Liquid chromatographic determination of ampicillin in bovine and dog plasma by using a tandem solid-phase extraction method.

The determination of ampicillin in plasma and serum by reversed-phase high-performance liquid chromatography with UV detection suffers from poor selectivity and sensitivity. Currently, the most common approach to overcoming these problems consists of improving the compound's detectability via pre- or postcolumn derivatization. In the method that we describe, however, enhanced selectivity is afforded by sample purification by a tandem solid-phase extraction method (ion-exchange and reversed-phase). This approach permits detection at wavelengths of as low as 210 nm, which results in enhanced sensitivity (detection limit, 0.01 microgram/ml). A second factor that affects selectivity is the addition to the chromatographic eluent of a crown ether to optimize the separation between ampicillin and polar endogenous plasma constituents. This combination of improved sample pretreatment and a more selective chromatographic system in conjunction with internal standardization forms the basis of a new assay for the quantitation of ampicillin in plasma. The overall recovery of ampicillin was 76.4% +/- 4.9% (n = 24), and the within-run and between-run coefficients of variation ranged from 1.6 to 7.2%. The method was applied to pharmacokinetic studies in cows and dogs after intramuscular or oral administration of the drug.