## 2 Results and Discussion
### Analysis of Plastic Additives  
Raw data were processed with Compound Discoverer 3.1 software. The first step generated more than 50,000 potential compounds. Among them, 8095 structures were identified at level 5. Further filtration provided 1661 compounds identified at confidence level 4; among them, 1340 compounds were tentatively identified at level 3. The comparison with MS/MS spectrum of the further fragmented compounds provided 563 compounds tentatively identified at level 2. The selection of plastic additives provided 157 polar plastic additives. At the same time, the rest of the structures were natural compounds or compounds that cannot be directly related to the plastic industry, such as pharmaceutical compounds (pain killers or antibiotics), personal care products (PCPs), antibacterial or antifungal) and other contaminants in the aquatic environment, which probably were adsorbed onto the plastic surface. The different groups of compounds can be seen in Figure 4.  
Plastic additives were identified in 95% of the samples. Among the different types of plastic additives, only medium- to non-polar compounds were detected because of the analytical method used. Plasticizers were the most-found compounds (present in 80% of the samples and 82% of the samples showing the presence of MNPL particles). Adipates and phthalates were the most common groups, and among them should be highlighted the presence of adipic acid, dimethyl phthalate, mono(2-ethylhexyl) phthalate (MEHP) and phthalimide, respectively. Another important group was the stabilizers, among which should be highlighted N, N-Bis(2-hydroxyethyl) dodecanamide, which is used as an anti-caking agent, and was present in 25% of the samples. Another frequently tentatively identified compound was 1,2-Benzisothiazolin-3-one, used as a preservative, which was in 17% of the samples. The groups of lubricants, colorants, and flame retardants were detected at lower rates (20%, 12%, and 7%, respectively), probably because these compounds are not properly extracted and analyzed under these conditions. In this case, another isolation and analytical approach would be required. Furthermore, some of them could require a more targeted analysis to meet the sensitivity requirements and conditions for their extraction and analysis.  
Figure 1: Summary of the polymers and plastic additive groups found in the GIT samples.  
Figure 3: Concentrations of polymers in the samples expressed in equivalent concentrations of \(\mathrm{\SIUnitSymbolMicro g}\)/g.  
Figure 2: Frequency of polymer detection in the GIT samples.  
In addition to plastic additives, other compounds detected in the samples were natural products and other anthropogenic-origin compounds that cannot be associated with plastic formulations. These contaminants were probably present in surrounding waters and some proportions were probably adsorbed onto plastic particles, before their ingestion by fish. Among these contaminants, some pharmaceutical compounds, such as primobolan (24%), which is an anabolic steroid; sarsasapogenin (17%), used in the synthesis of steroids; or citalopram hydrobromide (15%), used as an antidepressant, were detected in the samples. Another relevant group of detected compounds was PCPs such as civetone (63%), used as a perfume fixative and as a flavouring; undecanoic acid (17%), used as an antifungal agent; or compounds used as biocides such as N, N-Diethyl-meta-toluamide (DEET) (63.4%), used as an active compound in insect repellents.