A gas chromatographic method was devised for determining residues of Ciba C-9491 [O-(2,5-dichloro-4-iodophenyl) O,O-dimethyl phosphorothioate], its oxygen analog [2,5-dichloro-4-iodophenol dimethyl phosphate], and its phenolic hydrolysis product [2,5-dichloro-4-iodophenol] in sweet corn and milk. The three compounds were separated by liquid chromatography on two columns, silica gel and alumina. The columns were operated first in tandem, which allowed the Ciba C-9491 to pass through, and then separately to elute the oxygen analog from the silica gel with acetone and the phenol from the alumina with methanol. The gas chromatography of the Ciba C-9491 and its oxygen analog fractions was carried out with a flame photometric detector sensitive to phosphorus, while that of the phenol fraction was conducted with an electron-capture detector. Other than the separation of the three compounds, cleanup was minimal. Recoveries of Ciba C-9491 were 93 to 100%, those of the oxygen analog were 82 to 93%, and those of the phenol were 53 to 71%. Sensitivity was better than 0.01 p.p.m. © 1968, American Chemical Society. All rights reserved.
