Liquid chromatographic determination of amoxicillin concentrations in bovine plasma by using a tandem solid-phase extraction method.

We report a means of determining amoxicillin in bovine plasma by liquid chromatography with UV detection at 235 nm. Purification and concentration of extracts were accomplished by a tandem solid-phase extraction procedure with two reversed-phase columns. Separation of amoxicillin from interferences was improved by the incorporation of a crown ether in the solvent systems used both for the solid-phase extraction and the final high-pressure liquid chromatography. Cefadroxil was added as an internal standard. The average recovery of amoxicillin from plasma (n = 23) was 78.2 +/- 3.0%, and the within-run and between-run coefficients of variation ranged from 1.8 to 7.0%. The detection limit was estimated at 0.1 microgram/ml. This method was used to determine amoxicillin in bovine plasma after intramuscular administration of the drug.