CONFIRMATION OF CHLORAMPHENICOL RESIDUES IN BOVINE-MILK BY GAS-CHROMATOGRAPHY MASS-SPECTROMETRY

A gas chromatographic/mass spectrometric method was developed to confi rm the presence of chloramphenicol (CAP) at 0.5 ng/mL in bovine milk m eta-Nitrochloramphenicol (20 ng) is added to 10 mL milk as a surrogate standard. The milk is mixed with ethyl acetate and then loaded onto a diatomaceous earth-filled, solid-phase extraction (SPE) column; CAP i s eluted with additional ethyl acetate. A solution of 4% NaCl in water is added to the eluant, and ethyl acetate is removed by evaporation u nder a flow of N-2. The sample is defatted with hexane and then loaded onto a C-18-SPE column. The column is washed with water, and CAP is e luted with methanol. The methanol is evaporated under a flow of Na, an d the trimethylsilyl derivative is prepared with Sylon HTP xamethyldis ilazane-trimethylchlorosilane-pyridine, 3 + 1 + 9). The excess reagent is evaporated under N-2, and the residue is taken up in cyclohexane-h exane (60 + 40). CAP is separated on a 30 m x 0.25 mm id methytsilicon e column having a film thickness of 0.25 mu m and analyzed by methane negative chemical ionization with selected ion monitoring. The ions wi th m/z 304, 322, 358, 360, 466, and 468 are monitored. Confirmation is based on the presence of 5 ions with relative ion abundances within 1 0% of that obtained by using standards.