## 1 Introduction  
The term \"phenolic compound\" is used for aromatic organic substances containing one or more hydroxyl groups directly bonded to a benzene ring. Typical natural sources of these compounds are plants, from which they are further processed into plant products [1]. Phenolic compounds significantly affect the color, taste, and aroma of products [2], and have a positive effect on the human body [3]. Reactive oxygen species such as superoxide, hydrogen peroxide, and hydroxyl radicals affect lipids, membranes, proteins, and DNA in the human body. This can result in accelerated ageing, cancer formation, and brain degeneration [4]. Phenolic compounds have high antioxidant activity, which can help the human body prevent oxidative stress [5]. Phenolic compounds have been shown to help the body fight cancer [6] and cardiovascular diseases [7]. Balsamic vinegar is a rich source of phenolic compounds [8]. The tradition and history of its production are closely linked to the Italian provinces of Modena and Reggio Emilia. Therefore, in 2000, it obtained a protected designation of origin, which allows products called \"traditional balsamic vinegar of Modena\" (TBVM) or \"of Reggio Emilia\" (TBVRE) and \"balsamic vinegas of Modena\" (BVM) or \"of Reggio Emilia\" (BVRE) to be produced only in these Italian provinces and by using specified methods. Other products called \"Balsamic vinegar\" can be produced anywhere in the rest of the world [9]. The production of traditional balsamic vinegas begins with the concentration of grape must on an open fire. This is followed by a first fermentation (conversion of sugars to ethanol by yeast)and a second fermentation (conversion of ethanol to acetic acid by acetic acid bacteria). Subsequently, the vinegar is aged in a series of five wooden barrels (each of a different type of wood) for at least 12 years. During this time, the barrels are refilled to compensate for liquid losses during evaporation. No additives may be added to the vinegar and no processes may be carried out to accelerate the ageing of the vinegar [10; 11; 12]. The production of BVM and BVRE is much simpler and, above all, several times faster. It is based on mixing of cooked wine must with wine vinegar, and additives (e.g., caramel) may be used during production [13]. There are several studies focused on the analysis of phenolic compounds in balsamic vinegas using reversed-phase (RP) liquid chromatography. [PERSON] et al. [14] used the HPLC-DAD method to detect phenolic acids in 23 samples of vinegar, including balsamic vinegar. A Zorbax Extend-C18 column and gradient elution (methanol-formic acid 0.1%) were used for separation. The HPLC-DAD method was used by [PERSON] et al. [15] for the analysis of phenolic compounds in vinegars. Separation was performed using a Luna C18 column in gradient elution mode with aqueous/acetonitrile mobile phase containing 0.1% trifluoroacetic acid. [PERSON] et al. [16] carried out the analysis of phenolic compounds using the HPLC-DAD method with a LiChrospher 100 RP-18 column, and the mobile phases used were 5% methanol with 95% water-95% methanol with 5% water at pH 2.5. [PERSON] et al. [17] performed phenolic compounds analysis using the HPLC method coupled with a UV/VIS detector. The separation column used was a LiChroCART 250-4 Supersphere 100 RP-18. Gradient elution was utilized using acetonitrile/water with the addition of acetic acid yielding pH 2.65. All these methods utilized columns packed with 5 um particles and relatively long analysis times (45-75 min). [PERSON] et al. [8] conducted an extensive study of phenolic compounds. The HPLC-MS/MS method with online solid phase extraction (SPE) was used for the analysis of 56 phenolic compounds in various spirits, wines, and vinegars. Several types of SPE cartridges were tested, and the HyperSepTM Retain PEP cartridge packed with porous polystyrene-divinylbenzene material modified with urea functional groups showing the most balanced retention for the tested compounds.