Colorimetric determination of arprinocid in feed

An analytical method has been developed for the determination of arprinocid (9-(2-chloro-6-fluorophenylmethyl)-9H-purin-6-amine) in feed, based upon measurement of the absorbance of the diazo chromophore formed from a product of zinc reduction of the drug in acidic solution. The analyte is extracted from the feed into chloroform in the presence of a pH 7 phosphate buffer and isolated by adsorption chromatography on alumina, followed by partitioning between hexane and 0.15M HCl. The reduction product in the aqueous phase is then treated for colorimetric measurement. This procedure has been applied to determining 0.0010--0.0080% arprinocid in feed with a precision of less than 5% relative standard deviation near the middle of this concentration range. Of 32 feed additives examined, only zoalene and sulfamethazine were serious interferences. A study and discussion of several factors, e.g., reaction time, pH, and amount of zinc metal, that affect the analytical reactions are also included.