Determination of quinoxalines and their major metabolites residues in fishmeal by ultra-performance liquid chromatography tandem mass spectrometry

A sensitive and reliable ultra-high performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS) method was eatablished for the determination of olaquindox (OLA), carbadox (CBX), cyadox (CYA), quinocetone (QCT), mequindox (MEQ) and their main metabolites (QCA and MQCA) in fishmeal. The samples were extracted by acetonitrile-ethyl acetate mixture (1∶1, V∶V) and hydrochloric acid solution (1 mol·L^–1), then the analyte which dissolved in the aqueous phase was re-extracted by ethyl acetate. The extraction was concentrated and reconstituted with acetonitrile. After purification with the Oasis PRiME HLB SPE cartridge, the sample was subjected to the following analytical procedure. The mobile phase containing acetonitrile and 0.1% formic acid solution with a linear gradient elution was utilized to separate all compounds on a Phenomenex Kinetex C₁₈ column. The quantitative analysis of metabolites was carried out with an internal standard method and the others with an external standard method in the multiple reaction monitoring mode using positive electrospray ionization. The calibration curves for all compounds were linear (R≥0.994) within their corresponding concentration range. The recoveries were 64.4%−102.2% at different spiking levels with RSDs of 3.2%−10.2%. The limits of detection and quantification of MQCA and QCA were 2 μg·kg^–1and 5 μg·kg^–1; MEQ was 10 μg·kg^–1 and 20 μg·kg^–1; the others were 1 μg·kg^–1 and 2 μg·kg^–1, respectively. This method with high sensitivity and good precision can be applied to the simultaneous determination of quinoxaline drugs and their major metabolite in fishmeal samples.