LIQUID-CHROMATOGRAPHIC METHOD FOR DETERMINATION OF TOTAL AVERMECTIN B-1 AND 8,9-Z-AVERMECTIN B-1 RESIDUES IN HOPS

A liquid chromatographic method has been developed and validated for d etermination of avermectin B-1 and 8,9-Z-avermectin B-1 residues in dr ied hops. The dried hops are rehydrated and subsequently extracted wit h a methanol-water mixture. The avermectins are partitioned into hexan e and the hexane extract is concentrated (purified) on an aminopropyl solid-phase extraction column. The purified extract is derivatized wit h trifluoroacetic anhydride, and the derivatized avermectins are analy zed by reversed-phase liquid chromatography with fluorescence detectio n. Recoveries of the avermectins from dried hops fortified with approx imately 5-1000 ng/g avermectin B-1a or 8,9-Z-avermectin B-1a ranged fr om 73 to 108% with an overall average recovery of 95%. The limit of qu antitation 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 procedur e provides a simple, rapid, and sensitive method for monitoring the to tal avermectin residues in hops.