Determination of chlorinated methylthiobenzenes and their sulfoxides and sulfones in fish.

A procedure was developed to determine chlorinated methylthiobenzenes and their respective sulfur oxidation products in fish. Perch samples fortified at the 0.1 ppm level with 2,4,5-trichloromethylthiobenzene, pentachloromethylthiobenzene, and their sulfoxides and sulfones were extracted and cleaned up using an adaptation of the official AOAC method for multiple residues of organochlorine pesticides. The Florisil column cleanup was modified; 200 mL 6% petroleum etherethyl ether eluted the methylthiobenzenes, 200 mL 50% PE-EE eluted the sulfones, and 200 mL EE eluted the sulfoxides. Recoveries determined by electron capture (ECD) gas chromatography (GC) were 75-101% for the methylthiobenzenes and their sulfones and 63-93% for the sulfoxides. Co-extracted materials in the Florisil eluates that interfered with the ECD/GC quantitation were removed by partitioning the sulfoxides and sulfones into sulfuric acid and by thin layer chromatography on silica gel, using methylene chloride-hexane (50 + 50) as the developing solvent. Seven fish samples containing residues of chlorinated benzenes or polychlorinated biphenyls (PCBs) were examined for chlorinated methylthiobenzenes, methylthio-PCBs, and their oxidation products by matching GC retention times obtained with the EC detector and a flame photometric detector operated in the sulfur mode. These analytes were not found in the fish samples above a detection level equivalent to 0.02 ppm 2,4,5-trichloromethylthiobenzene.