Thermal and long-term freezing stability of ivermectin residues in sheep milk

The stability of ivermectin residues in sheep milk under conditions of pasteurization (74 degreesC, 40 s), high pasteurization (80 degreesC, 1 min), and boiling (100 degreesC, 10 s) based on residue content (at a level of concentration of about 2 mug/kg and 30 mug/kg of H2B1a) and sort of ingestion (in vitro, in vivo) was studied. None of these factors influenced concentrations of ivermectin in the matrix investigated. Residues of ivermectin were also stable in milk after one year of freezing at -20 degreesC. After two years of freezing they had diminished by approximately one quarter. The concentration of the H2B1a component was analyzed using a combined HPLC method with fluorescence detection. Samples were extracted with acetonitrile and cleaned-up using a solid phase extraction on C-8 minicolumns. Eluate was concentrated under stream of nitrogen and dry extracts were pre-column derivatized with trifluoroacetic anhydride and N-methyl-imidazole. The method used was extensively validated.