Detection of arsanilic acid in animal feeds

The animal feed sample is extracted with alcohol, the extract is evaporated to a small volume, and an aliquot is spotted on a silica gel H plate. After development of the plate in chloroform-methanol (80+20), the spotting area is scraped and extracted with ethanol, which is evaporated to dryness. The residue is dissolved in 3 ml alcohol, and an aliquot is reacted with p-dimethylaminobenzaldehyde solution; another aliquot of the extract is used as a sample blank. A control is prepared by spiking another portion of the same feed with about 5 ppm arsanilic acid. The method is applicable in the presence of procaine and sulfonamides.