Determination of tetracyclines in animal feeds in the presence of other drugs by thin-layer chromatography and microbiological method.

This method was developed to separate, detect, and quantitate oxytetracycline (OTC) or chlortetracycline hydrochloride (CTC.HCl) in animal feeds in the presence of 11 other drugs: 3 nitrofurans, 2 macrolide antibiotics, 3 sulfonamides, 2 coccidiostatics, and 1 antibacterial growth promoter. OTC or CTC.HCl was separated from coexisting drugs and detected by thin-layer chromatography, then quantitated microbiologically by the agar diffusion method. Analysis of 125 experimental animal feed samples fortified at 5 levels (7.5-400 ppm) with OTC or CTC.HCl and at 1 level (50 ppm) with the rest of the drugs, respectively, gave a limit of quantitation of 1.25 or 0.625 ppm, a recovery of 90.6 or 92.9%, and a coefficient of variation of 2.9-6.1 or 2.3-4.4%.