Liquid chromatography/fluorescence method for emamectin B1a and desmethylamino-emamectin B1a residues in lobster...

A liquid chromatography (LC)/fluorescence procedure was validated for emamectin (EM B1a) and desmethylamino-emamectin (DMAEM B1a) residues in lobster tissue. They were extracted by shaking and sonicating with 1% ammonium acetate-methanol in the presence of sand. The extract was concentrated, partitioned with ethyl acetate, and cleaned up on a propylsulfonic cation exchange cartridge. The analytes were eluted from the cartridge with 5% ammonium hydroxide-methyl acetate, the eluate was concentrated, and the solvent was changed to dry 20% ethyl acetate-acetonitrile before derivatization with trifluoroacetic anhydride-N-methylimidizole. The products were analyzed by LC-fluorescence, and no interference [>limit of detection (LOD)] was detected in the control samples. Lobster tissues fortified with EM B1a and DMAEM B1a at 0.5, 5, 10, 25, and 50 ng/g gave overall mean recoveries of 96.7 +/- 12.4%, relative standard deviation (RSD) = 12.8% for EM B1 and 83.6 +/- 12.1%, RSD = 14.5% for DMAEM B1a. Regression analysis of the calibration data gave slopes of 0.90 (EM B1a) and 0.71 (DMAEM B1a) with an r2 = 0.99 for both compounds. The calculated LOD and limit of quantification (LOQ) for EM B1a were 1.10 and 3.32 ng/g, respectively, and for DMAEM B1a were 0.762 and 2.31 ng/g, respectively. Residues of EM B1a and DMAEM B1a in fortified lobster tissues stored at -20 degrees C showed that residues were stable for 10-12 months. No loss of EM B1a and DMAEM B1a residues was observed after 3 freeze/thaw cycles of fortified tissue in a 5-day period.