HPLC-INTEGRATED SOLID-PHASE EXTRACTION WITH PHOTOCHEMICAL POSTCOLUMN DERIVATIZATION FOR THE DETERMINATION OF OXACILLIN, CLOXACILLIN AND DICLOXACILLIN IN RAW-MILK

An automated reversed-phase high-performance liquid chromatography (HP LC) method with UV detection at 300 nm alter on-line coupled solid-pha se extraction (SPE) and photochemical post-column derivatization was d eveloped for the residue analysis of or;acillin, cloxacillin and diclo xacillin in raw milk. After a centrifugal-ion step for defatting and u ltrafiltration, a HPLC-integrated SPE using a restricted access sorben t, is performed. By operating the system in parallel mode, SPE and HPL C run are performed simultaneously. The procedure allows the analysis of about 30 samples within 24 h and was used to investigate the milk o f cows which had been treated with oxacillin. The detection limit was 3 mu g/kg for oxacillin and cloxacillin and 5 mu g/ kg for dicloxacill in. The recoveries were between 66% and 81% with coefficients of varia tion between 3.3% and 7,7%. Using an acetonitrile extraction procedure better recoveries of greater than or equal to 90% were obtained with fortified samples, but more or less identical results were obtained fo r the samples with incurred residues after correction for recovery.