2.6. Determination of Individual Phenolic Compounds by GC-MS

Simple phenolic compounds were isolated from vinegar samples by solid phase extraction (SPE) on C8 Isolute columns, essentially as described by Soleas et al. [18]. Columns were preconditioned with ethyl acetate (3 mL), methanol (3 mL) and ultrapure water (3 + 2 mL). Then, vinegar samples (100 μL) were loaded on the SPE columns and allowed to drain by gravity. Solvent was subsequently removed under reduced pressure and the phenolics were recovered by ethyl acetate (3 mL). After solvent removal in a centrifuge evaporator (Speed Vac) the residues were dissolved in methanol (500 μL) and aliquots of the methanolic solutions were evaporated to dryness and silylated with N,O–bis-trimethylsilyl-trifluoroacetamide (BSTFA). Simple phenolics in the form of trimethylsilyl (TMS) ethers, were quantitated by selective ion monitoring GC-MS employing 3-(4-hydroxyphenyl)-1-propanol as internal standard [19]. The target and qualifier ions for the trimethylsilyl ethers (TMS) of the phenolic compounds and the internal standard (IS) are given in Table S1.