Abstract
Dihydrostreptomycin (DHS) is an aminoglycoside antibiotic used in veterinary medicine in combination with benzylpenicillin for the treatment of bacterial infections in cattle, pigs and sheep. A method to determine its residues in edible tissues of cattle, as well as in milk, was developed and validated. Extraction of DHS from the tissues was performed using a liquid extraction with a 10 mM phosphate buffer containing 2% (w/v) trichloroacetic acid, while milk samples were treated with a 50% (w/v) trichloroacetic acid solution, followed by a solidβphase cleanβup procedure on a carboxypropyl (CBA) weak cation exchange column. Ionβpair chromatography, using a mixture of 20 mM pentafluoropropionic acid in water and acetonitrile as the mobile phase, was used to retain DHS and the internal standard streptomycin (STR) on a Nucleosil (5 Β΅m) reversedβphase C18 column. The components were detected and quantified by electrospray ionization (ESI) tandem mass spectrometry. The method could be validated according to EC (European Community) requirements with respect to linearity, trueness and precision, the latter evaluated at the maximum residue limit (MRL)β1000 ng g^β1^ for kidney, 500 ng g^β1^ for muscle, liver and fat, and 200 ng g^β1^ for milkβ, at oneβhalf of the MRL and at one and a half times the MRL. A limit of quantification of 10 ng g^β1^ and 1 ng ml^β1^ was obtained for all tissues and for milk, respectively, which is far below oneβhalf of the MRL as requested, while the limit of detection was in the low ppb range, varying between 1.9 and 4.2 ng g^β1^ for the different tissues tested, and being 0.6 ng ml^β1^ for milk. The method was used for the monitoring of DHS residues in incurred tissue and milk samples coming from cattle medicated with DHS in combination with benzylpenicillin by intramuscular injection, in order to evaluate withdrawal times. Copyright Β© 2007 John Wiley & Sons, Ltd.