HPLC-MS/MS Determination of Residual Amounts of 12 Antibiotics in Prawn with Separation by Solvent Extraction

The prawn sample was freeze-dried and ball-milled to powdery form, and 1.00 g of the powdered sample was taken for analysis. The sample was extracted thrice with a mixture of Mcllvaine buffer solution containing 0.1 mol·L^-1 Na₂EDTA (pH 4.00±0.05) and acetoritrile at a volume ratio of 1:4 by ultrasonication and 10 mL of the mixed extraction solvent were used for each extraction, to extract the 12 drugs (including 5 sulphonamides, 2 quinolones, 2 macrolides and 3 tetracyclines) from the sample into the solvent. The 3 extracts were combined and shaken thoroughly with 20 mL of n-hexane saturated with acetontrile to dissolve the fatty impurities, that might be carried over in the extract, into the n-hexane. After centrifuging, the upper layer of n-hexane was discarded and the lower layer was evaporated at 40℃ until the volume of the solution was kept unchanged. The water was added to make the volume of the solution to 200 mL. It should be examined at this instant if the total volume of organic solvent in the 200 mL solution exceeded 5% by volume. Because when the organic solvent in the solution exceeded 5%, some of the analytes in the solution might pass through the SPE column in the follow-up SPE purification, thus leading to missing errors. The diluted 200 mL solution was then purified by passing through the activated CNWBOND LC-C₁₈ micro-column at a flow-rate of 1.0-1.5 mL·min^-1. When all of the solution was passed through the micro-column, 5 mL of 5% (volume fraction) methanol solution were used to rinse the column. All the effluent was discarded and the SPE micro-column was dried under reduced pressure. The analytes on the column were then eluted with 6 mL of methanol, and the eluate was evaporated to near-dryness by N₂-blowing. The residue was taken up and its volume was made up to 1.0 mL with methanol. The solution was filtered through 0.22 μm needle-filter and the filtrate was used for analysis under the instrumental working conditions. ZORBAX Eclipse XDB-C₁₈ chromatographic column was selected as stationary phase in HPLC separation, and mixtures of (A) 0.1% (volume fraction) acetic acid solution and (B) methanol in various ratios were used as mobile phases in the gradient elution. Peak areas of the 12 drugs were measured by MS/MS at their respective retention times, under the MS/MS conditions of ESI ionization source and mode of MRM. Linearity ranges of the standard curves of the 12 drugs were found same between 1-100 μg·L^-1, with detection limits (3S/N) in the range from 0.52 to 3.02 ng·g^-1. Recovery was tested by standard addition method, giving results of recovery ranged from 70.2% to 102%. Values of RSDs (n=5) found were all less than 14%.