Determination of tilmicosin in bovine milk by liquid chromatography with ultraviolet detection

A method is described for determining tilmicosin in bovine milk by liquid chromatography with ultraviolet (UV) detection. Samples are defatted by centrifugation. The lower skim layer is cleaned on a C18 solid-phase extraction cartridge. The extract is concentrated and analyzed with a reversed-phase phenyl column with UV detection at 280 nm. The method was validated with control milk fortified at 50, 100, and 200 ng/mL. Average recoveries (and intralaboratory coefficients of variation) were 97% (9%), 98% (5%), and 101% (3%), respectively. The limit of quantitation is approximately 20 ng/mL, and the limit of detection is approximately 13 ng/mL. The method was tested on milk from a cow dosed with tilmicosin.