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 milk was developed/modified on the basis of published papers. Mean recoveries were 87 and 95% for STR and DHS, respectively. Recoveries are dependent on the concentration level and batch of solid-phase extraction columns used, and independent of fat content and homogenization. The relative standard deviations are 15.6 and 9.6% for STR and DHS, respectively, at a level of 100 micrograms kg-1. Limits of detection (8 and 12 micrograms kg-1, respectively) and quantification (12 and 18 micrograms kg-1, respectively) are far below the EU maximum residue limit of 200 micrograms kg-1. Lyophilized DHS samples can be used for internal control of the analysis as the DHS concentration is not influenced by the lyophilization process and subsequent storage at 6 degrees C. In incurred milk samples no false negative results of preliminary confirmation tests (Charm II Aminoglycoside test, Ridascreen Streptomycin ELISA) with respect to DHS concentrations > or = 20 micrograms kg-1 as determined by the LC method are observed. DHS concentrations of incurred samples determined by ELISA are higher than those obtained by the LC method. These differences were more pronounced with incurred than with spiked milk samples, thus leading to the conclusion that in incurred samples substances are present which co-react in the ELISA and which are not detected by the LC method.