## 1 Introduction  
Polyesters (PES) play an important role in the production of food contact materials (FCM), being among the most used type of polymers in the field. Different types of PES can be found in the market as base materials for several FCM applications, with polyethylene terephthalate (PET) as the most relevant. This polymer has been used for years due to its advantageous technological properties and characteristics over other polymers, such as possessing generally low levels of migration and being an efficient functional barrier against organic molecules [1; 2; 3; 4]. These properties make PET one of the most inert food-contact polymers and the polymer of choice for applications such as water bottles [5]. Another notable PES in use nowadays as FCM is the polybutylene terephthalate (PBT), in applications such as coffee capsules, kitchenware utensils, microwaveable dishware and beverage cups [5]. PET and PBT have a huge recyclability potential, with the former being currently the most recycled food plastic in EU [4]. There is a growing interest in the determination and quantification of potential contaminants that can originate due to or during recycling processes. In fact, for new recycling processes to be approved in EU, they must (1) be in accordance with Regulation (EU) 2022/2016 that replaced the Commission Regulation (EU) 282/2008 on recycled plastic materials intended to come into contact with foods [6; 7], and (2) be submitted to the European Food Safety Authority (EFSA). This agency then carries out assessments of the risks originating from the migration of substances from a given recycled food contact plastic material into food and to deliver a scientific opinion on related recycling process. The aforementioned regulatory framework clearly highlights an absolute need not only for the identification of potential migrants (currently estimated in the several thousands), but even for a proper definition of all the categories to which those substances belong [8]. Among the most studied plastic migrants are a group of substances known as oligomers. In 2016, [PERSON] and colleagues hypothesized that polymeric oligomers (_oligo_ = a few and _mers_ = parts) could be defined as molecules consisting of a few of the monomer units used in the production of the plastic [9]. In other fields an oligomer is usually considered to contain between two and forty repeating units [10] (depending on the chemical composition of the building block), but it is not entirely clear (1) from how many repeating units a substance can be considered an oligomer and (2) up to what number. Some oligomers can be used as additives in the production of plastic polymers, but the majority are considered as non-intentionally added substances (NIAS). They can be formed through the degradation of the polymers due to thermal effects during production or recycling, by degradation due to various thermo-mechanical processes and as byproducts of the polymeric synthesis [1, 4, 9, 11, 12, 13, 14]. The use of irradiation energy during sterilization treatments of packed foods (gamma or beta radiation), as well as high pressure, UV-light or ozone, can also promote the formation of oligomers [15]. Generally, species with a molecular weight below 1000 Da are considered as relevant potential migrants [9]. The 1000 Da threshold has been set as a non-binding limit based on toxicological interest in their absorption in the gastrointestinal system. Nevertheless, the absorption can be related and correlated with several factors (i.e., molecular mass, polarity), though in general, the smaller the molecule the higher its diffusion rate [3, 12]. Several proficiency tests have been organized by the Joint Research Centre of the European Commission (JRC-EC) in recent years on the migration of PET and PBT oligomers, which highlights the importance of this category of organic molecules [16, 17, 18].