Liquid chromatographic method for determination of total avermectin B1 and 8,9-Z-avermectin B1 residues in hops

A liquid chromatographic method has been developed and validated for determination of avermectin B1 and 8,9-Z-avermectin B1 residues in dried hops. The dried hops are rehydrated and subsequently extracted with a methanol-water mixture. The avermectins are partitioned into hexane and the hexane extract is concentrated (purified) on an aminopropyl solid-phase extraction column. The purified extract is derivatized with trifluoroacetic anhydride, and the derivatized avermectins are analyzed by reversed-phase liquid chromatography with fluorescence detection. Recoveries of the avermectins from dried hops fortified with approximately 5-1000 ng/g avermectin B1a or 8,9-Z-avermectin B1a ranged from 73 to 108% with an overall average recovery of 95%. The limit of quantitation is 5 ng/g (signal-to-noise ratio [S/N] > 10), and the limit of detection is 2 ng/g (S/N > 3) for each analyte. The assay procedure provides a simple, rapid, and sensitive method for monitoring the total avermectin residues in hops.