Development and validation of a method for the determination of sub-additive levels of virginiamycin in compound animal feeds by liquid chromatography.

A method for the detection of virginiamycin M1 as a marker compound of virginiamycin at sub-additive level in pig, calf, piglet, sow, poultry, cattle and laying hen feeds was developed and validated. Both UV detection at 230 nm and MS detection were applied. Virginiamycin M1 was extracted from animal feeds with ethyl acetate after wetting of the feed with water followed by clean-up on Sep-Pak silica gel and OASIS HLB cartridges. Analysis of extracts was carried out on an Inertsil ODS-2 column with acetonitrile-water-formic acid as the mobile phase and UV detection at 230 nm. The limit of quantification (LOQ) of the method was 2.7 mg kg(-1). The proposed method was validated at a target species dependent minimum required performance limit (MRPL), at 2MRPL and at 5MRPL levels in pig, calf, piglet, sow, poultry, cattle and laying hen feeds. Recoveries at target species dependent MRPL levels ranged from 38 to 67%, within-day repeatabilities from 7 to 19% and within-laboratory reproducibilities from 13 to 27%. The proposed UV method is primarily suitable for screening purposes at subadditive levels, but semi-quantitative data can also be produced. Three MS detection modes (ion-source CID, full MS and MS2) were tested as an alternative and/or extension to UV detection. The selectivity and sensitivity of both LC-MS2 and LC-MS were much better than those of UV detection at 230 nm.