Multi-residue analysis of avermectins and moxidectin by ion-trap LC-MSn

A multi-residue method was developed and validated for the quantitation and confirmation of avermectins and moxidectin residues in bovine liver. Target analytes were extracted from liver homogenate using C8 solid phase cartridges, chromatographed under basic pH conditions in order to promote the formation of analyte anions, and detected by ion-trap mass spectrometry (MS) in negative ion mode using an atmospheric pressure chemical ionization interface (APCI). The method provided detection capabilities (CC beta, where beta = 0.05) for eprinomectin, abamectin, doramectin, moxidectin and ivermectin of 3.1, 3.2, 2.2, 4.0 and 3.2 ng g-1 liver respectively, well below their respective maximum residue limits (MRLs). The critical concentrations for MRL compliance (CC alpha, where alpha = 0.01) were 840, 28, 130, 130 and 130 ng g-1 respectively. Analysis of liver fortified at the appropriate MRLs gave recoveries (% +/- RSD) of 70.9 +/- 11.6 (n = 14), 69.1 +/- 3.9 (n = 13), 65.9 +/- 6.4 (n = 19), 69.7 +/- 9.3 (n = 19) and 73.2 +/- 10.5 (n = 19), respectively, for each analyte. Calibration curves fitted a second order polynomial function (R2 > or = 0.9978) over a wide range of concentrations (0 to 10,000 ng ml-1). The detection of two daughter-ions for each analyte allowed for quantitation and the confirmation of identity. The method is suitable for application in European Union statutory veterinary drug residue surveillance programmes, since it fulfills appropriate analytical criteria, and has the particular advantage of enabling high throughput multi-residue quantitation and confirmation of the target analytes.