Gas-liquid chromatographic determination of melengestrol acetate in cattle feed supplements: collaborative study

The assay for melengestrol acetate (MGA) in cattle feed supplements was the subject of a collaborative study using 6 laboratories. Two feed formulations were each fortified with 0, 0.0625, 0.125, 0.500, 1.00, and 1.50 mg MGA/lb of supplement, and each laboratory assayed 4 samples of each level of each formulation by electron capture gas chromatography after liquid-liquid extraction of each sample followed by solvent partition and column chromatography cleanup procedures. Overall recovery was 83.2% with a pooled within-laboratory sample-to-sample standard deviation of 9.5%. This includes the variability due to formulation, batches, days, levels of MGA, samples, and their interactions. Including the collaborators as a source of variance results in an overall standard deviation of 15.0%. The major sources of variability were associated with collaborator, formulation, and level of MGA. Lower than expected recoveries were attributed to difficulty in recovering MGA from samples containing concentrations of MGA below the range of the method and the inadvertent substitution by some laboratories of ethanol-free chloroform for the ethanol-containing reagent grade chloroform specified in the methodology. On the basis of the collaborative results, the method has been adopted as official first action for MGA in cattle feed supplements at concentrations from 0.125 to 1.00 mg MGA/lb.