A typical chromatogram obtained from separation of the quinolones by LC-ESI-MS-MS-QToF from a fortified blank fish fillet sample is presented in Fig. 3. Table 1 shows the m/z ratios of ions used for identify each quinolone, presenting the isotope and experimentally found m/z ratios, and the error (ppm) between them. In general, the error of the ions was lower than 10 ppm, which points out the high quality of the results. As already discussed, to fulfill EU requirements for residue analysis using ToF, to identification ions only (the molecular ion plus one product ion) are necessary characterize the method as suitable for identity confirmation. Table 1 presents at least three ions for confirmation of the Identity of each quinolone.