Environmental Petroleomics by ESI(-) FT-ICR MS to assess weathering effects on the spilled oils beached at the Campos Basin area, SE Brazil

Once oil enters the ocean, significant physical and chemical changes occur due to weathering processes, such as photooxidation and biodegradation. Therefore, it is crucial for forensic geochemical studies to understand how these processes alter the chemical composition of petroleum spilled into the sea. This composition can be assessed by Environmental Petroleomics using FT-ICR MS to monitor and understand the fate of spilled oils. In this context, this study aimed to evaluate the acidic polar composition of spilled oils collected in the Campos Basin area and compare it with that of crude oils from this basin, assessing which weathering processes modify their composition and how these changes occur. Twenty spilled oil samples were collected from three beaches in Arraial do Cabo/RJ (Grande, Pontal, and Prainha) during six collection times: 06/2023, 09/2023, 10/2023, 01/2024, 06/2025, and 07/2025. Previous studies showed that 15 of the 20 samples geochemically correlate with the Campos Basin. The oils were extracted and analyzed using a Bruker SolariX 2xR FT-ICR 7 T mass spectrometer with negative-mode electrospray ionization [ESI(-)]. The crude oils contain nine heteroatom classes, with N₁ being the most abundant class (> 58%), followed by O₂ and O₁ classes, and no classes with more than two oxygen atoms, except O₃S. In contrast, the spilled oils present up to 34 classes, with O₂ as the most abundant (23-66%), followed by O₃, as well as highly oxygenated classes (i.e., O₄, O₅, and O₆). These results show that weathering significantly increases the abundance of oxygenated compounds and their acidity and toxicity. The O_x≥2/O₁, O_x>2/O₁, Ox_≥2/N₁, and O_x>2/N₁ ratios, together with the O₂/O₁ and O₂/N₁ ratios, are proposed to evaluate the extent of photooxidation and biodegradation, respectively. Oils from the first two collections present lower values, indicating a lower degree of weathering, except for one sample, which exhibits the greatest degree of biodegradation and was likely spilled before the others. In addition, the five spilled oils that did not correlate with Campos Basin show a distinct composition, indicating that oil origin also influences the distribution of oxygenated classes. Biodegradation was also assessed by A/C [DBE 1/(DBE 2 + 3 + 4)] and modified A/C ratios [DBE 1/(DBE 2 + 3 + 4 + 5 + 6)]. These indices confirm that biodegradation is an effective weathering process and indicate that the spilled oils from the first collections were less biodegraded than those collected later. Some spilled oils that did not correlate with the Campos Basin also show strong evidence of biodegradation. In contrast, others remained less altered, possibly due to a more paraffinic character or lower exposure to weathering. Overall, this study highlights the temporal increase in oxygenated compounds in spilled oils, contributing to greater acidity and toxicity, and confirms that biodegradation and photooxidation drive these compositional changes.