possible additives, our preliminary data excluded colorants as the main cause of isotopic variation in the investigated samples. These results were confirmed by the lack of significant difference among polymers of different colors (p > 0.001). The independence of the δ13C value from the plastic color could provide an important analytical advantage to the isotopic approach over some of the other analytical methods used for plastic characterization. In particular, the spectroscopic methods have been proved to be limited by the color of the plastic samples, because of the occurrence of interferences due to a decrease of the diffuse reflection intensity in dark color samples [31]. Further investigation and larger analytical data set are required in order to strength these results.
