W. D. Rowe1 , M. R. Madson1 , J. N. Sofos1 , F. J. Schenck2 , V. A. Vega2 , L. H. Lagman3 , D. M. Altwein4 , 1FDA, Denver, CO, 2FDA, Atlanta, GA, 3FDA, Jefferson, AR, 4FDA, Bothell, WA
Background: FDA uses Laboratory Information Bulletin (LIB) #3461 to monitor for ivermectin (IVR), a potent antiparasitic agent, in raw milk.Lagman, et al. demonstrated in LIB #4155 a faster, more sensitive method for IVR and for three related compounds, doramectin (DOR), abamectin (ABA), and moxidectin (MOX). All four may be administered to cattle.
Method: A modified version of LIB #4155 was validated by a two-laboratory method trial with additional validation by the Denver lab. Method extracts the avermectins from milk with ammonia, ethanol, ethyl acetate and iso-octane. The ethyl acetate - iso-octane layer is evaporated to an oily residue.The oily residue is partitioned between hexane and acetonitrile and the acetonitrile is evaporated to near dryness. The residue is dissolved in acetonitrile and derivatized with trifluoroacetic anhydride in the presence of N-methylimidazole.The derivatized extracts are injected on an LC (C-18 column, fluorescence detector).
Results: Average recoveries are typically greater than 60% with CV's less than 20%.MDL calculates to less than 0.1 ppb. Incurred milk samples were positive for the expected analytes at the expected ranges, 0.5 to 4 ppb.Ruggedness was demonstrated by varying extraction volume (± 20%), extending drying times, and substituting glassware with plasticware.
Conclusion: This method, published as LIB #4347 and LIB #4348, is faster than LIB #3461, sensitive to 0.2 ppb and rugged for the determination of MOX, ABA, DOR and IVR residues in milk.