PARTICLE BEAM LIQUID-CHROMATOGRAPHY MASS-SPECTROMETRY WITH NEGATIVE-ION CHEMICAL IONIZATION FOR THE CONFIRMATION OF IVERMECTIN RESIDUE IN BOVINE-MILK AND LIVER

Particle beam liquid chromatography/mass spectrometry (LC/MS) using negative ion chemical ionization was applied to the analysis of ivermectin residue in bovine milk and liver. Samples were prepared by liquid/liquid extraction followed by alumina B solid-phase extraction clean-up. On-line LC/MS of extracts was carried out on a C-18 bonded silica column. Signals were observed from on-column injections of 4 ng dihydro-avermectin B1a (H2B1a) in extracts equivalent to 2 ml milk or 0.3 g liver. The specificity required for a regulatory confirmation procedure was achieved by monitoring the H2B1a molecular ion and four fragment ions. Ion chromatogram peak areas were at least three times greater than control samples integrated over the same time window. Coeluting matrix compounds enhanced the response and altered the abundance pattern of H2B1a. To compensate for this matrix effect, control milk extracts were spiked with H2B1a standard and used for the abundance matching requirement of regulatory confirmation.