Comparison of liquid chromatography with microbial inhibition assay for determination of incurred amoxicillin and ampicillin residues in milk

A comparison was made between liquid chromatography (LC) methods and a microbial inhibition (MI) method (Bacillus stearothermophilus disk assay) for the determination of amoxicillin and ampicillin residues in milk from cows dosed with the drugs. One cow was injected intramuscularly with ampicillin, and a second cow received an intramammary infusion of amoxicillin. Milk samples were collected at various depletion time periods following administration of the drugs. The LC methods using formaldehyde and salicylaldehyde derivatization were;applied in the determination of ampicillin and amoxicillin residues, respectively. The LC salicylaldehyde derivatization method was also applied to mixed milk samples for determination of both antibiotics, and the results were in agreement with those determined separately. The MI method was applied to each type of incurred milk. No significant differences were found between the LC and MI assay methods for residue levels within the reliable detection range of the MI method. The LC method was more sensitive than the MI method for residues <10 ng/mL.