2-[(DiMethoxyphosphinothioyl)thio]butanedioic Acid 1-Ethyl Ester Malathion β-Monocarboxylic Acid O,O-DiMethyl S-(1-Carbethoxy-2-carboxy)ethyl Phosphorodithioate S-[2-Carboxy-1-(ethoxycarbonyl)ethyl]O,O-di-Methyl Phosphorodithioate Butanedioic acid, 2-[(dimethoxyphosphinothioyl)thio]-, 1-ethyl ester Malathion β-Monoacid Malathion Impurity 3(Malathion β-Monoacid)