## 1 Introduction  
Normally, metal cans intended to come into contact with food and beverages have an internal coating to protect the food from the metal and vice versa during pasteurization, sterilization, and long-time storage. Epoxy resins are the type of coating most widely used in metal substrates. However, can manufacturers have started to innovate and develop substitutes to replace food-contact materials based on epoxy resins as a consequence of reported toxic effects of their components and the limitations established by the regulatory authorities. Polyester-based coatings are currently used as a first-generation alternative to epoxy-based resins [1]. Polyester polymer coatings are produced from polycondensation reactions between carboxylic acid monomers, such as phthalic, isophthalic, or terephthalatic acids, and polyfunctional alcohols in excess such as ethylene glycol, diethylene glycol, and neopentyl glycol. Polyesters are commonly used in food packaging materials, such as polyethylene terephthalate (PET), and many monomeric raw materials have previously been fully evaluated for toxicity and are compliant with food packaging regulations [2]. However, there is not European, harmonized legislation specified to date for varnishes and coatings for food contact. There is only one specific regulation in order to control bisphenol A diglycidyl ether (BADGE), as well as its hydrolysis and hydrochlorination products, in epoxy resins, among others [3]. Additionally, additives can be incorporated into the polyester formulation to provide the functionality that is necessary such as cross-linking agents, catalysts, lubricants, pigments, solvents, etc. [4; 5]. During the manufacturing, some substances, such as monomers, additives, or oligomers, may remain unreacted in the polymeric network and migrate into the food [6]. The many possible combinations of the different monomers in polyester-based coatings provide a large number of possible oligomers that have the potential to migrate from can coatings into food [5]. These low-molecular-weight compounds, such as small linear or cyclic polyester oligomers, polyester hydrolysis products, and manufacturing byproducts of polyester coatings, are considered as non-intentionally added substances (NIAS) [2]. Several analytical methods have been proposed in the literature for the analysis of polyester composition, mostly applying liquid chromatography coupled to different detection techniques for acid compounds analysis and gas chromatography for alcohols after a hydrolysis, transesterification, or aminolysis step [5; 6; 7; 8; 9; 10]. Migration tests are required to identify and quantify potential migrants in order to assess the risks and safety of these coatings. These migration tests are challenging because the formulation used in the manufacture of can coatings is typically unknown for most analytical laboratories, and NIAS, such as polyester oligomers, can migrate into the food simulant, increasing the complexity of the analyses since reference standards are not commercially available. Therefore, it is necessary to perform a previous, non-targeted analysis in order to collect as much information as possible about the coating formulation [2]. Some studies have been published reporting the migration of polyester monomers and oligomers from polyester can coatings in the literature [6; 8; 9; 10], but there is very limited information on non-targeted analysis of this type of resin. In this work, several analytical techniques were used in order to characterize the type of coating on the food-contact surface of the analyzed can samples. Firstly, infrared spectra were acquired using an ATR-FTIR spectrometer, and the identification of the polymer type was achieved by comparison with polymer spectrum libraries. Confocal Raman microscopy was also applied to provide a complete characterization of the coating layers of the samples.