Quantitation and validation of macrolide endectocides in raw milk by negative ion electrospray MS/MS.

Macrolide endectocides (abamectin, doramectin, emamectin, eprinomectin, ivermectin, and moxidectin) are used to treat animals against a variety of parasites. They are approved for use with food animals, but require long withdrawal times. Two compounds, eprinomectin and moxidectin, are approved for use with lactating cattle and have established maximum residue limit values of 20 and 40 microg/kg, respectively. The remaining compounds may appear in milk if there is off-label or accidental use. This method is capable of quantitating and confirming the presence of six of the macrolide endectocides over the concentration range 1-60 microg/kg (parts per billion). Selamectin is used as the internal standard. The compounds are extracted with C18 solid-phase extraction under basic conditions and quantitated by negative ion LC-MS/MS using one selected reaction monitoring transition, with a second transition used for verification. The limits of detection were determined from between-day experiments as 0.14, 0.14, 0.18, 0.24, 0.25, and 0.25 microg/kg for abamectin, doramectin, emamectin, eprinomectin, ivermectin, and moxidectin, respectively. The detection capability CCbeta for eprinomectin was measured as 28.5 microg/kg, and 50.0 microg/kg for moxidectin using five milk matrices.