Global carbon isotope variability of commercial plastics: implications for microplastic source attribution and...

Microplastic source attribution remains a major challenge because conventional analytical techniques primarily identify polymer types while providing limited information on the origin and environmental history of plastic materials. This study evaluates the potential and limitations of stable carbon isotope analysis (δ¹³C) for tracing common plastic polymers at the global scale. A total of 460 commercial plastic samples, including 233 polyethylene terephthalate (PET) bottles and 227 polyethylene (PE) caps, were collected from 59 countries across six continents and analysed by elemental analyzer - isotope ratio mass spectrometry (EA-IRMS). A preliminary thermal additive removal experiment showed systematic but minor isotopic shifts (∼0.3‰ for PET and ∼0.5‰ for PE), indicating a minor influence of additives on bulk δ¹³C values. Among petroleum-derived samples, PE exhibited a broad δ¹³C range from -35.2‰ to -24.0‰, with significant interregional variability and country-averaged values ranging from -33.9 ± 0.1‰ (Tunisia) to -25.9 ± 0.1‰ (New Zealand). Continental-scale comparisons revealed significant differences between several regions, consistent with regional differences in petrochemical feedstocks and production pathways. In contrast, PET displayed a narrower δ¹³C range (-33.1‰ to -26.2‰) and no significant interregional differences, consistent with a more homogeneous and globally integrated production system. Overall, the results demonstrate that stable carbon isotopes provide useful first-order constraints on broad PE source domains, while also highlighting important limitations related to overlapping isotopic domains and the globalization of polymer supply chains. Stable isotope analysis therefore represents a promising complementary tool for improving microplastic source attribution within multi-tracer environmental forensic frameworks.