## 1 Introduction
However, items made of compostable _bio-based_ plastics are often accidentally or intentionally found dispersed in terrestrial and aquatic environments as a result of improper disposal due to the widespread false belief that compostable plastics are completely degradable under environmental (uncontrolled) conditions. While many manufacturers claim the degradability of _bio-based_ materials under marine conditions based on the available degradation standard methods (e.g., ASTM D6691-17, ASTM D7081-05; \"OK biodegradation MARINE\" label based on the criteria of ASTM D7081), the scientific community is concerned that these approaches are not effectively assessing biodegradation under realistic scenarios, as they do not account for the heterogeneous and variable properties of marine environments potentially impacting degradation processes [9; 12; 13]. In this paper, we aim to explore the degradation process of compostable starch-based (i.e., _bio-based_) plastic shopping bags in the marine environment by using Fourier-transform infrared spectroscopy (FTIR) and isotope ratio mass spectrometry (IRMS). The methods commonly used for assessing degradability for certification purposes rely on measurements of carbon to CO\({}_{2}\) conversion (or to CH\({}_{4}\) under anaerobic conditions), biological demand for oxygen and secondary parameters (e.g., the evolution of weight loss, the decrease of molecular weight and the deterioration of mechanical properties) under different degradation environments (e.g., industrial compost, home compost, soil, sea, water and aerobic/anaerobic conditions) [9]. FTIR is an established methodology used to identify the polymeric composition of plastic material. The obtained infrared spectrum allows for the determination of the chemical structure of the mixture focusing on the observation of specific functional groups. IRMS has only recently begun to be explored for applications in plastics research and nowadays is an established methodology used to identify the polymeric composition of plastic materials [1]. It measures the relative abundances of stable isotopes, which depend on natural isotopic fractionation (the partitioning of the heavier and lighter isotopes due to different biotic and abiotic processes). The approach used in this study provides an easy and time/cost-saving technique that does not require laborious preparative steps and thus is more easily applicable in routine laboratory activities (compared to other techniques like _GC-MS and LC-MS_). The coupled application of these two analytical methodologies is intended to gain complementary evidence regarding: (i) the time-course degradability of different polymers usually present in the blend (i.e., starch and polyester) by measuring changes in the FTIR spectral signatures, and (ii) potential degradation mechanisms by analyzing changes in carbon isotopes rates (i.e., \({}^{13}\)C vs. \({}^{12}\)C, via IRMS). To our knowledge, there is no literature addressing the biodegradability of plastic polymers in marine environments when a coupling of these two analytical methods is used. The outputs of this study will contribute to a better understanding of degradation processes and the end-of-life of compostable (and, more generally, degradable) plastics in marine environments and thus support advancements in the development of more sustainable-by-design plastic materials.