Detection of incurred dihydrostreptomycin residues in milk by liquid chromatography and preliminary confirmation methods

An LC method for the determination of the aminoglycosides streptomycin (STR ) and dihydrostreptomycin (DHS) in milli was developed/modified on the basi s of published papers. Mean recoveries were 87 and 95% for STR and DHS, res pectively. Recoveries are dependent on the concentration level and batch of solid-phase extraction columns used, and independent of fat content and ho mogenization. The relative standard deviations are 15.6 and 9.6% for STR an d DHS, respectively, at a level of 100 mu g kg(-1). Limits of detection (8 and 12 mu g kg(-1), respectively) and quantification (12 and 18 mu g kg(-1) , respectively) are far below the EU maximum residue limit of 200 mu g kg(- 1). Lyophilized DHS samples can be used for internal control of the analysi s as the DHS concentration is not influenced by the lyophilization process and subsequent storage at 6 degrees C. In incurred milk samples no false ne gative results of preliminary confirmation tests (Charm II Aminoglycoside t est, Ridascreen Streptomycin ELISA) with respect to DHS concentrations grea ter than or equal to 20 mu g kg(-1) as determined by the LC method are obse rved. DHS concentrations of incurred samples determined by ELISA are higher than those obtained by the LC method. These differences were more pronounc ed with incurred than with spiked milk samples, thus leading to the conclus ion that in incurred samples substances are present which co-react in the E LISA and which are not detected by the LC method.