Session 2
The Determination of Oxytetracycline Hydrochloride in Animal and Fish Feeds, Premixes and Animal Remedies by Liquid Chromatography with Fluorescence Detection: A Collaborative Study Report

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Nancy Thiex & Richard Larson
South Dakota State University, Olson Biochemistry Laboratories, Brookings, SD, USA

A method for determining oxytetracycline in animal feed, fish feed and animal remedies at medicating and trace levels was studied. Oxytetracycline hydrochloride (OTC.HCl) is extracted from ground feed material in acid-methanol solution using mechanical agitation.  After centrifugation for 5 minutes at 1230 g, an aliquot of the extract is diluted with water and/or acid-methanol so that the concentration of OTC.HCl is approximately the same as in the working standard, and the solutions contain at least 50 % water. 
Injectable animal remedy materials are diluted with water and/or extractant to reach the target concentration.  The extracts are filtered and analyzed by reversed phase LC with fluorescence detection with excitation at 390 nm and emission at 512 nm.  Twenty-eight test samples of medicated feeds, supplements and drug premixes, including four test samples for trace level analysis, were sent to 17 collaborators in Canada, The Netherlands, and the United States.
Results were received from 11 laboratories.  The RSDr values (within-laboratory repeatability) ranged from 1.26 %– 9.21 %, RSDR values (among- laboratory reproducibility) ranged from 2.14 % -12.9 % and HorRat values ranged from 0.54 to 3.02.  The method is recommended for AOAC Official First Action.