Determination of penicillins in animal tissues at trace residue concentrations Part I. Determination of benzylpenicillin in milk by reversed-phase liquid chromatography with solid phase extraction and liquid Chromatographic fractionation clean-up

A liquid chromatographic (LC) method for the determination of benzylpenicillin (penicillin G) in milk has been developed. Milk was decreamed by centrifugation and deproteinised using sulphuric acid/sodium tungstate. Initial clean-up was achieved using C18 solid phase extraction (SPE). The SPE eluate was cleaned-up by reversed-phase LC fractionation at neutral pH. Final determination was by reversed-phase LC at acidic pH with post-column derivatisation with imidazole/mercury(II) chloride reagent followed by UV absorbance determination at 320 nm. Average recoveries at 2 and 10 μg kg−1 were 70–73%. The lower limit of quantification was 2 μg kg−1.