SIMULTANEOUS DETERMINATION OF CHLORAMPHENICOL, FLORFENICOL AND THIAMPHENICOL RESIDUES IN MILK BY GAS-CHROMATOGRAPHY WITH ELECTRON-CAPTURE DETECTION

A gas chromatographic (GC) method is described for determining residue s of chloramphenicol (CAP), florfenicol (FF), and thiamphenicol (TAP) in raw milk, with meta-nitrochloramphenicol (mCAP) as internal standar d. Milk is extracted with acetonitrile, centrifuged, evaporated, recon stituted in water, and passed through a C-18 solid-phase extraction (S PE) column. The SPE column is eluted with 60% methanol, and then the e luate is evaporated and derivatized with Sylon BFT {N,O-bis(trimethyls ilyl)trifluoroacetamide [BSTFA]-trimethylchlorosilane [TMCS], 99 + 1}. After derivatization, toluene is added directly to the sample, follow ed by water, to quench the derivatization process. After centrifugatio n, the organic layer is carefully removed. Analytes are determined by GC with electron capture detection (ECD). Milk was fortified with feni cols (the collective name for CAP, FF, and TAP) at 5, 10, 20, 40 and 8 0 ng/mL (target level = 10 ng/ml). Overall recoveries were 92, 100, an d 104% for CAP, FF, and TAP, respectively. Overall interassay (between -day) variabilities were 6.1, 6.7, and 6.0% for CAP, FF, and IAP, resp ectively. Raw milk samples containing incurred residues of FF were als o analyzed.