HPLC-integrated solid-phase extraction with photochemical post-column derivatization for the determination of oxacillin, cloxacillin and dicloxacillin in raw milk

An automated reversed-phase high-performance liquid chromatography (HPLC) method with UV detection at 300 nm alter on-line coupled solid-phase extraction (SPE) and photochemical post-column derivatization was developed for the residue analysis of or;acillin, cloxacillin and dicloxacillin in raw milk. After a centrifugal-ion step for defatting and ultrafiltration, a HPLC-integrated SPE using a restricted access sorbent, is performed. By operating the system in parallel mode, SPE and HPLC run are performed simultaneously. The procedure allows the analysis of about 30 samples within 24 h and was used to investigate the milk of 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 dicloxacillin. The recoveries were between 66% and 81% with coefficients of variation 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 for the samples with incurred residues after correction for recovery.