The procedures (purification, developing solvent, medium) used in the original thinlayer bioautographic method of Donoho and Kline for the determination of monensin in chick tissues were modified to improve the recovery and sensitivity of the assay. Recoveries of monensin from fortified tissue samples were 92.9% from fat, 86.0% from liver and 104.4% from muscle. The assay sensitivity was improved to give a detection limit of 0.01ppm in fat and 0.0125ppm in other tissues. Feed containing monensin at 80, 100 or 120ppm was given to chicks for 9 weeks, and the residual levels of monensin in tissues were assayed. The residual levels of monensin at 0 hour after withdrawal were 0.057 to 0.110ppm in fat, none to 0.035ppm in muscle, none to 0.039ppm in liver and none to 0.014ppm in kidney. No detectable amount of monensin was found in fat at 48 hours or more after its withdrawal from the feed, or in liver, muscle and kidney at 24 hours or more after withdrawal. When monensin was given for seven days at significantly higher levels (300 and 600 ppm) than recommended, the tissue levels did not increase proportionally with the levels in the feed. The half-life of monensin disappearance from fat was estimated to be 3.4 to 4.0 hours.
A paper disc method is described for determination of residual cephalexin (CEX) in chick tissues. A trichloroacetic acid extract of plasma and tissues is chromatographed on a macroreticular resin (Diaion HP-20) column to remove endogenous antibacterial substances interfering with the assay. The eluate is evaporated to dryness and the residue, dissolved in methanol-water (1 + 2), is subjected to a paper disc assay using Bacillus stearothermophilus var. calidolactis C953 NIZO as a test organism. The detection limit was 0.0375 ppm in tissue; the average recovery of CEX ranged from 72.4% in skin to 90.4% in plasma. Water containing 200 or 500 mg/L of CEX was given ad libitum to 2-week-old chicks for 10 days; the highest levels of CEX were found in the kidney, and the lowest were found in muscle at 0 h of withdrawal. CEX disappeared from most tissues at 24 h after withdrawal except from skin of chicks given 500 mg/ L. However, the drug was not detected in the skin at 48 h after withdrawal.
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