Quantitative analysis of amoxycillin and its major metabolites in animal tissues by liquid chromatography combined with electrospray ionization tandem mass spectrometry

A sensitive and specific method for the quantitative determination of amoxycillin and its major metabolites (amoxycilloic acid, amoxycillinpiperazine-2',5'-dione) in animal tissue samples using liquid chromatography combined with positive electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) is presented. A liquid extraction using an aqueous 0.01 M potassium dihydrogenphosphate solution as the extraction solvent was performed for a preliminary sample cleanup. The extracts were further purified by a solid-phase extraction using an octadecyl (C18) column. Ampicillin was used as the internal standard. Chromatographic separation of the analytes of interest was achieved on a reversed-phase Hypersil column (100 x 3 mm i.d., dp, 5 microm), using a mixture of 9.6 mM pentafluoropropionic acid in water and acetonitrile as the mobile phase. Gradient elution was performed. To obtain as high sensitivity and selectivity as possible, the mass spectrometer was operated in the multiple reaction monitoring mode. The method was validated for the analysis of amoxycillin and its investigated metabolites in various porcine tissues, kidney, liver, muscle, and fat, according to the requirements defined by the European Community. Calibration graphs were prepared for all tissues, and good linearity was achieved over the concentration range tested (25-500 ng/g, r > or = 0.9974, and goodness of fit < or = 9.6). A limit of quantification of 25 ng/g was obtained for amoxycillin and its metabolites in all tissues, which corresponds to half the maximum residue limit for amoxycillin. Limits of detection ranged from 2.3 to 12.0 ng/g for amoxycillin and from 1.1 to 15.1 ng/g and 0.2 to 2.4 ng/g for amoxycilloic acid and amoxycillinpiperazine-2',5'-dione, respectively. The results for the within-day precision and the trueness fell within the ranges specified. The method has been successfully used for the quantitative determination of amoxycillin and its major metabolites in tissue samples from pigs medicated via the drinking water, proving the usefulness of the developed method for application in the field of residue analysis.