Ultra-performance liquid chromatography–tandem mass spectrometry determination and depletion profile of flunixin residues in tissues after single oral administration in rabbits

An ultra-performance liquid chromatography with tandem mass spectrometric detection (UPLC–MS/MS) method was developed for the detection of flunixin residues in rabbit tissues. The samples were extracted with acidic acetonitrile, defatted with n-hexane, and then purified by HLB solid-phase extraction cartridge. Analysis was carried out on UPLC–ESI-MS/MS working with multiple reaction monitoring (MRM) mode. The limits of detection (LODs) of the method were 0.3–0.8 μg kg−1 and limits of quantification (LOQs) were 1.0–3.0 μg kg−1 in rabbit tissues, respectively. In all fortified samples at a concentration range of 1.0–300.0 μg kg−1, mean recoveries were 61.7–115.7% with relative standard deviations (RSDs) below 16%. Residue depletion of flunixin in rabbit was conducted after oral administration at a dose of 5 mg kg−1 of body weight. The average concentrations for flunixin measured 2 h post-administration in kidney and intestine were significantly higher than in liver, heart and muscle. The concentrations for flunixin in all rabbit tissues were below the LOD or not detected in all tissues after 96 h administration of drug. A minimum withdrawal time of 21 h was indicated for residue levels in heart, liver, kidney, intestine and muscle below the maximum residue limits (MRLs).