Determination of quinolone residues in shrimp using liquid chromatography with fluorescence detection and residue confirmation by mass spectrometry

The quinolones, oxolinic acid (OXO), flumequine (FLU), and nalidixic acid (NAL), are antibacterial drugs effective against Gram-negative bacteria. Quinolones are used in both human and veterinary medicine, but are currently not approved by the U.S. Food and Drug Administration for use in food fish. A liquid chromatography-fluorescence (LC-FL) method was developed to determine OXO, FLU, and NAL residues in shrimp. An additional liquid chromatography-mass spectrometry (LC-MSⁿ) method was created to confirm these residues using the same sample extract. Samples were prepared with a simple ethyl acetate extraction followed by solvent exchange into 0.2% formic acid and cleaned-up with hexane. Reverse phase chromatography was used to separate the three compounds in both procedures. For the LC-FL determinative method, fluorescence emission was monitored at 369 nm with excitation at 327 nm. With electrospray ionization, the three most abundant ions from the MS³ product ion spectrum were used to identify OXO, FLU, and NAL in the confirmation procedure. Shrimp samples fortified at levels ranging from 7.5 to 100 ng g⁻¹ were used to validate both methods.