Confirmation of Chloramphenicol Residues in Bovine Milk by Gas Chromatography/Mass Spectrometry

Abstract

Abstract A gas chromatographic/mass spectrometric method was developed to confirm the presence of chloramphenicol (CAP) at 0.5 ng/mL in bovine milk. meta-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 is eluted with additional ethyl acetate. A solution of 4% NaCI in water is added to the eluant, and ethyl acetate is removed by evaporation under a flow of N2. The sample is defatted with hexane and then loaded onto a C18-SPE column. The column is washed with water, and CAP is eluted with methanol. The methanol is evaporated under a flow of N2, and the trimethylsilyl derivative is prepared with Sylon HTP (hexamethyldisilazane–trimethylchlorosilane–pyridine, 3 + 1 + 9). The excess reagent is evaporated under N2, and the residue is taken up in cyclohexane-hexane (60 + 40). CAP is separated on a 30 m × 0.25 mm id methylsilicone column having a film thickness of 0.25 pm and analyzed by methane negative chemical ionization with selected ion monitoring. The ions with 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 10% of that obtained by using standards.