Liquid chromatographic determination of tilmicosin in swine feed at 200-400 mg/kg level: interlaboratory study

An analytical method for the determination of tilmicosin at 200-400 mg/kg, the intended use concentration range, was evaluated in an interlaboratory study involving 5 laboratories, including the sponsor. The interlaboratory study evaluated the intra- and interlaboratory precision and accuracy of a tilmicosin feed method. The method procedure involved extracting tilmicosin from feed by adding 200 mL extractant to 20 g feed and shaking for 1 h. The extract is filtered and analyzed by gradient liquid chromatography which separates tilmicosin from feed matrix in 30 min. Each laboratory assayed 5 replicates of fortified feed at concentrations of 0, 100, 200, 400, and 600 mg/kg. The mean recovery among fortified samples ranged from 81.4 to 98.8%, with a percent coefficient of variation (%CV) ranging from 0.3 to 4.0%. For all blank control feed samples no significant interferences were observed. In addition, each laboratory assayed 5 replicates of medicated feed samples prepared at 2 levels (200 and 400 mg/kg) with either a horizontal or vertical mixer. Along with the medicated feed samples were included 5 replicates of a blank control feed. The identities of the medicated and blank control feed samples were blinded to the analysts. The results for the medicated feed samples ranged from 95.8 to 106% of label claim, with a %CV ranging from 2.1 to 6.7%.