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One-day-old chickens were individually orally treated with chloramphenicol at a dose of 100 mg per kg of body weight per day for three consecutive days. After the final treatment, the groups of six birds were sacrificed in seven-day intervals up to 42 days. The muscle tissue collected from the breasts and legs of each bird was individually examined for the presence of chloramphenicol residues using a GC/MS-NCI analytical method, which was validated according to Commission Decision 2002/657/EC. The decision limit (CCα) obtained for the method was 0.05 ng g. The results showed a rapid decrease of chloramphenicol concentration in the muscle tissue after termination of the treatment, but also showed a relatively long persistence of low residue concentrations. Levels of chloramphenicol in muscle tissue averaged 64 ng g seven days after the final treatment and fell to 0.21 ng g after 35 days. All animals tested on the 35th day after the final treatment showed detectable chloramphenicol concentrations above the decision limit of the method used. No residues were detected in the animal tissues 42 days after the end of the treatment.

Amoxicillin concentrations were determined by two independent laboratories for a pig starter feed medicated from a coated amoxicillin premix. The analytical method was previously transferred from one laboratory to the other one. The data between the two laboratories were consistent, showing ruggedness of the assay. Mean amoxicillin feed concentrations before and after pelletization were higher than 90% of the theoretical content, confirming satisfactory stability of this active ingredient in the coated form tested.