RESUMEN
A method was developed for determining residual dexamethasone and betamethasone in swine liver by HPLC-MS/MS with isotope dilution. The samples were digested by β-glucuronidase/aryl sulfatase, and extracted) with acetonitrile. Further cleanup was performed on C_(18)) cartridge and liquid-liquid extraction with sodium) carbonate solution. Then the supernatant was dried under nitrogen and residues were dissolved in mobile) phase. The samples were analyzed by HPLC-MS/MS on a Hypercarb C_(18)) column with a mixture of acetonitrile-water-formic acid as the mobile phase. The samples were quantified with the internal standard calibration curve method using isotope dilution. The limit of detection for dexamethasone and betamethasone in swine liver was 0.12 μg/kg and 0.14 μg/kg, respectively. The limits of quantification were 0.42 μg/kg and0.47 μg/kg), respectively. The average recoveries were 97.3%-111.0%, the intra-assay relative standard deviations were 1.85%-5.65% and the inter-assay relative standard deviations were 2.8%-8.0% at spiked levels of 0.75-2.00 μg/kg. There was a good linear correlation (the correlation coefficient is above 0.9997) between the ratio of peak areas of quantitative ion-pair to internal standard and concentration of analyte) in the range of 10-500 μg/L.