Collaborative study of a modified extraction procedure for lasalocid sodium in feeds by microbiological assay

A collaborative study was conducted on a modified extraction technique in the microbiological determination of lasalocid sodium in finished poultry feeds. The feed is warmed briefly with pH 4.7 buffer and the drug is extracted with ethyl acetate. The extract is washed with HCl and NaOH and evaporated to dryness, and the residue is dissolved in hexane, partitioned into methanol-water (3+1), and diluted to assay range at a final alcohol concentration of 25% (v/v). Seven laboratories participated in the study. Average recoveries of lasalocid sodium added at 0.01035, 0.00863, and 0.0069% ranged from 93 to 99% for mash feeds and from 86 to 95% for pellets. The coefficients of variation averaged 11.2% for mashes and 9.8% for pellets. The modified extraction technique has been adopted as official first action.