Gas chromatographic determination of residues of thyreostatic drugs in bovine muscle tissue using combined resin mediated methylation and extraction

A method is described for the assay of residues of the thyreostatic substances 2-thiouracil, 6-methyl-2-thiouracil, 5- and 6-propyl-2-thiouracils and 6-phenyl-2-thiouracil in beef muscle at levels of quantitation down to 25 mu g/kg for thiouracil and 15 mu g/kg for substituted thiouracils. Analytes are extracted from the muscle matrix with acetonitrile and methylated by absorption onto a macroporous anion exchange resin followed by treatment with methyl iodide in acetonitrile at room temperature. Supercritical carbon dioxide is also a suitable derivatization solvent. Determination is carried out using gas chromatography with mass spectrometric detection. The methyl ester of 2,4,5-trichlorophenoxyacetic acid serves as an internal standard while 5-methyl-2-thiouracil is a suitable surrogate. 5-Ethyl-2-thiouracil is added to minimise losses of the analytes during the analysis. The method exhibits a linear range of 25-625 mu g/kg for the substited thiouracils and 50-1250 mu g/kg for 2-thiouracil. Recovery of the analytes ranges from 50% for 6-phenyl-2-thiouracil to 90% for 2-thiouracil with coefficients of variation of less than 10%. (C) 1997 Elsevier Science B.V.