Assay of oxytetracycline in animal feed by liquid chromatography and microbiological plate assay.

In a proposed method for determining oxytetracycline (OTC) in animal feed, OTC is extracted with acidic methanol, and the extract is centrifuged and assayed by microbiological plate assay and by reversed-phase liquid chromatography (LC). Riboflavin and furazolidone elute at retention times similar to that of OTC. Successful separations are made by using both dimethylformamide and acetonitrile mobile phases. Variation between microbiological and LC results is comparable with variation within the microbiological assay. The relative standard deviation for 6 replicates of feed at the 100 ppm level is 3.6% for the LC assay and 3.1% for the microbiological assay.