Optimization and ruggedness testing of the determination of residues of carbadox and metabolites in products of animal origin. Stability studies in animal tissues.

A method developed for the determination of residues of carbadox and its metabolites in swine tissues using high-performance liquid chromatography with on-line precolumn enrichment and postcolumn derivatization with UV-VIS detection was optimized and the applicability of the method was extended to plasma and eggs. With the optimized method, more than twenty samples per person per day can be analysed. In the matrices investigated, the observed limit of determination for carbadox is 0.5-1 micrograms/kg and for desoxy-carbadox 0.5-2 micrograms/kg. The mean recovery for desoxy-carbadox in kidney, muscle and liver as established by two laboratories over a 2-month period is 95% (relative standard deviation = 14%, N = 37, 10 micrograms/kg). In other matrices the recoveries are between 83 and 91%. The recovery for carbadox is 70-80% in muscle, plasma and eggs. The method has been used routinely in pharmacokinetic and surveillance studies. Stability studies of kidney and liver samples spiked with carbadox showed that carbadox is rapidly decomposed (in vitro metabolism). After storage for about 1 h at 4 degrees C, more than 50% of the added amount is converted by reduction to desoxy-carbadox. In contrast, carbadox is stable in eggs and muscle under spiking conditions and during storage at -20 degrees C. Desoxy-carbadox is stable during spiking and storage at -20 degrees C in eggs and muscle. In kidney and liver, its stability was good under spiking conditions but could not be proved unequivocally during storage.