LIQUID-CHROMATOGRAPHIC DETERMINATION OF MELENGESTROL ACETATE IN FEEDS

A method for melengestrol acetate (MGA) was developed to accurately determine 0.08-220 ppm MGA in animal feeds. MGA is extracted from an aqueous slurry of feed sample with hexane and partitioned from hexane into aqueous methanol and then into dichloromethane. After evaporation of the dichloromethane, the dried extract is transferred with chloroform into a volumetric flask. An aliquot is pipetted into a mixed column of silica gel and acid aluminum oxide, and the MGA is eluted with hexane-acetone. The residue is dissolved in methanol, and the MGA is quantitated by reversed-phase liquid chromatography (LC). The method was validated by using commercial feeds and laboratory-fortified feeds. The method was also applied successfully to com silages supplemented with MGA. Recoveries [and associated coefficients of variation (CVs)] for feeds fortified in the laboratory with MGA 100 at 0.2, 0.8, and 5 ppm were 96.9 (6.28),102 (3.59), and 109% (2.06%), respectively. CVs for a variety of commercial feeds varied from 0.88 to 3.84%. An interlaboratory study was also conducted to compare the LC method to the AOAC gas chromatographic method.