Microbiological determination of lincomycin in feeds and supplements containing high concentrations of bentonite

A microbiological method is described for determining lincomycin in complete feeds, supplements, and pre-mixes greater than 100 g/ton and containing high levels of bentonite. The AOAC method currently used is unsatisfactory for analyzing feeds with concentrations of bentonite greater than 5.5%. Study indicates that low lincomycin recoveries from high level bentonite feeds are a function of aqueous contact and subsequent binding. The present method involves an alternative extraction technique using formamide as the primary extractant. A binary solvent system of ethanol and phosphate buffer aids in the extraction, miscibility, and conversion to water solubility for subsequent testing. Forty-one feeds containing as high as 60% bentonite were assayed by the reported method and gave a mean recovery of 106% and a range of 94-114%. An inter-laboratory confirmation study produced a mean recovery of 103%, with a range of 93-114%. A factorial analysis of variance of the interactive effects between lincomycin level and bentonite level within the AOAC method and within the bentonite method showed no interactions which would influence percent recovery in either assay method.